[Transcriber's notes:

The text of this e-book has been preserved in its original form
apart from correction of the typographic errors listed below.
Illustrations have been repositioned adjacent to relevant tabulated
data, and the List of Illustrations adjusted accordingly. On
p.72 an image of the Xanthin formula incorrectly shows a double
bond between a carbon and nitrogen atom – the correct formula
is shown on the next page – and there is a date discrepancy on
p. 248 between the text and the illustration caption (November
18/February 27). Footnotes have been repositioned below the
relevant paragraphs.

Typographic corrections:

  enyzmes → enzymes
  oxgyen → oxygen
  enyzme → enzyme
  Futher → Further
  mechancial → mechanical
  rythmical → rhythmical
  economcially → economically
  circulirinden → circulirenden
  SUBJECT → SUBJECTS
  equibrium → equilibrium
  availibility → availability
  (166) grams → (166 grams)
  accusstomed → accustomed
  Glassner → Glässner
  strach → starch
]




THE NUTRITION OF MAN




  THE

  NUTRITION OF MAN

  BY

  RUSSELL H. CHITTENDEN, Ph.D., LL.D., Sc.D.

  AUTHOR OF “PHYSIOLOGICAL ECONOMY IN NUTRITION,” ETC.
  PROFESSOR OF PHYSIOLOGICAL CHEMISTRY
  AND DIRECTOR OF THE SHEFFIELD
  SCIENTIFIC SCHOOL OF YALE UNIVERSITY


  WITH ILLUSTRATIONS


  NEW YORK
  FREDERICK A. STOKES COMPANY
  PUBLISHERS




  _Copyright, 1907_,
  BY FREDERICK A. STOKES COMPANY

  _All rights reserved_


  May, 1907


  _FIFTH PRINTING_




PREFACE


The present book is the outcome of a course of eight lectures delivered
before the Lowell Institute of Boston in the early part of 1907.

In this presentation of the subject the attempt has been made to
give a systematic account of our knowledge regarding some of the
more important processes of nutrition, with special reference to the
needs of the body for food. In doing this, the facts accumulated by
painstaking observations and experiments during recent years in our
laboratory have been incorporated with data from other sources and
brought into harmony, so far as possible, with the modern trend of
physiological thought.

Numerous experimental results, hitherto unpublished, have been
introduced, notably in Chapter VII, in which a few of the data recently
obtained in our laboratory with dogs are presented in some detail,
since they afford evidence of the error of the current arguments
concerning the necessity of a high proteid intake by man, as based on
the results of earlier investigators with high proteid animals.

It is hoped that the facts and arguments here presented will help to
arouse a more general interest in the subject of human nutrition, as
right methods of living promise so much for the health and happiness of
the individual and of the community.




CONTENTS


  CHAPTER I                                                         PAGE

  FOODS AND THEIR DIGESTION                                            1

  TOPICS: The purpose of nutrition. The food of man. Proteid foods.
  Carbohydrate foods. Fats. Food as fuel. Composition of foodstuffs.
  Availability of foods. Food as source of energy. Various factors in
  the nourishment of the body. Processes of digestion. Secretion of
  saliva. Function of saliva. Enzymes. Reversible action of enzymes.
  Specificity of enzymes. Mastication. Gastric secretion. Components
  of gastric juice. Action of gastric juice. Muscular movements
  of stomach. Time foods remain in stomach. Importance of stomach
  digestion. Processes of the small intestine. Secretion of pancreatic
  juice. Chemical changes in small intestine. Destruction of proteid
  food. Significance of the breaking down of proteid. Change of fatty
  foods and carbohydrates in intestine. Digestion practically complete
  at end of small intestine. Putrefaction held in check. Digestion a
  prelude to utilization of food.


  CHAPTER II

  ABSORPTION, ASSIMILATION, AND THE PROCESSES OF METABOLISM           39

  TOPICS: Physiological peculiarities in absorption. Chemical changes
  in epithelial walls of intestine. Two pathways for absorbed
  material. Function of the liver as a regulator of carbohydrate.
  Absorption of proteid products. Assimilation of food products.
  Anabolism. Katabolism. Metabolism. Processes of metabolism. Older
  views regarding oxidation. Discoveries of Lavoisier. The views of
  Liebig. Theory of luxus consumption. Oxidation in the body not simple
  combustion. Oxygen not the cause of the decompositions. Oxidation not
  confined to any one place. Intracellular enzymes. Living cells the
  guiding power in katabolism. Some intermediary products of tissue
  metabolism. Chemical structure of different proteids. Decomposition
  products of nucleoproteids. Relation to uric acid. Action of specific
  intracellular enzymes. Creatin and creatinin. Relation to urea.
  Proteid katabolism a series of progressive chemical decompositions.
  Intracellular enzymes as the active agents.


  CHAPTER III

  THE BALANCE OF NUTRITION                                            77

  TOPICS: Body equilibrium. Nitrogen equilibrium. Carbon equilibrium.
  Loss of nitrogen during fasting. Influence of previous diet on loss
  of nitrogen in fasting. Output of carbon during fasting. Influence
  of pure proteid diet on output of nitrogen. Influence of fat on
  proteid metabolism. Effect of carbohydrate on nitrogen metabolism.
  Storing up of proteid by the body. Transformation of energy in the
  body. Respiration calorimeter. Basal energy exchange of the body.
  Circumstances influencing energy exchange. Effect of food on heat
  production. Respiratory quotient and its significance. Influence of
  muscle work on energy exchange. Elimination of carbon dioxide during
  work and with different diets. Effect of excessive muscular work
  on energy exchange. Oxygen consumption under different conditions.
  Output of matter and energy subject to great variation. Body
  equilibrium and approximate nitrogen balance to be expected in health.


  CHAPTER IV

  SOURCE OF THE ENERGY OF MUSCLE WORK, WITH SOME THEORIES OF
    PROTEID METABOLISM                                               119

  TOPICS: Relation of muscle work to energy exchange. Views of Liebig.
  Experimental evidence. Relation of nitrogen excretion to muscle
  work. Significance of the respiratory quotient in determining
  nature of the material oxidized. Fats and carbohydrates as source
  of energy by muscles. Utilization of proteid as a source of energy.
  Formation of carbohydrate from proteid. Significance of proteid
  metabolism. Theories of Carl Voit. Morphotic proteid. Circulating
  proteid. General conception of proteid metabolism on the basis of
  Voit’s theories. Pflüger’s views of proteid metabolism. Rapidity of
  elimination of food nitrogen. Methods by which nitrogen is split off
  from proteid. Theories of Folin. Significance of creatinin and of the
  percentage distribution of excreted nitrogen. Endogenous or tissue
  metabolism. Exogenous or intermediate metabolism. Needs of the body
  for proteid food possibly satisfied by quantity sufficient to meet
  the demands of tissue or endogenous metabolism. Bearings of Folin’s
  views on current theories and general facts of proteid metabolism.
  Large proteid reserve and voluminous exogenous metabolism probably
  not needed. Importance of feeding experiments in determining the true
  value of different views.


  CHAPTER V

  DIETARY HABITS AND TRUE FOOD REQUIREMENTS                          153

  TOPICS: Dietetic customs of mankind. Origin of dietary standards.
  True food requirements. Arguments based on custom and habit.
  Relationship between food consumption and prosperity. Erroneous
  ideas regarding nutrition. Commercial success and national wealth
  not the result of liberal dietary habits. Instinct and craving not
  wise guides to follow in choice and quantity of food. Physiological
  requirements and dietary standards not to be based on habits and
  cravings. Old-time views regarding temperate use of food. The sayings
  of Thomas Cogan. The teachings of Cornaro. Experimental results
  obtained by various physiologists. Work of the writer on true proteid
  requirements. Studies with professional men. Nitrogen equilibrium
  with small amounts of food. Sample dietaries. Simplicity in diet.
  Nitrogen requirement per kilogram of body-weight. Fuel value of the
  daily food. Experiments with University athletes. Nitrogen balance
  and food consumption. Sample dietaries. Adequacy of a simple diet.


  CHAPTER VI

  FURTHER EXPERIMENTS AND OBSERVATIONS BEARING ON TRUE FOOD
    REQUIREMENTS                                                     191

  TOPICS: Dietary experiments with a detail of soldiers from the United
  States Army. General character of the army ration. Samples of the
  daily dietary adopted. Rate of nitrogen metabolism attained. Effect
  on body-weight. Nitrogen balance with lowered proteid consumption.
  Influence of low proteid on muscular strength of soldiers and
  athletes. Effect on fatigue. Effect on physical endurance. Fisher’s
  experiments on endurance. Dangers of underfeeding. Dietary
  observations on fruitarians. Observations on Japanese. Recent dietary
  changes in Japanese army and navy. Observations of Dr. Hunt on
  resistance of low proteid animals to poisons. Conclusions.


  CHAPTER VII

  THE EFFECT OF LOW PROTEID DIET ON HIGH PROTEID ANIMALS             229

  TOPICS: A wide variety of foods quite consistent with temperance in
  diet. Safety of low proteid standards considered. Arguments based
  on the alleged effects of low proteid diet on high proteid animals.
  Experiments of Immanuel Munk with dogs. Experiments of Rosenheim.
  Experiments of Jägerroos. Comments on the above experiments. The
  experiments of Watson and Hunter on rats. The writer’s experiments
  with dogs. Details of the results obtained with six dogs. Comparison
  of the results with those of previous investigators. Effect of a
  purely vegetable diet on dogs. Different nutritive value of specific
  proteids considered. Possible influence of difference in chemical
  constitution of individual proteids. Effect of low proteid diet on
  the absorption and utilization of food materials in the intestine
  of dogs. General conclusions from the results of experiments with
  animals.


  CHAPTER VIII

  PRACTICAL APPLICATIONS WITH SOME ADDITIONAL DATA                   266

  TOPICS: Proper application of the results of scientific research
  helpful to mankind. Dietary habits should be brought into conformity
  with the true needs of the body. The peculiar position of proteid
  foods emphasized. The evil effects of overeating. What the new
  dietary standards really involve. The actual amounts of foodstuffs
  required. Relation of nutritive value to cost of foods. The
  advantages of simplicity in diet. A sample dietary for a man of
  70 kilograms body-weight. A new method of indicating food values.
  Moderation in the daily dietary leads toward vegetable foods. The
  experiments of Dr. Neumann. The value of fruits as food. The merits
  of animal and vegetable proteids considered in relation to the
  bacterial processes in the intestine. A notable case of simplicity
  in diet. Intelligent modification of diet to the temporary needs of
  the body. Diet in summer and winter contrasted. Value of greater
  protection to the kidneys. Conclusion.


  INDEX                                                              303




LIST OF ILLUSTRATIONS


                                                             FACING PAGE

  Photograph of one of the athletes                                  190

  Photograph of soldiers taken at the close of the experiment        194

  Photograph of soldiers taken at the close of the experiment        195

  Photograph of Fritz at the close of the experiment                 200

  Photographs of the dogs experimented with

    Subject No. 5 August 19, 1905                                    248
    Subject No. 5 November 18, 1905                                  248
    Subject No. 5 April 24, 1906                                     248
    Subject No. 5 June 27, 1906                                      248

    Subject No. 3 August 19, 1905                                    251
    Subject No. 3 November 18, 1905                                  251
    Subject No. 3 April 24, 1906                                     251
    Subject No. 3 June 27, 1906                                      251

    Subject No. 13 January 2, 1906                                   252
    Subject No. 13 February 27,1906                                  252
    Subject No. 13 April 24, 1906                                    252
    Subject No. 13 June 19, 1906                                     252

    Subject No. 15 January 2, 1906                                   252
    Subject No. 15 February 27, 1906                                 252
    Subject No. 15 April 24, 1906                                    253
    Subject No. 15 June 19, 1906                                     252

    Subject No. 20 January 2, 1906                                   252
    Subject No. 20 February 27, 1906                                 252
    Subject No. 20 April 24, 1906                                    252
    Subject No. 20 June 19, 1906                                     252

    Subject No. 17 January 2, 1906                                   256
    Subject No. 17 February 27, 1906                                 256
    Subject No. 17 April 24, 1906                                    252
    Subject No. 17 June 27, 1906                                     252




THE NUTRITION OF MAN




CHAPTER I

FOODS AND THEIR DIGESTION

  TOPICS: The purpose of nutrition. The food of man. Proteid foods.
  Carbohydrate foods. Fats. Food as fuel. Composition of foodstuffs.
  Availability of foods. Food as source of energy. Various factors in
  the nourishment of the body. Processes of digestion. Secretion of
  saliva. Function of saliva. Enzymes. Reversible action of enzymes.
  Specificity of enzymes. Mastication. Gastric secretion. Components
  of gastric juice. Action of gastric juice. Muscular movements
  of stomach. Time foods remain in stomach. Importance of stomach
  digestion. Processes of the small intestine. Secretion of pancreatic
  juice. Chemical changes in small intestine. Destruction of proteid
  food. Significance of the breaking down of proteid. Change of fatty
  foods and carbohydrates in intestine. Digestion practically complete
  at end of small intestine. Putrefaction held in check. Digestion a
  prelude to utilization of food.


One of the great mysteries of life is the power of growth, that
harmonious development of composite organs and tissues from simple
protoplasmic cells, with the ultimate formation of a complex organism
with its orderly adjustment of structure and function. Equally
mysterious is that wonderful power of rehabilitation by which the cells
of the body are able to renew their living substance and to maintain
their ceaseless activity through a period, it may be of fourscore
years, before succumbing to the inevitable fate that awaits all organic
structures. This bodily activity, visible and invisible, is the result
of a third mysterious process, more or less continuous as long as life
endures, of chemical disintegration, decomposition, and oxidation, by
which arises the evolution of energy to maintain the heat of the body
and the power for mental and physical work.

These three main functions constitute the purpose of nutrition. The
growth of the adult man from the tiny cell or germ that marks his
simple beginning is at the expense of the food material he absorbs and
assimilates. The rehabilitation of the cells, or the composite tissues
of the fully developed organism, is accomplished through utilization
of the daily food, whereby cell substance is renewed and all losses
made good. The energy which manifests itself in the form of heat and
mechanical or mental work, _i. e._, the energy by which the vital
machinery is maintained in ceaseless activity, comes from the breaking
down of the food materials by means of which, as the saying goes, the
body is nourished. The body thus becomes the centre of different lines
of activity, the food serving as the material out of which new cells
and tissues are constructed, old cells revivified, and energy for
running the bodily machinery derived. Development, growth, and vital
activity all depend upon the availability of food in proper amounts and
proper quality.

The food of man is composed mainly of organic materials, for while,
as Dr. Curtis[1] has expressed it, “the plant can make organic matter
out of inorganic elements, just this the animal cannot do at all.
The thing of legs and locomotion, of spine and speech, can build
his organic walls only out of organic bricks ruthlessly ripped from
existing walls of other animals or plants.” It is true that man has
need of certain inorganic salts in his daily diet, but they are in
the nature of aids to nutrition (aside from such as are necessary for
the formation of bone and teeth), contributing in some measure toward
regulation and control of nutritive processes rather than as a source
of energy to the body. Inorganic substances, however, are an integral
part of the essential tissues and organs of the body, being combined
with the organic constituents of the living cells. Indeed, electrolytes
are perhaps the substances that put life into the proteids of the
protoplasm, and it is truly important for the integrity and functional
power of living cells that the proportion of inorganic constituents
therein be kept in a constant condition of quality and quantity.
Still, the food of mankind is essentially organic in nature, and while
it may be exceedingly varied in character, ranging from the simple
vegetable dietary of the natives of India and the Far East to the
voluminous admixture of varied forms of animal and vegetable foodstuffs
so acceptable to the _bon vivant_ of our western civilization, the
principles contained therein are few in number.

  [1] Edward Curtis, M.D. Nature and Health: Henry Holt & Co., New
  York. 1906. p. 39.

The organic foodstuffs are of three distinct types and are classified
under three heads, viz.: Proteids or Albuminous foodstuffs,
Carbohydrates, and Fats. All animal and vegetable foods, whatever
their nature and whatever their origin, are composed simply of
representatives of one or more of these three classes of food
principles.

Proteid substances are characterized by containing about 16 per cent
of nitrogen. In addition, they contain on an average 52 per cent of
carbon, 7 per cent of hydrogen, 23 per cent of oxygen, and 0.5–2.0 per
cent of sulphur. A certain class of proteids, known as nucleoproteids
because of their occurrence in the nuclei of cells, contain likewise
a small amount of phosphorus in organic combination. Proteid or
albuminous substances constitute the chemical basis of all living
cells, whether animal or vegetable. This means, expressed in different
language, that the organic substance of all organs and tissues, whether
of animals or plants, is made up principally of proteid matter. Proteid
substances occupy, therefore, a peculiar position in the nutrition of
man and of animals in general. They constitute the class of essential
foodstuffs without which life is impossible. For tissue-building and
for the renewal of tissues and organs, or their component cells,
proteid or albuminous foodstuffs are an absolute requirement. The
vital part of all tissue is proteid, and only proteid food can serve
for its growth or renewal. Hence, no matter how generous the supply
of carbohydrates and fats, without some admixture of proteid food the
body will weaken and undergo “nitrogen starvation.” It is to be noted,
however, that while the element nitrogen (16 per cent) gives character
to the proteid or albuminous foodstuffs, so that they are frequently
spoken of or classified as the “nitrogenous foodstuffs,” it is not the
nitrogen _per se_ that is so essential for the nutrition of the body.
Man lives in an atmosphere of oxygen and nitrogen. He can and does
absorb and utilize the free oxygen of the air he breathes; indeed,
it is absolutely essential for his existence, but the free nitrogen
likewise drawn into the lungs at each inspiration is of no avail for
the needs of the body. Further, there are many compounds of nitrogen,
some of them closely allied to the proteid foodstuffs in chemical
composition, which are just as useless as free nitrogen in meeting the
wants of the body for nitrogenous foods.

Dame Nature is very discriminating; she demands a definite form of
nitrogenous compound, some peculiar or specific grouping of the
nitrogen element with other elements in the food that can make good
the waste of proteid tissue. In the inactive and fibrous tissues of
animals, such as are found in bones, tendons, and ligaments, there is
present a substance known as collagen, which, when boiled with water,
as in the making of soups, is transformed into gelatin. This body,
because of its close chemical relationship to proteid or albuminous
substances, is known as an albuminoid. Yet, though it has essentially
the same chemical composition as ordinary albuminous substances and
shows many of the reactions characteristic of the latter, it cannot
take the place of true proteid in building up or repairing the tissues
of the body. To quote again from Dr. Curtis: “Tissue is nitrogenous, so
that, of course, only nitrogenous food can serve for its making; but
of the two kinds of nitrogenous principles, proteids and albuminoids,
behold, proteids only are of avail! Why this is so is unknown, since
albuminoid is equally nitrogenous with proteid; but so it is--proteid
and proteid alone can fulfil the high function of furnishing the
material basis of life. Gelatin cannot even go to make the very kind
of tissue of which itself is a derivative. Alongside of its brother
proteid, gelatin stands as a prince of the blood whose escutcheon bears
the ‘bend sinister.’ Such a one, though of royal lineage, may never
aspire to the throne.” It is thus quite clear that the true proteid
foods are tissue builders in the broadest sense of the term, and it is
equally evident that they are absolutely essential for life, since no
other kind or form of foodstuff can take their place in supplying the
needs of the body. Every living cell, whether of heart, muscle, brain,
or nerve, requires its due allowance of proteid material to maintain
its physiological rhythm. No other foodstuff stands in such intimate
relationship to the vital processes, but so far as we know at present
any form of true proteid, whether animal or vegetable, will serve the
purpose.

Carbohydrates include two closely related classes of compounds, viz.,
sugars and starches. They are entirely free from nitrogen, containing
only carbon (44.4 per cent), hydrogen (6.2 per cent), and oxygen
(49.4 per cent), and hence are classified as non-nitrogenous foods.
Obviously, they cannot serve as tissue builders, but by oxidation they
yield energy for heat and work. They constitute an easily oxidizable
form of fuel, and when supplied in undue amounts they may undergo
transformation within the body into fat, which is temporarily
deposited in tissues and organs for future needs.

Fats, like carbohydrates, are free from nitrogen, but differ from
them in containing a much larger percentage of carbon, and hence have
greater fuel value per pound. Fats contain on an average 76.5 per cent
of carbon, 11.9 per cent of hydrogen, and 11.5 per cent of oxygen. With
their larger content of carbon and smaller proportion of oxygen, fats
are less easily oxidizable than sugars, requiring a larger intake of
oxygen for their combustion, but when oxidized they yield more heat per
pound than carbohydrates.

Fats and carbohydrates are thus seen to be the natural fuel foodstuffs
of the body. They cannot serve for the upbuilding or renewal of tissue,
but by oxidation they constitute an economical fuel for maintaining
body temperature and for power to run the bodily machinery. It should
be remembered, however, that anything capable of being burned in
the body may serve as fuel material; hence proteid food, though of
specific value as a tissue builder, may likewise by its oxidation yield
energy for heat and work, but its combustion, owing to the content of
nitrogen, is never complete. Further, its use as fuel is uneconomical
and undesirable for reasons to be discussed later, but it is well to
know that its oxidation, though incomplete, is accompanied by the
liberation of energy, as in the oxidation of non-nitrogenous foods. A
portion of the carbon, hydrogen, and oxygen of the proteid molecule
will burn within the body to gaseous products, as do sugars and fats,
but there remains a nucleus of nitrogen, with some carbon, hydrogen,
and oxygen, which resists combustion and must be gotten rid of by the
combined labors of liver and kidneys. Fats and carbohydrates, on the
other hand, undergo complete combustion to simple gaseous products,
carbon dioxide and water, which are easily removed by the lungs, skin,
etc.

These three classes of foodstuffs exist in a great variety of
combinations or admixtures in nature. In many cases, noticeably in
milk, all three occur together in fairly large quantities. In animal
foods, such as meats, fish, etc., proteid and fat alone are found,
while in perfectly lean meat proteid only is present, excepting a
small amount of fat. Again, the white of the egg contains proteid
alone. Hence, a meat and egg diet would be essentially a proteid diet.
In vegetable foods, as in the cereals, there is found an admixture
of proteid and starch, the latter predominating in many cases, as in
wheat flour. The following table,[2] showing the chemical composition
of various food materials, may be of service in throwing light on the
relative distribution of the three classes of foodstuffs in natural
products.

  [2] The data composing this table are taken from Bulletin 28 (Revised
  Edition), United States Department of Agriculture, Office of
  Experiment Stations.


THE CHEMICAL COMPOSITION OF SOME COMMON FOOD MATERIALS

  +----------------------+--------+--------+--------+--------+--------+----------+
  |   Food Materials.    |Proteid.| Carbo- |  Fat.  | Water. |Mineral |Fuel Value|
  |                      |        |hydrate.|        |        | Matter.|per pound.|
  +----------------------+--------+--------+--------+--------+--------+----------+
  |                      |per cent|per cent|per cent|per cent|per cent| calories |
  |Fresh beef, loin,     |        |        |        |        |        |          |
  | lean, edible portion |  24.2  |   0    |   3.7  |  70.8  |   1.3  |    615   |
  |Fresh beef, round,    |        |        |        |        |        |          |
  | lean, edible portion |  22.3  |   0    |   2.8  |  73.6  |   1.3  |    540   |
  |Fresh Porterhouse     |        |        |        |        |        |          |
  | steak, edible portion|  21.9  |   0    |  20.4  |  60.0  |   1.0  |   1270   |
  |Fresh beef liver      |  21.0  |   1.7  |   4.5  |  71.2  |   1.6  |    605   |
  |Fresh beef tongue     |  19.0  |   0    |   9.2  |  70.8  |   1.0  |    740   |
  |Fresh sweetbreads     |  16.8  |   0    |  12.1  |  70.9  |   1.6  |    825   |
  |Fresh beef kidney     |  16.9  |   0.4  |   4.8  |  76.7  |   1.2  |    520   |
  |Cooked beef, roasted  |  22.3  |   0    |  28.6  |  48.2  |   1.3  |   1620   |
  |Cooked round steak    |  27.6  |   0    |   7.7  |  63.0  |   1.8  |    840   |
  |Broiled tenderloin    |        |        |        |        |        |          |
  | steak                |  23.5  |   0    |  20.4  |  54.8  |   1.2  |   1300   |
  |Dried beef, canned    |  39.2  |   0    |   5.4  |  44.8  |  11.2  |    960   |
  |Stewed kidneys,       |        |        |        |        |        |          |
  | canned               |  18.4  |   2.1  |   5.1  |  71.9  |   2.5  |    600   |
  |Fresh corned beef,    |        |        |        |        |        |          |
  | edible portion       |  15.3  |   0    |  26.2  |  53.6  |   4.9  |   1395   |
  |Fresh breast of veal, |        |        |        |        |        |          |
  | lean                 |  21.2  |   0    |   8.0  |  70.3  |   1.0  |    730   |
  |Fresh leg of lamb,    |        |        |        |        |        |          |
  | edible portion       |  19.2  |   0    |  16.5  |  63.9  |   1.1  |   1055   |
  |Lamb chops, broiled   |  21.7  |   0    |  29.9  |  47.6  |   1.3  |   1665   |
  |Roast leg of lamb,    |        |        |        |        |        |          |
  | edible portion       |  19.4  |   0    |  12.7  |  67.1  |   0.8  |    900   |
  |Roast leg of mutton,  |        |        |        |        |        |          |
  | edible portion       |  25.9  |   0    |  22.6  |  50.9  |   1.2  |   1420   |
  |Fresh lean ham        |  25.0  |   0    |  14.4  |  60.0  |   1.3  |   1075   |
  |Smoked ham, fat,      |        |        |        |        |        |          |
  | edible portion       |  14.8  |   0    |  52.3  |  27.9  |   3.7  |   2485   |
  |Chicken, broilers,    |        |        |        |        |        |          |
  | edible portion       |  21.5  |   0    |   2.5  |  74.8  |   1.1  |    505   |
  |Turkey, edible portion|  21.1  |   0    |  22.9  |  55.5  |   1.0  |   1360   |
  |Roast turkey, edible  |        |        |        |        |        |          |
  | portion              |  27.8  |   0    |  18.4  |  52.0  |   1.2  |   1295   |
  |Fricasseed chicken,   |        |        |        |        |        |          |
  | edible portion       |  17.6  |   2.4  |  11.5  |  67.5  |   1.0  |    855   |
  |Fresh cod, dressed    |  11.1  |   0    |   0.2  |  58.5  |   0.8  |    215   |
  |Fresh mackerel, edible|        |        |        |        |        |          |
  | portion              |  18.7  |   0    |   7.1  |  73.4  |   1.2  |    645   |
  |Fresh halibut, steaks |  18.6  |   0    |   5.2  |  75.4  |   1.0  |    565   |
  |Fresh shad, edible    |        |        |        |        |        |          |
  | portion              |  18.8  |   0    |   9.5  |  70.6  |   1.3  |    750   |
  |Fresh smelt, edible   |        |        |        |        |        |          |
  | portion              |  17.6  |   0    |   1.8  |  79.2  |   1.7  |    405   |
  |Cooked bluefish,      |        |        |        |        |        |          |
  | edible portion       |  26.1  |   0    |   4.5  |  68.2  |   1.2  |    670   |
  |Broiled Spanish       |        |        |        |        |        |          |
  | mackerel, edible     |        |        |        |        |        |          |
  | portion              |  23.2  |   0    |   6.5  |  68.9  |   1.4  |    715   |
  |Salt codfish, edible  |        |        |        |        |        |          |
  | portion              |  25.4  |   0    |   0.3  |  53.5  |  24.7  |    410   |
  |Salt mackerel, edible |        |        |        |        |        |          |
  | portion              |  22.0  |   0    |  22.6  |  42.2  |  13.2  |   1345   |
  |Canned salmon, edible |        |        |        |        |        |          |
  | portion              |  21.8  |   0    |  12.1  |  63.5  |   2.6  |    915   |
  |Canned sardines,      |        |        |        |        |        |          |
  | edible portion       |  23.0  |   0    |  19.7  |  52.3  |   5.6  |    162   |
  |Fresh round clams     |   6.5  |   4.2  |   0.4  |  86.2  |   2.7  |    215   |
  |Fresh oysters, solid  |   6.0  |   3.3  |   1.3  |  88.3  |   1.1  |    230   |
  |Fresh hen’s eggs      |  13.4  |   0    |  10.5  |  73.7  |   1.0  |    720   |
  |Boiled hen’s eggs     |  13.2  |   0    |  12.0  |  73.2  |   0.8  |    765   |
  |Butter                |   1.0  |   0    |  85.0  |  11.0  |   3.0  |   3605   |
  |Full cream cheese     |  25.9  |   2.4  |  33.7  |  34.2  |   3.8  |   1950   |
  |Whole cow’s milk      |   3.3  |   5.0  |   4.0  |  87.0  |   0.7  |    325   |
  |Corn meal, unbolted   |   8.4  |  74.0  |   4.7  |  11.6  |   1.3  |   1730   |
  |Oatmeal               |  16.1  |  67.5  |   7.2  |   7.3  |   1.9  |   1860   |
  |Rice                  |   8.0  |  79.0  |   0.3  |  12.3  |   0.4  |   1630   |
  |Wheat flour, entire   |        |        |        |        |        |          |
  | wheat                |  13.8  |  71.9  |   1.9  |  11.4  |   1.0  |   1675   |
  |Boiled rice           |   2.8  |  24.4  |   0.1  |  72.5  |   0.2  |    525   |
  |Shredded wheat        |  10.5  |  77.9  |   1.4  |   8.1  |   2.1  |   1700   |
  |Macaroni              |  13.4  |  74.1  |   0.9  |  10.3  |   1.3  |   1665   |
  |Brown bread           |   5.4  |  47.1  |   1.8  |  43.6  |   2.1  |   1050   |
  |Wheat bread or rolls  |   8.9  |  56.7  |   4.1  |  29.2  |   1.1  |   1395   |
  |Whole wheat bread     |   9.4  |  49.7  |   0.9  |  38.4  |   1.3  |   1140   |
  |Soda crackers         |   9.8  |  73.1  |   9.1  |   5.9  |   2.1  |   1925   |
  |Oyster crackers       |  11.3  |  70.5  |  10.5  |   4.8  |   2.9  |   1965   |
  |Ginger bread          |   5.8  |  63.5  |   9.0  |  18.8  |   2.9  |   1670   |
  |Sponge cake           |   6.3  |  65.9  |  10.7  |  15.3  |   1.8  |   1795   |
  |Lady fingers          |   8.8  |  70.6  |   5.0  |  15.0  |   0.6  |   1685   |
  |Apple pie             |   3.1  |  42.8  |   9.8  |  42.5  |   1.8  |   1270   |
  |Custard pie           |   4.2  |  26.1  |   6.3  |  62.4  |   1.0  |    830   |
  |Squash pie            |   4.4  |  21.7  |   8.4  |  64.2  |   1.3  |    840   |
  |Indian meal pudding   |   5.5  |  27.5  |   4.8  |  60.7  |  1.5   |    815   |
  |Tapioca pudding       |   3.3  |  28.2  |   3.2  |  64.5  |  0.8   |    720   |
  |Fresh asparagus       |   1.8  |   3.3  |   0.2  |  94.0  |  0.7   |    105   |
  |Fresh lima beans      |   7.1  |  22.0  |   0.7  |  68.5  |  1.7   |    570   |
  |Dried lima beans      |  18.1  |  65.9  |   1.5  |  10.4  |  4.1   |   1625   |
  |Dried beans           |  22.5  |  59.6  |   1.8  |  12.6  |  3.5   |   1605   |
  |Cooked beets          |   2.3  |   7.4  |   0.1  |  88.6  |  1.6   |    185   |
  |Fresh cabbage, edible |        |        |        |        |        |          |
  | portion              |   1.6  |   5.6  |   0.3  |  91.5  |  1.0   |    145   |
  |Green corn, edible    |        |        |        |        |        |          |
  | portion              |   3.1  |  19.7  |   1.1  |  75.4  |  0.7   |    470   |
  |Dried peas            |  24.6  |  62.0  |   1.0  |   9.5  |  2.9   |   1655   |
  |Green peas            |   7.7  |  16.9  |   0.5  |  74.6  |  1.0   |    465   |
  |Raw potatoes, edible  |        |        |        |        |        |          |
  | portion              |   2.2  |  18.4  |   0.1  |  78.3  |  1.0   |    385   |
  |Boiled potatoes       |   2.5  |  20.9  |   0.1  |  75.5  |  1.0   |    440   |
  |Fresh tomatoes        |   0.9  |   3.9  |   0.4  |  94.3  |  0.5   |    105   |
  |Baked beans, canned   |   6.9  |  19.6  |   2.5  |  68.9  |  2.1   |    600   |
  |Apples, edible portion|        |        |        |        |        |          |
  | steak                |   0.4  |  14.2  |   0.5  |  84.6  |  3.0   |    290   |
  |Bananas, yellow,      |        |        |        |        |        |          |
  | edible portion       |   1.3  |  22.0  |   0.6  |  75.3  |  0.8   |    460   |
  |Fresh cranberries     |   0.4  |   9.9  |   0.6  |  88.9  |  0.2   |    215   |
  |Oranges, edible       |        |        |        |        |        |          |
  | portion              |   0.8  |  11.6  |   0.2  |  86.9  |  0.5   |    240   |
  |Peaches, edible       |        |        |        |        |        |          |
  | portion              |   0.7  |   9.4  |   0.1  |  89.4  |  0.4   |    190   |
  |Fresh strawberries    |   1.0  |   7.4  |   0.6  |  90.4  |  0.6   |    180   |
  |Dried prunes, edible  |        |        |        |        |        |          |
  | portion              |   2.1  |  73.3  |   0.0  |  22.3  |  2.3   |   1400   |
  |Almonds, edible       |        |        |        |        |        |          |
  | portion              |  21.0  |  17.3  |  54.9  |   4.8  |  2.0   |   3030   |
  |Peanuts, edible       |        |        |        |        |        |          |
  | portion              |  25.8  |  24.4  |  38.6  |   9.2  |  2.0   |   2560   |
  |Pine nuts, edible     |        |        |        |        |        |          |
  | portion              |  33.9  |   6.9  |  49.4  |   6.4  |  3.4   |   2845   |
  |Brazil nuts, edible   |        |        |        |        |        |          |
  | portion              |  17.0  |   7.0  |  66.8  |   5.3  |  3.9   |   3265   |
  |Soft-shell walnuts,   |        |        |        |        |        |          |
  | edible portion       |  16.6  |  16.1  |  63.4  |   2.5  |  1.4   |   3285   |
  +----------------------+--------+--------+--------+--------+--------+----------+

In commenting on these figures, reference to which will be made from
time to time in other connections, it may be wise to emphasize the
large amount of water almost invariably present in natural foodstuffs.
Further, it is to be noted that, in animal products especially, the
variations in proteid-content are in large measure coincident with
variations in the amount of water present. In other words, foods
of animal origin if freed entirely of water would, as a rule, show
essentially the same percentage of proteid matter. Fat is naturally
variable, according to the condition of the animal at the time it was
slaughtered. Among the vegetable products, carbohydrate, mainly in the
form of starch, becomes exceedingly conspicuous, though proteid is by
no means lacking. Indeed, in some cereals, as in oatmeal, in dried peas
and beans, the content of proteid will average as high as in fresh
beef, while in addition 50–70 per cent of the entire substance is made
up of carbohydrate. Again, in the edible nuts, the content of proteid
runs high, in some cases higher than in fresh beef, while at the same
time carbohydrate and fat are noticeably large. Further, it is to be
noted that in nuts there is here and there some striking individuality,
as in pine nuts and Brazil nuts, both of which show a noticeable lack
of carbohydrate as contrasted with peanuts, almonds, and walnuts; a
fact of some importance in cases where a vegetable food rich in proteid
is desired, but with freedom from starch.

Another generality, to be thoroughly understood, is that while the
figures given for proteid express quite clearly and with reasonable
degree of accuracy the relative amounts of proteid matter present in
the foodstuffs in question, there may be important differences in
availability of which the percentage figures give no suggestion. In
other words, the analytical data deal solely with the total content
of proteid, while there is needed in addition information as to the
relative digestibility, or availability by the body, of the different
kinds of proteid food. For example, roast mutton, cream cheese, and
dried peas contain approximately the same amount of proteid. Are we
then to infer that these three foods have the same nutritive value so
far as proteid is concerned? Surely not, since no account is taken of
the relative digestibility of the three foods. It is one of the axioms
of physiology that the true nutritive value of any proteid food is
dependent not alone upon the amount of proteid contained therein, but
upon the quantity of proteid that can be digested and absorbed; or,
in other words, made available for the needs of the body. The same
rule holds good for both fats and carbohydrates, but as proteid is the
more important foodstuff, and is as a rule taken more sparingly, the
question of availability has greater import with the proteid foods.

The availability or digestibility of foods can be determined only by
physiological experiment. By making a comparison for a definite period
of time of the amount of a given food ingredient consumed and the
amount that passes unchanged through the intestine, an estimate of its
digestibility can be made. The result, to be sure, is not wholly free
from error, since we cannot always distinguish between the undigested
food and so-called metabolic products coming from the digestive juices
and from the walls of the intestine; but the errors are not large, and
results so obtained are full of meaning. In a general way it may be
stated that with animal foods, such as meats, eggs, and milk, about
97 per cent of the contained proteid is digested and thereby rendered
available for the body. With ordinary vegetable foods, on the other
hand, as they are usually prepared for consumption, only about 85 per
cent of the proteid is made available. This is partially due to the
presence in the vegetable tissue of cellulose, which in some measure
prevents that thorough attack of the proteid by the digestive juices
which occurs with animal foods. With a mixed diet, _i. e._, with
a variable admixture of animal and vegetable foods, it is usually
considered that about 92 per cent of the proteid contained therein will
undergo digestion.

Regarding differences in the availability of fats, it may be stated
that, as a rule, the fatty matter contained in vegetable foods is less
readily, or less thoroughly, digested than that present in foods of
animal origin. In the latter, about 95 per cent of the fat is digested
and absorbed. This figure, however, is generally taken as representing
approximately the digestibility or availability of the fat contained
in man’s daily dietary, since by far the larger proportion of the fat
consumed is of animal origin. Carbohydrates, on the other hand, are
much more easily utilized by the body. Naturally, sugars, owing to
their great solubility and ready diffusibility, offer little difficulty
in the way of easy digestion; but starches likewise, though not so
readily assimilable, are digested, as a rule, to the extent of 98 per
cent or more of the amount consumed. It is thus evident that in any
estimate of the food value of a given diet, chemical composition is to
be checked by the digestibility or availability of the food ingredients.

As has been stated several times, the proteid foodstuffs are the more
important, since proteid matter is essential to animal life. Man
must have a certain amount of proteid food to maintain the body in a
condition of strength and vigor. The other essential is that the daily
food furnish sufficient energy to meet the needs of the body for heat
and power. This means that in addition to proteid, which primarily
serves a particular purpose, there must be enough non-nitrogenous food
(either carbohydrate or fat or both) to provide the requisite fuel
for oxidation or combustion to meet the demands of the body for heat
and for work; both of which are subject to great variation owing to
differences in the temperature of the surrounding air, and especially
because of variations in the degree of bodily activity. The energy
which a given foodstuff will yield can be ascertained by laboratory
experiment, in which a definite weight of the substance is burned or
oxidized in a calorimetric bomb under conditions where the exact amount
of heat liberated can be accurately measured. The fuel, or energy,
value so obtained is expressed in calories or heat units. A calorie may
be defined as the amount of heat required to raise 1 gram of water 1°
C., or, to be more exact, the amount of heat required to raise 1 gram
of water from 15° to 16° C. This unit is usually spoken of as the small
calorie, to distinguish it from the large calorie, which represents
the amount of heat required to raise 1 kilogram of water 1° C. Hence,
the large calorie is equal to one thousand small calories. When burned
in a calorimeter, 1 gram of carbohydrate yields on an average 4100
gram-degree units of heat, or small calories; 1 gram of fat yields 9300
small calories. Both of these non-nitrogenous foods burn or oxidize to
the same products--viz., carbon dioxide and water--when utilized in
the body as when burned in the calorimeter; hence, the figures given
represent the physiological heat of combustion, per gram, of the two
classes of foodstuffs. Obviously, the fuel values of different foods
belonging to the same group or class will show slight variation, but
the above figures represent average values.

Unlike fats and carbohydrates, proteids are not burned completely
in the body; hence, the physiological fuel value of a proteid is
less than the value obtained by oxidation in a bomb calorimeter. In
the body, proteids yield certain decomposition products which are
removed through the excreta, and which represent a certain quantity of
potential energy thus lost to the economy. The average fuel value of
proteids burned outside of the body is placed at 5711 calories per
gram,[3] or 5.7 large calories. Deducting the heat value of the proteid
decomposition products contained in the excreta, the physiological fuel
value of proteids is reduced on an average to about 4.1 large calories
per gram.[4] Rubner considers that the physiological fuel value of
vegetable proteids is somewhat less than that of animal proteids;
conglutin, for example, yielding 3.96 calories, as contrasted with 4.3
calories furnished by egg-albumin, or 4.40 calories from casein. On a
mixed diet, where 60 per cent of the ingested proteid food is of animal
origin and 40 per cent vegetable, the fuel value available to the body
would be about 4.1 calories per gram of proteid, on the assumption
that the physiological heat value of vegetable proteids averages 3.96
calories per gram and that of animal proteids 4.23 calories per gram
(Rubner).

  [3] Stohmann: Ueber den Wärmewerth der Bestandtheile der
  Nahrungsmittel. Zeitschr. f. Biol., Band 31, p. 373.

  [4] See Rubner: Calorimetrische Untersuchungen. Zeitschr. f. Biol.,
  Band 21, p. 250. Also, Rubner: Die Quelle der thierischen Wärme.
  Ibid., Band 30, p. 73.

At present, we accept for all purposes of computation the following
figures as representing the physiological or available (to the body)
fuel value of the three classes of organic foodstuffs:

  1 gram of proteid          4.1 Large Calories
  1 gram of fat              9.3   "      "
  1 gram of carbohydrate     4.1   "      "

From these data, it is evident at a glance that 1 gram of fat is
isodynamic with 2.27 grams of either carbohydrate or proteid; and
since carbohydrate and fat are of use to the body mainly because
of their energy value, it is obvious that 50 grams of fat taken as
food will be of as much service to the body as 113 grams of starch.
In view of the relatively high fuel value of fats, it follows that
the physiological heat of combustion of any given food material will
correspond largely with the content of fat therein. This is quite
apparent from the data given in the table showing chemical composition
of food materials, where the fuel value per pound is seen to run more
or less closely parallel with the percentage of fat. Experience, as
well as direct physiological experiment, teaches us, however, that fat
and carbohydrate cannot be interchanged indefinitely, because of the
difficulty in utilization of fat when the amount is increased beyond
a certain point. Personal experience provides ample evidence of the
difference in availability between the two classes of foodstuffs.
Carbohydrates are easily utilizable, fats with more difficulty. Palate,
as well as stomach, rebels at large quantities of fat; a statement that
certainly holds good for most civilized people, though exceptions may
be found, as in the Esquimeaux and certain savage races.

In the nourishment of the body, the various factors that aid in the
utilization of food are of great moment and must not be overlooked. It
is not enough that the body be supplied with the proper proportion of
nutrients, with sufficient proteid to meet the demand for nitrogen, and
with carbohydrate and fat adequate to yield the needed energy; but all
those physiological processes which have to do with the preparation
of the foodstuffs for absorption into the circulating blood and lymph
must be in effective working order. There is an intricacy of detail
here which calls for careful oversight, and it is one of the functions
of the nervous system to control and regulate both the mechanical and
the chemical processes that are concerned in this seemingly automatic
progression of foodstuffs from their entry into the mouth cavity to
their final discharge from the alimentary tract, after removal of the
last vestige of true nutritive material.

Mastication; deglutition; secretion of the various digestive juices,
saliva, gastric juice, pancreatic juice, bile, intestinal juice, etc.;
peristalsis, or the rhythmical movements of the muscular walls of the
gastro-intestinal tract; the solvent action of the several digestive
fluids on the different types of foodstuffs; the absorption of the
products formed as a preliminary step in their transportation to the
tissues and organs of the body, where they are to serve their ultimate
purpose in nutrition; the interaction of these several processes one on
the other; and, finally, the influence of the various nerve fibres and
nerve centres concerned in the control of these varied activities,--all
must work together in harmony and precision if the full measure of
available nitrogen and energy-yielding material is to be extracted
and absorbed from the ingested food, without undue expenditure of
physiological labor. Further, the various processes of cell and tissue
metabolism, by which the absorbed food material is built up into living
protoplasm, and the chemical processes of oxidation, hydrolysis,
reduction, etc., by which the intra and extra cellular material is
broken down progressively into varied katabolic or excretory products,
with liberation of energy; all these must move forward harmoniously
and with due regard to the preservation of an even balance between
intake and outgo, if the nutrition of the body is to be maintained at
a proper level, and with that degree of physiological economy which is
coincident with good health and high efficiency.

We may well pause here and consider briefly some of these processes
which play so prominent a part in the proper utilization of the three
classes of organic foodstuffs. The first digestive fluid which the
ingested food comes in contact with is the saliva. Sensory nerve
fibres, chiefly of the glossopharyngeal and lingual nerves which supply
the mouth and tongue, are stimulated by the sapid substances of the
food, and likewise by mere contact of the food particles with the
mucous membrane lining the mouth cavity as the food is masticated and
rolled about prior to deglutition. Impulses communicated in this way
to the above sensory nerves are transmitted to certain nerve centres
in the medulla oblongata, whence impulses are reflected back through
secretory nerves going to the individual salivary glands, thereby
calling forth a secretion. The production of saliva is thus a simple
reflex act, in which the food consumed serves as a true stimulant or
excitant. Pawlow,[5] indeed, claims a certain degree of adaptability
of the secretion to the character of the food taken into the mouth.
Thus, he finds that dry, solid food excites a large flow of saliva,
such as would be needed to masticate it properly and bring it into a
suitable condition for swallowing. On the other hand, foods containing
an abundance of water cause only a scanty flow of saliva. The situation
of this secretory centre in the medulla, and the many branchings of
nerve cells in this locality would naturally suggest the possibility
of salivary secretion being incited by stimuli from a variety of
sources. This is indeed the case, and it is worthy of note that a flow
of saliva may result from stimulation of the sensory fibres of the
vagus nerves as well as of the splanchnic and sciatic, thus indicating
how a given secreting gland may be called into activity by impulses
or stimuli which come to the centre through very indirect and devious
pathways. Further, the secretory centre may be stimulated, and likewise
inhibited, by impulses which have their origin in higher nerve centres
in the brain. These facts are of great importance in throwing light
upon the ways in which a secretion like saliva is called forth and its
digestive action thus made possible. The thought and the odor of savory
food cause the mouth to water, the flow of saliva so incited being
the result of psychical stimulation. Similarly, fear, embarrassment,
and anxiety frequently cause a dry mouth and parched throat through
inhibition of the secretory centre by impulses which have their origin
in higher centres in the brain.

  [5] Pawlow: The Work of the Digestive Glands. Translated by Thompson.
  London, 1902.

The application of these facts to our subject is perfectly obvious,
since they suggest at once how the production or secretion of an
important digestive fluid--upon which the utilization of a given class
of foodstuffs may be quite dependent--is controlled and modified
through the nervous system by a variety of circumstances. We might
reason that the appearance, odor, and palatability of food are factors
of prime importance in its utilization by the body; that the æsthetics
of eating are not to be ignored, since they have an important influence
upon the flow of the digestive secretions. A peaceful mind, pleasurable
anticipation, freedom from care and anxiety, cheerful companionship,
all form desirable table accessories which play the part of true
psychical stimuli in accelerating the flow of the digestive juices
and thus pave the way for easy and thorough digestion. Further, it is
easy to see how thorough mastication of food may prolong mechanical
stimulation of the salivary glands and thus increase the flow of the
secretion, while the longer stay of sapid substances in the mouth
cavity increases the duration of the chemical stimulation of the
sensory fibres of the lingual and glossopharyngeal nerves. In this
connection, we may cite the view recently advanced by Pawlow that the
individual salivary glands respond normally to different stimuli. Thus,
there are three pairs of salivary glands concerned in the production
of saliva,--the submaxillary, parotid, and sublingual,--all of which
pour their secretions through separate ducts into the mouth cavity. By
experiment, Pawlow has found that in the dog the submaxillary gland
yields a copious flow of saliva when stimulated by acids, the chewing
of meats, the sight of food, etc., while the parotid gland fails to
respond. On the other hand, the latter gland responds with an abundant
secretion when dry food, such as dry powdered meat, dried bread, etc.,
is placed in the mouth. With this gland, the inference is that dryness
is the active stimulus.

As a digestive secretion, saliva serves several important purposes. By
moistening the food it renders mastication and deglutition possible;
its natural alkalinity tends to neutralize somewhat such acidity as may
be present in the food; it dissolves various solid substances, thus
making a solution capable of stimulating the taste nerves; lastly,
and most important, it has a marked digestive and solvent action on
starchy foods. A large proportion of the non-nitrogenous food consumed
by man--in most countries--is composed of some form of starch, and this
the body cannot use until it has undergone conversion into soluble
forms, such as dextrins and sugar. This it is the function of saliva to
accomplish, and it owes its activity in this direction to the presence
of a soluble ferment or enzyme known as ptyalin.

Enzymes, which play so important a part in all digestive processes,
are a peculiar class of substances produced by the living cells
which constitute the various secreting glands. They are of unknown
composition, and are peculiar in that the chemical changes they induce
are the result of what is termed catalysis, _i. e._, contact. That
is, the enzyme or catalyzer does not enter into the reaction, it is
not destroyed or used up, but by its mere presence sets in motion or
accelerates a reaction between two other substances. The ordinary
illustration from the inorganic world is spongy platinum, which, if
placed in contact with a mixture of oxygen and hydrogen, causes the two
gases to unite with formation of water, although the two gases alone at
ordinary temperature will not so combine. In this reaction the platinum
is not altered, neither does it apparently enter into the reaction; it
is a simple catalyzer. The chemical nature of the change which most
digestive enzymes produce is usually defined as hydrolytic, in which
the substance undergoing transformation is made to combine with water,
thus becoming hydrolyzed, this reaction generally being accompanied
by a cleavage or splitting of the molecule into simpler substances.
It is to be noted further that enzymes are specific in their action.
An enzyme that acts upon starch, for example, cannot act on proteids
or fats. Some digestive fluids have the power of producing changes
in different classes of foodstuffs, but such diversity of action is
always assumed to be due to the presence in the same fluid of different
enzymes. Emil Fischer[6] has advanced the theory that the specificity
of an enzyme is related to the geometrical structure of the substance
undergoing change; _i. e._, that each enzyme is capable of acting upon
or attaching itself only to such molecules as have a definite structure
with which the enzyme is in harmony. Or, the enzyme may be considered
as a key which will fit only into the lock (structure) of the molecule
it acts upon.

  [6] Emil Fischer: Bedeutung der Stereochemie für die Physiologie.
  Zeitschr. für physiologische Chemie, Band 26, p. 60.

One characteristic feature of enzymes is the incompleteness of their
action. Thus, the enzyme of saliva transforms starch by a series of
progressive changes into soluble starch, two or more dextrins, and
the sugar maltose as the chief end-product. A mixture of starch paste
and saliva under ordinary conditions, however, never results in the
formation of a hundred per cent of maltose, but there always remains
a variable amount of dextrin which appears to resist further change.
This is apparently due to what is known as the reversible action of
enzymes. Thus, the chemical reactions involved here are reversible
actions, _i. e._, they take place in opposite directions. The catalyzer
not only accelerates or incites a reaction in the direction of breaking
down the substance acted upon, but it also aids in the recomposition
of the products so formed into the original or kindred substance. With
reversible reactions of this sort the opposite changes sooner or later
strike an equilibrium, which remains constant until some alteration in
the conditions brings about an inequality and the reactions proceed
until a new equilibrium is established. In the body, however, where
the circulating blood and lymph provide facilities for the speedy
removal by absorption of the soluble products formed, the reaction may
proceed until the original substance undergoing change is completely
transformed into the characteristic end-product. This reversible
action of enzymes is an important feature, and helps explain certain
nutritional changes to be referred to later. Whether all enzymes behave
in this way is not as yet determined.

Another peculiarity of digestive enzymes is their extreme sensitiveness
to changes in their environment. Powerful in their ability to transform
relatively large quantities of a given foodstuff into simple products
better adapted for absorption and utilization by the body, they are,
however, quickly checked in their action, and even destroyed, when
the conditions surrounding them are slightly interfered with. They
require for their best action a temperature closely akin to that of the
healthy body, and any great deviation therefrom will result at once
in an inhibition of their activity. Further, they demand a certain
definite reaction of the fluid or mixture, if their working power
is to be maintained at the maximum. Indeed, many enzymes, like the
ptyalin of saliva, are quickly destroyed if the reaction is greatly
changed. Enzymes are thus seen to be more or less unstable substances,
endowed with great power as digestive agents, but sensitive to a high
degree and working advantageously only under definite conditions. Many
perversions of digestion and of nutrition are connected not only with
a lack of the proper secretion of some one or more digestive enzyme,
but also with the lack of proper surroundings for the manifestation of
normal or maximum activity.

With these statements before us, we can readily picture for ourselves
the initial results following the ingestion of starch-containing foods
properly cooked; and it may be mentioned here that the cooking is an
essential preliminary, for uncooked starch cannot be utilized in any
degree by man. With the mind in a state of pleasurable anticipation,
with freedom from care and worry, which are so liable to act as
deterrents to free secretion, and with the food in a form which appeals
to the eye as well as to the olfactories, its thorough mastication
calls forth and prolongs vigorous salivary secretion, with which the
food becomes intimately intermingled. Salivary digestion is thus at
once incited, and the starch very quickly commences to undergo the
characteristic change into soluble products. As mouthful follows
mouthful, deglutition alternates with mastication, and the mixture
passes into the stomach, where salivary digestion can continue for a
limited time only, until the secretion of gastric juice eventually
establishes in the stomach-contents a distinct acid reaction, when
salivary digestion ceases through destruction of the starch-converting
enzyme. Need we comment, in view of the natural brevity of this
process, upon the desirability for purely physiological reasons of
prolonging within reasonable limits the interval of time the food
and saliva are commingled in the mouth cavity? It seems obvious, in
view of the relatively large bulk of starch-containing foods consumed
daily, that habits of thorough mastication should be fostered, with
the purpose of increasing greatly the digestion of starch at the very
gateway of the alimentary tract. It is true that in the small intestine
there comes later another opportunity for the digestion of starch; but
it is unphysiological, as it is undesirable, for various reasons, not
to take full advantage of the first opportunity which Nature gives for
the preparation of this important foodstuff for future utilization.
Further, thorough mastication, by a fine comminution of the food
particles, is a material aid in the digestion which is to take place in
the stomach and intestine. Under normal conditions, therefore, and with
proper observance of physiological good sense, a large proportion of
the ingested starchy foods can be made ready for speedy absorption and
consequent utilization through the agency of salivary digestion.

Nowhere in the body do we find a more forcible illustration of
economical method in physiological processes than in the mechanism
of gastric secretion. Years ago, it was thought that the flow of
gastric juice was due mainly to mechanical stimulation of the gastric
glands by contact of the food material with the lining membrane of the
stomach. This, however, is not the case, as Pawlow has clearly shown,
and it is now understood that the flow of gastric juice is started
by impulses which have their origin in the mouth and nostrils; the
sensations of eating, the smell, sight, and taste of food serving
as psychical stimuli, which call forth a secretion from the stomach
glands, just as the same stimuli may induce an outpouring of saliva.
These sensations, as Pawlow has ascertained, affect secretory centres
in the brain, and impulses are thus started which travel downward to
the stomach through the vagus nerves, and as a result gastric juice
begins to flow. This process, however, is supplemented by other forms
of secretion, likewise reflex, which are incited by substances, ready
formed in the food, and by substances--products of digestion--which
are manufactured from the food in the stomach. Soups, meat juice, and
the extractives of meat, likewise dextrin and kindred products, when
present in the stomach, are especially active in provoking secretion.
Substances which in themselves have less flavor, as water, milk, etc.,
are far less effective in this direction, while the white of eggs and
bread are entirely without action in directly stimulating secretion.
When the latter foods have been in the stomach for a time, however,
and the proteid material has undergone partial digestion, then
absorption of the products so formed calls forth energetic secretion of
gastric juice. It is thus seen that there are three distinct ways--all
reflex--by which gastric juice is caused to flow into the stomach as a
prelude to gastric digestion. Further, it has been shown by Pawlow that
there is a relationship between the volume and character of the gastric
juice secreted and the amount and composition of the food ingested,
thus suggesting a certain adjustment in the direction of physiological
economy well worthy of note. A diet of bread, for example, leads to the
secretion of a smaller volume of gastric juice than a corresponding
weight of meat produces, but the juice secreted under the influence of
bread is richer in pepsin and acid, _i. e._, it has a greater digestive
action than the juice produced by meat. The suggestion is that gastric
juice assumes different degrees of concentration, with different
proportions of acid and pepsin, to meet the varying requirements of a
changing dietary.

As has been indicated, pepsin and hydrochloric acid are the important
constituents of gastric juice. It is noteworthy, however, that it is
the combination of the two that is effective in digestion. Pepsin
without acid is of no avail, and acid without pepsin can accomplish
little in the digestion of food. Pepsin and acid are secreted by
different gland cells in the stomach, and gastric insufficiency, or
so-called indigestion, may arise from either a condition of apepsia
or from hypoacidity. It is worthy of comment that the amount of
hydrochloric acid secreted during 24 hours by the normal individual,
under ordinary conditions of diet, amounts to what would constitute
a fatal dose of acid if taken at one time in concentrated form. At
the outset of gastric secretion, the fluid shows only a slight degree
of acidity, but as secretion proceeds, the acidity rises to 0.2–0.3
per cent of hydrochloric acid. The main action of gastric juice is
exerted on proteid foods, which under its influence are gradually
dissolved and converted into soluble products known as proteoses and
peptones. It is a process of peptonization, in which the proteid of
the food is gradually broken down into so-called hydrolytic cleavage
products. The enzyme, like the ptyalin of saliva, is influenced by
temperature, maximum digestive action being manifested at about 38° C.,
the temperature of the body. Further, a certain degree of acidity is
essential for procuring the highest degree of efficiency. Ordinarily,
it is stated that digestive action proceeds best in the presence of 0.2
per cent hydrochloric acid, but what is more essential for vigorous
digestion is a certain relationship between the acid, pepsin, and
proteid undergoing digestion. As pepsin and the amount of proteid
are increased, the amount of acid, and its percentage somewhat, must
be correspondingly increased if digestion is to be maintained at the
maximum.

Another important function of gastric juice is that of curdling milk,
due to the presence in the secretion of a peculiar enzyme known as
rennin. The latter ferment acts upon the casein of milk,--the chief
proteid constituent,--transforming it into a related substance commonly
called paracasein. This then reacts with the calcium salts present in
milk, forming an insoluble curd or calcium compound. From this point
on, the digestion of milk-casein by gastric juice is the same as that
of any other solid proteid, it being gradually transformed by the
pepsin-acid into soluble cleavage products. Why gastric juice should
be provided with this special enzyme, capable of acting solely on
the casein of milk, can only be conjectured, but we may assume that
it has to do with the economical use of this important food. As the
sole nutriment of the young, milk occupies a peculiar position as a
foodstuff, and being a liquid, its proteid constituent might easily
escape complete digestion were it to pass on too hastily through the
gastro-intestinal tract. Experiment has shown that when liquid food
alone is taken into the stomach it is pushed forward into the small
intestine in a comparatively short time. Curdled as it is by rennin,
however, casein must stay for a longer period in the stomach, like any
other solid food, and its partial digestion by gastric juice thereby
made certain. For the reasons above stated, it is apparent why milk
should not be treated as a drink in our daily diet. Remembering that
when milk reaches the stomach it is converted into a solid clot or
curd, there is obvious reason for sipping it, instead of taking it by
the glassful, thereby favoring the formation of small, individual clots
instead of one large curd, and thus facilitating instead of retarding
digestion.

Among other factors in gastric digestion, the muscular movements of the
stomach walls are to be emphasized, since we have here a mechanical aid
to digestion of no small moment, and likewise a means of accomplishing
the onward movement of the stomach contents. The outer walls of the
stomach are composed of a thick layer of circular muscular fibres,
especially conspicuous at the pyloric end of the organ, where the
latter is joined on to the intestine; a smaller, less conspicuous
layer of longitudinal muscle fibres, and some oblique fibres. At the
pylorus, the circular fibres are so arranged as to form a structure
which, aided by a peculiar folding of the inner mucous membrane,
serves as a sphincter, closing off the stomach from the duodenum, the
beginning of the small intestine. The movements of the stomach were
first made the subject of careful investigation by Dr. Beaumont in his
study of the celebrated case of Alexis St. Martin, a French Canadian,
who, in 1822, was accidentally wounded by the discharge of a musket,
with the resultant formation of a permanent fistulous opening in the
stomach. Dr. Beaumont, in the description[7] of his observations,
writes that “by the alternate contractions and relaxations of these
bands (of muscle) a great variety of motion is induced on this
organ (the stomach), sometimes transversely, and at other times
longitudinally. These alternate contractions and relaxations, when
affecting the transverse diameter, produce what are called _vermicular_
or _peristaltic_ motions.... When they all act together, the effect is
to lessen the cavity of the stomach, and to press upon the contained
aliment, if there be any in the stomach. These motions not only produce
a constant disturbance, or _churning_ of the contents of this organ,
but they compel them, at the same time, to revolve around the interior,
from point to point, and from one extremity to the other.” Of more
recent investigations, the most important are those made by Cannon,[8]
with the X-ray apparatus. From these later studies, it is evident
that Dr. Beaumont’s view of the entire stomach being involved in a
general rotary movement is not correct, since in reality the movements
are confined mainly to the pyloric end of the stomach, the fundus or
portion nearer the œsophagus not being directly involved. This means
that when food material passes into the stomach, it may remain at the
fundic end for some time more or less undisturbed before admixture with
the gastric juice occurs, and under such conditions, until acidity
creeps in, the salivary digestion of starch can continue.

  [7] The Physiology of Digestion. By William Beaumont, M.D. Second
  Edition, 1847, p. 100.

  [8] W. B. Cannon: The Movements of the Stomach studied by means of
  the Röntgen Rays. American Journal of Physiology, vol. 1, p. 359.

According to the observations of Cannon, the contractile movements
of the stomach commence shortly after the entrance of food, the
contractions starting from about the middle of the stomach and passing
on toward the pylorus. These waves of contraction follow each other
very closely, certainly not more than one or two minutes apart, and
perhaps less, while the resulting movements bring about an intimate
commingling of food and gastric juice in the pyloric portion of the
stomach; followed by a gradual diffusion of the semi-fluid mixture into
the fundus accompanied by a gradual displacement of the more solid
food in the latter region. These movements of the stomach are more or
less automatic, arising from stimuli--the acid secreted--originating
in the stomach itself, although it is considered that the movements
are subject to some regulation from extrinsic nerve fibres, such as
the vagi and the splanchnics. As digestion proceeds and the mass in
the stomach becomes more fluid, the pyloric sphincter relaxes and a
certain amount of the fluid material is forced into the intestine by
the pressure of the contraction wave. This is repeated at varying
intervals, depending presumably in some measure upon the consistency
of the mass in the stomach, until after some hours of digestion the
stomach is completely emptied.

Especially interesting and suggestive are the experiments made by
Cannon[9] on the length of time the different types of foodstuffs
remain in the stomach. Using cats as subjects, he found that fats
remain for a long period in the stomach; they leave that organ slowly,
the discharge into the intestine being at about the same rate as the
absorption of fat from the small intestine or its passage into the
large intestine. Carbohydrate foods, on the other hand, begin to leave
the stomach soon after their ingestion. They pass out rapidly, and at
the end of two hours reach a maximum amount in the small intestine
almost twice the maximum for proteids, and two and a half times the
maximum for fats, both of which maxima are reached only at the end of
four hours. Carbohydrates remain in the stomach about half as long as
proteids. Proteids, Cannon finds, frequently do not leave the stomach
at all during the first half-hour after they are eaten. After two
hours, they accumulate in the small intestine to a degree only slightly
greater than that reached by carbohydrates an hour and a half earlier.
The departure of proteids from the stomach is therefore slower at
first than that of either fats or carbohydrates. When a mixture of
equal parts of carbohydrates and proteids is fed, the discharge from
the stomach is intermediate in rapidity. When fat is added to either
carbohydrates or proteids it retards the passage of both foodstuffs
through the pylorus.

  [9] W. B. Cannon: The Passage of different Food-stuffs from the
  Stomach and through the Small Intestine. American Journal of
  Physiology, vol. 12, p. 387.

It is evident from what has been stated that the gastric digestion of
proteid foods is a comparatively slow process, involving several hours
of time; and further, that food material in general remains in the
stomach for varying periods, dependent upon its chemical composition.
It would appear further, that relaxation of the pyloric sphincter,
allowing passage of chyme into the intestine, must depend somewhat upon
chemical stimulation, as this offers the most plausible explanation
of the diversity of action seen with the different foodstuffs. As
has been pointed out, gastric digestion is primarily a process for
the conversion of proteid food into soluble products. It would be
a mistake, however, to assume that the digestion of proteid foods
is complete in the stomach. Stomach digestion is to be considered
more as a preliminary step, paving the way for further changes to be
carried forward by the combined action of intestinal and pancreatic
juice in the small intestine. The importance of gastric digestion
is frequently overrated. It is unquestionably an important process,
but not absolutely essential for the maintenance of life. Dogs have
lived and flourished with their stomachs removed, the intestine being
joined to the œsophagus. The intestine is a much more important part
of the alimentary tract; it is likewise far more sensitive to changing
conditions than the stomach, and undoubtedly one function of the
latter organ is to protect the intestine and preserve it from insult.
The stomach may be compared to a vestibule or reservoir, capable of
receiving without detriment moderately large amounts of food, together
with fluid, in different forms and combinations, with the power to hold
them there until by action of the gastric juice they are so transformed
that their onward passage into the intestine can be permitted with
perfect safety. Then, small portions of the properly prepared material
may be discharged from time to time through the pylorus without danger
of overloading the intestine, and in a form capable of undergoing rapid
and complete digestion. Further, the stomach as a reservoir is very
useful in bringing everything to a proper and constant temperature
before allowing its entry into the intestine. Another fact of some
importance is that, contrary to the general view, absorption from
the stomach of the products of digestion is not very rapid under
ordinary conditions. Even water and soluble salts pass very slowly into
the circulation from the stomach. Like the partially digested food
material, they are carried forward through the pyloric sphincter into
the intestine, where absorption of all classes of material is most
marked.

It is in the small intestine that both digestion and absorption are
seen at their best. It is here that all three classes of foodstuffs
are acted upon simultaneously through the agency of the pancreatic
juice, intestinal juice, and bile. Here, too, are witnessed some of the
most complicated and interesting reactions and changes occurring in
the whole range of digestive functions. Especially noteworthy is the
peculiar mechanism by which the secretion of pancreatic juice is set
up and maintained. On demand, pancreatic juice is manufactured in the
pancreas and poured into the intestine just beyond the pylorus through
a small duct--the duct of Wirsung. Secretion is started by contact of
the acid contents of the stomach with the mucous membrane of the small
intestine, so that as soon as the acid chyme passes through the pyloric
sphincter there commences an outflow of pancreatic juice into the
intestine. While acid is plainly the inciting agent in this secretory
process, its action is indirect. It does not cause secretion through
reflex action on nerve fibres, but it acts upon a substance formed in
the mucous membrane of the intestine, transforming it into _secretin_,
which is absorbed by the blood and carried to the pancreas, where it
excites secretory activity. As would be expected from the foregoing
statements, the secretion of pancreatic juice commences very soon after
food finds its way into the stomach, and naturally increases in amount
with the onward passage of acid chyme into the intestine, the maximum
flow being obtained in the neighborhood of the third or fourth hour,
after which the secretion gradually decreases. In man, it is estimated
on the basis of one or two observations that the amount secreted during
24 hours is about 700 cc., or a pint and a half. Careful experiments,
however, tend to show that the quantity of secretion depends in some
measure at least upon the character of the food, and also that the
composition of the secretion varies with the character of the food.
Thus, on a diet composed mainly of meat, the proteid-digesting enzyme
is especially conspicuous, while on a bread diet, with its large
content of starch, the starch-digesting enzyme is increased in amount.
In other words, there is suggested the possibility of an adaptation in
the composition of the secretion to the character of the food to be
digested.

Pancreatic juice is an alkaline fluid, rather strongly alkaline
in fact, from its content of sodium carbonate, and is especially
characterized by the presence of at least three distinct enzymes;
viz., trypsin, a proteid-digesting ferment; lipase, a fat-splitting
enzyme; and amylopsin, a starch-digesting enzyme. It has already been
pointed out how dependent the secretion of pancreatic juice is upon the
co-operation of the intestinal mucous membrane. A similar dependence
is found when the digestive activity of the secretion is studied. As
just stated, pancreatic juice contains a proteid-digesting enzyme.
This statement, however, is not strictly correct, for if the secretion
is collected through a cannula so that it does not come in contact
with the mucous membrane of the intestine, it is found free from any
digestive action on proteids. The secretion is activated, however, by
contact with the duodenal membrane. Expressed in different language,
pancreatic juice as it is secreted by the gland does not contain
ready-formed trypsin; it does contain, however, an inactive pro-enzyme,
which, under the influence of a specific substance contained in the
intestinal mucous membrane, known as enterokinase, is transformed
into the active enzyme trypsin. There is thus seen another suggestive
example of the close physiological relationship between the small
intestine and the activity of the pancreatic gland, or its secretion.

The chemical changes taking place in the small intestine are many
and varied. The acid chyme, with its admixture of semi-digested food
material, as it passes through the pyloric sphincter into the small
intestine, is at once brought into immediate contact with bile,
pancreatic juice, and intestinal juice, all of which are more or less
alkaline in reaction. As a result, the acidity of the gastric juice
is rapidly overcome, and the enzyme pepsin, which up to this point
could exert its characteristic digestive action, is quickly destroyed
by the accumulating alkaline salts. Pepsin digestion thus gives way
to trypsin digestion,--most effective in an alkaline medium,--and the
proteids of the food, already semi-digested by pepsin-acid, are further
transformed by trypsin; aided and abetted by another enzyme, known as
erepsin, secreted by the mucous membrane of the intestine. These two
enzymes are much more powerful agents than pepsin. It is true that they
begin work where pepsin left off, but most striking is the character
of the end-products which result from their combined action, since they
are small molecules and there is a surprising diversity of them. In
other words, while gastric digestion breaks down the proteid foodstuffs
into soluble bodies, such as proteoses and peptones closely related
to the original proteids, in pancreatic digestion as it takes place
in the intestine there is a profound breaking down, or disruption of
the proteid molecule into a row of comparatively simple nitrogenous
fragments, many of them crystalline bodies; such as leucin, tyrosin,
glutaminic acid, aspartic acid, arginin, lysin, histidin, etc., known
chemically as monoamino-acids and diamino-acids. We have no means
of knowing to how great an extent these more profound disruptive
changes of the proteid molecule take place in the intestine. Whether
practically all of the ingested proteid food is broken down into these
relatively simple compounds prior to absorption, or whether only a
small fraction suffers this change, cannot be definitely stated.

A few years ago, the majority of physiologists held to the view that
in the digestion of proteid food all that was essential was its
conversion into soluble and diffusible forms which would permit of
ready absorption into the blood. The belief was prevalent that, since
the proteid of the food was destined to make good the proteid of the
blood and through the latter the proteids of the tissues, any change
beyond what was really necessary for absorption of the proteid would
be uneconomical and indeed wasteful. On the other hand, due weight
must be given to the fact that in trypsin digestion, proteid can be
quickly broken down into simple nitrogenous compounds, and that in the
enzyme erepsin, present in the mucous membrane of the intestine, we
have an additional ferment very efficient in bringing about cleavage
of proteoses and peptone into amino-acids. From these latter facts it
might be argued that, in the digestion of proteid foodstuffs by the
combined action of gastric and pancreatic juice in the alimentary
tract, a large proportion of the proteid is destined to undergo
complete conversion into amino-acids, and that from these fragments the
body, by a process of synthesis, can construct its own peculiar type of
proteid.

This latter suggestion is worthy of a moment’s further consideration.
As is well known, every species of animal has its own particular
type of proteid, adapted to its particular needs. The proteids of
one species directly injected into the blood of another species are
incapable of serving as nutriment to the body, and frequently act as
poisons. Man in his wide choice of food consumes a great variety of
proteids, all different in some degree from the proteids of his own
tissues. Is it not possible, therefore, that it is the true function
of pancreatic and intestinal digestion to break down the different
proteids of the food completely into simple fragments, so that the body
can reconstruct after its own particular pattern the proteids essential
for its nourishment? Or, we can follow the suggestion contained in the
work of Abderhalden,[10] who finds that in the long continued digestion
of various proteids by pancreatic juice there results in addition to
the amino-acids a very resistant residue, non-proteid in nature, which
is termed polypeptid. In other words, Abderhalden believes that pepsin,
trypsin, and erepsin are not capable of bringing about a _complete_
breaking down of proteids into amino-acids, but that there always
remains a nucleus of the proteid not strictly proteid in nature, though
related thereto,--polypeptid,--which may serve as a starting-point for
the synthesis or construction of new proteid molecules, the various
amino-acids being employed to finish out the structure and give the
particular character desired. This view, however, is rendered somewhat
untenable by the more recent experiments of Cohnheim,[11] who claims
that proteids can be _completely_ broken down by pepsin, trypsin,
and erepsin, and consequently polypeptids would hardly be available
for the synthesis of proteids. Moreover, Bergell and Lewin[12] have
ascertained that there is present in the liver an enzyme or ferment
which has the power of digesting or breaking down certain dipeptids
and polypeptids into amino-acids. Hence, it follows that if any
polypeptids are absorbed from the intestine, they would naturally be
carried to the liver, where further cleavage into fragments suitable
for synthetical processes might occur. In any event, there is good
ground for the belief that the more or less complete disruption of the
proteid molecule into small fragments renders possible a synthetical
construction of new proteid to meet the demands of the organism; a fact
of great importance in our conception of the possibilities connected
with this phase of proteid nutrition.

  [10] Emil Abderhalden: Abbau und Aufbau der Eiweisskörper im
  thierischen Organismus. Zeitschr. f. physiologische Chemie, Band 44,
  p. 27.

  [11] Otto Cohnheim: Zur Spaltung des Nahrungseiweisses im Darm.
  Zeitschrift f. physiologische Chemie, Band 49, p. 64.

  [12] Bergell and Lewin: Zeitschrift für experimentelle Pathologie und
  Therapie, Band 3, p. 425.

Fatty foods undergo little or no chemical alteration until they reach
the small intestine. During their stay in the stomach they naturally
become liquid from the heat of the body, and there is more or less
liberation of fat from the digestive action of gastric juice on cell
walls, connective tissues, etc. Most food fat is in the form of
so-called neutral fat, which must undergo hydrolysis or saponification
before it can be absorbed and thus made available for the body. This
is accomplished by the enzyme lipase, or steapsin, of the pancreatic
juice, aided indirectly by the presence of bile. Under the influence
of this fat-splitting enzyme all neutral fats, whether animal or
vegetable, are broken apart, through hydrolysis, into glycerin and a
free fatty acid; the latter reacting in some measure with the sodium
carbonate of the pancreatic juice to form a sodium salt, or soluble
soap, while perhaps the larger part of the fatty acid is held in
solution by the bile present. Soap, free acid, and glycerin are then
absorbed from the intestine and are found again combined in the lymph
as neutral fat. In this way the fats of the food are rendered available
for the nourishment of the body.

The next important chemical change taking place in the small intestine
is that induced by the amylopsin of the pancreatic juice, which, acting
in essentially the same manner as the ptyalin of saliva, converts
any unaltered starch into dextrins and sugar. The latter substance,
maltose, is exposed to the action of another enzyme contained in the
intestinal secretion termed maltase, which transforms it into dextrose,
a monosaccharide.

In these ways the proteids, fats, and carbohydrates of the food are
gradually digested, so far as conditions will admit, digestion being
practically completed by the time the material reaches the ileocæcal
valve at the beginning of the large intestine. Throughout the length
of the small intestine absorption proceeds rapidly; water, salts, and
the products of digestion passing out from the intestine into the
circulating blood and lymph. At the ileocæcal valve, however, the
contents of the intestine are practically as fluid as at the beginning
of the small intestine, due to the fact that water is continually being
secreted into the intestine. In the large intestine, the contents
become less and less fluid through reabsorption of the water, and as
the propulsive movements of the circular and longitudinal muscle fibres
of the intestinal wall carry the material onward toward the rectum,
the last portions of available nutriment are absorbed. Finally, in
varying degree, certain putrefactive changes are observed in the large
intestine involving a breaking down of some residual proteid matter,
through the agency of micro-organisms almost invariably present, with
formation of such substances as indol, skatol, phenol, fatty acids,
etc. These processes, however, in health are held rigidly in check,
and count for little in fitting the food for absorption. Digestion, on
the other hand, extending as we have seen from the mouth cavity to the
ileocæcal valve, is the handmaiden of nutrition, preparing all three
classes of organic foodstuffs for their passage into the circulating
blood and lymph, and thus paving the way for their utilization by the
hungry tissue cells.




CHAPTER II

ABSORPTION, ASSIMILATION, AND THE PROCESSES OF METABOLISM

  TOPICS: Physiological peculiarities in absorption. Chemical changes
  in epithelial walls of intestine. Two pathways for absorbed
  material. Function of the liver as a regulator of carbohydrate.
  Absorption of proteid products. Assimilation of food products.
  Anabolism. Katabolism. Metabolism. Processes of metabolism. Older
  views regarding oxidation. Discoveries of Lavoisier. The views of
  Liebig. Theory of luxus consumption. Oxidation in the body not simple
  combustion. Oxygen not the _cause_ of the decompositions. Oxidation
  not confined to any one place. Intracellular enzymes. Living cells
  the guiding power in katabolism. Some intermediary products of tissue
  metabolism. Chemical structure of different proteids. Decomposition
  products of nucleoproteids. Relation to uric acid. Action of specific
  intracellular enzymes. Creatin and creatinin. Relation to urea.
  Proteid katabolism a series of progressive chemical decompositions.
  Intracellular enzymes as the active agents.


Digestion being completed, and the available portion of the foodstuffs
thereby converted into forms suitable for absorption, the question
naturally arises, In what manner are these products transported
from the alimentary tract to the tissues and organs of the body? In
attempting to answer this question, we shall find many illustrations
of the precise and undeviating methods which prevail in the processes
of nutrition. For example, it would seem plausible to assume that the
different forms of sugar entering into man’s ordinary diet, all of
them being soluble, would be directly absorbed and at once utilized,
but such is far from being the case. Milk-sugar and cane-sugar, both
appearing in greater or less degree in our daily dietaries, if
introduced directly into the blood, are at once excreted through the
kidneys unchanged. The body cannot use them, and they are gotten rid
of as speedily as possible, much as if they were poisons. When taken
by way of the mouth, however, they are utilized, simply because in
the intestine two enzymes are present there, known as lactase and
invertase, which break each of the sugars apart into two smaller
molecules. In other words, milk-sugar and cane-sugar are disaccharides,
and if they are to be absorbed in forms capable of being made use
of by the body they must be split apart into simpler sugars, viz.,
monosaccharides, such as dextrose, levulose, etc. The great bulk of the
carbohydrate food consumed by man is in the form of starch, and this,
as we have seen, is converted into maltose by the action of saliva and
pancreatic juice. Maltose, however, like cane-sugar, is a disaccharide,
and the body has no power to burn it or utilize it directly; but in the
intestine and elsewhere is an enzyme termed maltase, which breaks up
maltose into two molecules of the monosaccharide dextrose, and this the
body can use. Man frequently consumes starch to the extent of a pound a
day, and if utilized it must all undergo transformation into maltose,
and then into dextrose. There is no apparent reason why maltose should
not be absorbed and assimilated as readily as dextrose, but so urgent
is the necessity for this conversion into dextrose that in the blood
itself there is present maltase, to effect the transformation of any
maltose that may gain entrance there. We are here face to face with
a simple fact in nutrition. The body cannot utilize disaccharides
directly. Why it is so we cannot say, but the fact is a good
illustration of the principle that nothing can be taken for granted in
our study of nutrition.

For years, physiologists assumed that the ordinary physical laws of
osmosis, imbibition, and diffusion were quite adequate to explain
the passage of digested food materials into the blood and lymph.
If a substance was soluble and diffusible, that was sufficient; it
would quite naturally be absorbed in harmony with its diffusion
velocity. This, however, is not wholly true, since experiment shows
that the rapidity of absorption of diffusible substances through
the wall of the intestine is by no means always proportional to the
diffusion velocity of the substance. The lining membrane of the
small intestine, where absorption mainly takes place, is not to be
compared to a dead parchment membrane. On the contrary, it is made
up of living protoplasmic cells; absorption is not a physical, but a
physiological, process, in which the living epithelium cells stand as
guardians of the portals, ready to challenge and, if need be, modify
the rate of passage. Osmosis and diffusion undoubtedly play some part
in absorption, but they alone are not sufficient to account for what
actually takes place in the absorption of digestion products, and other
substances from the living intestine.

The primary products formed in the digestion of proteid foods--the
proteoses and peptones--afford another illustration of physiological
peculiarity in absorption. These bodies are readily soluble and
quite diffusible, yet they are never found to any extent in the
circulating blood and lymph during health. It is of course possible,
as has been previously suggested, that as soon as formed they undergo
transformation into simpler decomposition products in the small
intestine; but this is by no means certain. If proteoses and peptones
are injected directly into the blood, they cause a marked disturbance,
influencing at once blood-pressure, affecting the coagulability of
the blood, and in many other ways exhibiting a pronounced deleterious
action which at once indicates they are out of their normal
environment. They are not at home in the circulating blood, and the
latter medium gets rid of them as speedily as possible; they behave
like veritable poisons, and yet they are the primary products formed
in the digestion of all proteid foodstuffs. On the basis of all
physical laws governing diffusion they should be absorbed, and help to
renew the proteids of the blood and later the proteids of the tissues.
Yet, as we have said, they are not normally present in the blood or
lymph. Apparently, in the very act of absorption, as they pass through
the epithelial cells of the intestinal wall, before they gain entrance
to the blood stream, they undergo transformation into serum-albumin
and globulin, the characteristic blood proteids. The other alternative
is that, as previously mentioned, they are completely broken down
in the intestine into amino-acids, etc., and these simpler products
synthesized, as they pass through the intestinal wall toward the blood,
into serum-albumin and globulin. Certainly as yet, there is no evidence
that the amino-acids, as such, go through the epithelial cells of the
intestine; they are not found in the blood or lymph to any appreciable
extent, yet the proteids of the blood are reinforced in some manner by
the products of proteid digestion. Whichever view is correct, one thing
is perfectly obvious, viz., that in the act of absorption the products
resulting from the gastric and pancreatic digestion of proteid foods
are exposed to some influence, presumably in the epithelial cells of
the intestinal wall, by which there is a reconstruction of proteid.
Further, the proteid substances so formed are of the type peculiar to
the blood of that particular species of animal. The proteids of beef,
mutton, chicken, oatmeal, or bread go to make the proteids of human
blood.

From these statements, it is obvious that what we term absorption is
something more than a simple diffusion of soluble substances from
the alimentary tract into the blood current. The process is much
more complex than appears on the surface, and our lack of definite
knowledge, in spite of numerous efforts to unravel the mystery,
merely strengthens the view that we are dealing here with an obscure
physiological problem, and not a simple physical one. Digestion induces
a splitting up of the food proteid into fragments, large or small,
while incidental to absorption there is apparently a reconstruction,
or synthesis, of proteid from the fragments so formed. The process
seems somewhat costly, physiologically speaking, yet when one considers
the variety of proteids consumed as food, it is easy to comprehend
how essential it is that in some manner, as in absorption, there be
opportunity for construction of the specific proteids of the blood and
lymph.

We find an analogous process in the absorption of fats. As we have
seen, the fats of the food are broken apart in the small intestine into
glycerin and free fatty acid, a portion of the latter, and perhaps all,
combining with the alkali of the intestinal juices to form soluble
soaps, or sodium salts of the respective fatty acids. The neutral fats
present in animal and vegetable foods are all alike in containing
the glyceryl radicle, but they differ in the character of the fatty
acids present. Further, one form of animal fat, like that from beef,
may contain quite a different proportion of stearin, palmitin, and
olein than is present in the fat of another animal, like mutton. By
digestion, however, they are all broken apart into fatty acid and
glycerin. These acids and their salts can be readily detected in the
intestine, but they are not found in the blood or lymph, yet shortly
after fatty food is taken the lymph is seen to be milky from fat.
Obviously, the fatty acids liberated in the intestine are absorbed,
either as soluble soaps or as free fatty acids dissolved in bile, but
as they pass through the epithelial cells of the intestine into the
lacteal radicles, there is a synthesis or reconstruction of fat; and
as a result, neutral fats and not soaps are found in the lymph. Here,
then, we have a process quite analogous to what apparently occurs in
the absorption of proteid, though less complex; and it is possible that
this is one of the factors which aids in the formation of a specific
fat mixture corresponding, in a measure, to the type of fat present
in the particular species. It is well understood that the fat of an
animal’s tissues may be modified somewhat by the character of the
fat fed, yet in spite of this there is a certain degree of constancy
in composition which calls for explanation. Sheep and oxen feeding
in the same pasture have fat widely different in the proportion of
stearin, palmitin, etc. The fat of man’s tissues is fairly definite
in composition, yet he eats a great variety of fatty foods. One man
may consume large amounts of hard mutton fat with its relatively large
content of stearin, while another individual may take his fat mainly in
the form of the soft butter fats, with their relatively large content
of olein and palmitin. In both cases, the fat of the man’s tissues will
be essentially the same. To be sure, the changes that take place in the
tissue cells, reinforced by the construction of fat from other sources,
may be partly responsible for this constancy of composition, but the
transformations incidental to absorption are quite possibly, in some
measure, helpful thereto.

The great bulk of the digested food material is absorbed from the small
intestine, and there are two pathways open through which the absorbed
material can gain access to the blood. The one path leads directly to
the liver, and substances taking this course are exposed to the action
of this organ, before they enter into the general circulation. The
other path is through the lacteal or lymphatic system, and constitutes
a roundabout way for substances to enter the blood stream, since
they must first pass through the thoracic duct before entering the
main circulation. As a general truth, it may be stated that fats are
absorbed through the latter channel, while carbohydrates and proteids
follow the first path. The innumerable blood capillaries in the villi
of the intestine take up the products resulting from the digestion
of proteids and carbohydrates, through which they are passed into
the portal vein, and thereby distributed throughout the liver. This
means that both carbohydrates and proteids--or their decomposition
products--are exposed to a variety of possible changes in this large
glandular organ, before they can enter into the tissues of the body.
As we have seen, practically all carbohydrate food is converted into
a monosaccharide, principally dextrose, in the alimentary tract; and
it is in this form of a simple sugar that the carbohydrate passes into
the blood. This might easily mean a pound of sugar absorbed during
the twenty-four hours, and would obviously give to the blood a high
degree of concentration, unless the excess was quickly disposed of.
Sugar is very diffusible, and if it accumulates to any extent in the
blood it is quickly gotten rid of by excretion through the kidneys.
This, however, is wasteful, physiologically and otherwise, and does not
ordinarily occur except in diseased conditions. Further, physiologists
have learned that a certain small, but definite, amount of sugar in the
blood is a necessary requirement in nutrition, and it is the function
of the liver to maintain the proper carbohydrate level.

We must again emphasize the great importance of carbohydrate food;
there is a far larger amount of starchy food consumed than of any
other foodstuff, and it is more readily available as a source of
energy. Its presence in the blood, in the form of sugar, is constantly
demanded, but it must be kept within the proper limits for the uses
of the different tissues and organs of the body. The liver serves as
an effective regulator, maintaining, in spite of all fluctuations
in the supply and demand, a definite percentage of sugar such as is
best adapted to keep the tissues of the body in a normal and healthy
condition. This regulation by the liver is rendered possible through
the ability of the hepatic cells to transform the sugar brought to
the gland into glycogen, so-called animal starch, which is stored
up in the liver until such time as it is needed by the body. The
process is one of dehydration, the reverse of what takes place in the
intestine when ordinary starch is converted into maltose and dextrose.
The efficiency of this regulating mechanism depends also upon the
ability of the liver to transform glycogen into sugar, presumably
through the agency of an enzyme in the hepatic cells. Hence, glycogen
may be looked upon as a temporary reserve supply of carbohydrate,
manufactured and stored in the liver during digestion, when naturally
large amounts of sugar are passing into the portal blood, and to be
drawn upon whenever from any cause the content of sugar in the blood
threatens to fall below normal. Obviously, there must be some delicate
machinery for the adjustment of these opposite changes in the liver,
and we may well believe that it is associated with the composition
of the blood itself, which in some fashion stimulates and inhibits,
as may be required, the functional activity of the liver, or its
component cells. In any event, we have in this so-called glycogenic
function of the liver a most effective means for accomplishing the
complete and judicious utilization of all the sugar formed from the
carbohydrates of the food, after it has once passed beyond the confines
of the alimentary tract into the blood; preventing all loss, and at
the same time guarding against all danger, from undue accumulation
of sugar in the circulation. We see, too, how wise the provision
that all sugar should pass from the alimentary canal into the portal
circulation and not by way of the lymphatics, since by the latter
channel the regulating action of the liver would be mainly lost.
Further, recalling how soluble and diffusible sugar is, we may well
marvel that it practically all passes from the intestine by way of
the blood, and escapes entry into the lymphatics. Surely, this marked
shunning of the other equally accessible pathway affords a striking
illustration of selective action such as might be expected in a
physiological process, but not in harmony with the ordinary physical
laws of osmosis or diffusion. In conformity with this statement, it may
be mentioned that appropriate experiments have clearly demonstrated
that the different sugars available as food are not absorbed from the
intestine in harmony with their diffusion velocity, but show deviations
therefrom which can be explained only on the ground that the intestinal
wall exercises some selective action, due to the living cells composing
it. Likewise interesting in their bearing on nutrition are the
observations of Hofmeister,[13] who finds by experiments on dogs that
the assimilation limit of the different sugars shows marked variation.
Thus, dextrose, levulose, and cane-sugar have the highest assimilation,
while milk-sugar is far less easily and completely assimilated. If this
is equally true of man, it indicates that starchy foods, with their
ultimate conversion into dextrose, are to be ranked as having a high
assimilation limit, thus affording additional evidence of their high
nutritive value.

  [13] Franz Hofmeister: Ueber Resorption und Assimilation der
  Nährstoffe. Archiv f. d. exper. Pathol. u. Pharm., Band 25, p. 240.

In the absorption of proteid products, their passage from the intestine
by way of the portal circulation insures exposure to the action of the
hepatic cells, before they are distributed by the general circulation
throughout the body. It is only under conditions of an excessive
intake of proteid foods that their products are absorbed by way of the
lymphatics. These points are clearly established, and there is every
ground for believing that substantial reasons exist to account for
this single line of departure. Just what the liver does, however, is
uncertain. In fact, as already indicated, there is lack of definite
knowledge as to how far the proteid foods are broken down in digestion,
prior to absorption. The combined action of pepsin, trypsin, and
erepsin, if sufficiently long continued, can accomplish a complete
disruption of the proteid molecule. We are inclined to assume in a
general way that the “proteids taken as food cannot find a place in
the economy of the animal body till they have been, as it were, melted
down and recast.”[14] How far this melting down or disruption extends
in normal digestion, we do not at present know. As already stated,
neither proteoses and peptones, nor the amino-acids, are found in
the blood stream in sufficient amounts, or with that frequency, to
suggest absorption in these forms. Possibly, as some physiologists have
suggested, the amount of any of these products to be found at any one
time in a given quantity of blood is too small for certain recognition,
yet in the twenty-four hours the amount passing from intestine to
liver might be sufficiently large to equal the total proteid absorbed.
We can, however, at present only conjecture, and must rest content
with the simple statement that in the digestion of the proteid
foodstuffs, proteoses, peptones, and amino-acids are formed, and that
by transformation or total reconstruction of these products, special
types of proteid are manufactured either in the epithelial cells of
the intestinal walls during absorption, or elsewhere in the body after
absorption. If this latter is the case, the liver might readily be
regarded as a likely spot for the synthesis to occur.

  [14] J. B. Leathes: Problems in Animal Metabolism. Blakiston’s Son
  and Co., 1906, p. 123.

Bearing in mind what has been said regarding the production of specific
types of proteid by every species of animal, we can the more readily
conceive of a synthesis “out of fragments of the original molecules
rearranged and put together in new combinations, by processes in
which the intestine can hardly be supposed to play a part.” This,
the liver might well be assumed as capable of accomplishing, and if
we were disposed to accept this view we might use as an argument the
fact that the products of proteid digestion are taken directly to
this organ, before being cast loose in the tissues and organs of the
body. There is perhaps as good ground for assuming that a synthesis
or reconstruction of proteid takes place all over the body; that, as
suggested by Leathes, “the synthesis of proteids is a function of every
cell in the body, each one for itself, and that the material out of
which all proteids in the body are made is not proteid in any form,
but the fragments derived from proteids by hydrolysis, probably the
amido-acids, which in different combinations and different proportions
are found in all proteids, and into which they are all resolved by the
processes, autolytic or digestive, which can be carried out in every
cell in the body.” It is certainly a reasonable hypothesis, and since
we lack positive knowledge it cannot at present be disproved. All that
we can affirm in the light of established fact is that the products of
proteid digestion are absorbed from the intestine by way of the portal
circulation, and that either in their passage through the intestinal
wall, or later on in the liver or elsewhere, there is a construction of
new proteid to meet the wants of the body. The liver, indeed, may be
effective in both construction and destruction of proteid, but there is
no way of telling at present just how far it acts in either direction.

Regarding the absorption of fats, a single statement will suffice, in
addition to what has already been said. Fats gain access to the general
circulation by passing from the intestine into the lacteal radicles,
thence into the lymphatics, whence they move onward into the thoracic
duct, and from there are emptied into the great veins at the neck. A
small amount is apparently absorbed in the form of soap by the portal
circulation, but by far the larger amount of fat gains access to the
blood stream without going through the liver.

       *       *       *       *       *

In these ways, the blood and lymph are continually supplied with
proteid, fat, and carbohydrate from the ingested food, and as these
fluids surround and permeate the organized elements of the tissues, the
latter are enabled to gain what they need to maintain their nutritive
balance. Living matter is essentially unstable; it is the seat of
chemical changes of various kinds, anabolic or constructive, and
katabolic or destructive. The more comprehensive term “metabolic” is
applied to all of these changes that take place in living matter. In
anabolism, the dead, inert proteids, fats, and carbohydrates are more
or less assimilated and made a part of the living matter of the tissue
cells, while at the same time a certain amount of the food material,
probably the larger amount, is simply stored as such, or left to
circulate in the blood and lymph, without being raised to the higher
level of living protoplasm. In katabolism, this accumulated material,
and in some degree the living substance itself, is broken down or
disintegrated with liberation of the stored-up energy, which manifests
itself in the form of heat and mechanical work. At times, the anabolic
processes predominate and there is a relatively large accumulation
of stored-up materials; while at other times, katabolism, with its
attendant chemical decompositions, predominates, and the body loses
correspondingly. The point to be emphasized here is that the living
body, with its multitude of living cells, is the seat of incessant
change. Construction and destruction are continually going forward
side by side; sometimes the one and sometimes the other predominating,
according to existing conditions. The living protoplasm with its
attendant storage material is, under ordinary conditions, constantly
being made good from the assimilated food, a part of which is raised to
the dignity of living matter and becomes an integral part of the living
cells, while the larger portion is simply stored for future uses, or
circulates in the blood and lymph which bathe them. Doubtless, this
storage or circulating material is the main source of the energy which
constantly flows from the cells in the form of heat and of work, as a
result of the disruptive changes that constitute katabolism.

Worthy of special notice is the fact that cell protoplasm is
essentially proteid in nature; water and proteid make up the larger
part of its substance, to which are added small proportions of
carbohydrate, fat, and mineral matter. Proteid is the basis of cell
protoplasm; it is the chemical nucleus of living matter, and owing to
the large size of its molecule, with its large number of contained
atoms, is capable of many combinations and many alterations. Most of
the reactions characteristic of katabolism centre around this proteid,
but the disruptive changes that occur undoubtedly involve more largely
the circulating materials present in the blood and lymph, and which
bathe the cells, rather than the so-called fixed, or organ proteid, of
the cell substance itself. Still, while the circulating blood and lymph
furnish largely the substances which are made to undergo disintegration
in katabolism, the living protoplasmic cell is the controlling power
which regulates the extent and character of the decompositions,
and proteid matter is the chemical basis of protoplasm. From these
statements, we again have suggested the significant importance of the
proteid foods in nutrition, since they alone can furnish the material
which constitutes the chemical basis of living cells. The human body,
which represents the highest form of animal life, is merely, as stated
by another, “literally a nation of cells derived from a single cell
called the ovum, living together, but dividing the work, transformed
variously into tissues and organs, and variously surrounded by
protoplasm products” (Waller).

The processes involved in metabolism are not easily unravelled. The
word itself is simple, but it is employed to designate that complex of
“chemical changes in living organisms which constitute their life, the
changes by which their food is assimilated and becomes part of them,
the changes which it undergoes while it shares their life, and finally
those by which it is returned to the condition of inanimate matter.
Gathered together under this one phrase are some of the most intricate
and inaccessible of natural phenomena. It implies also, and gently
insists on the idea, that all the phenomena of life are at bottom
chemical reactions” (Leathes). Regarding the processes of anabolism, as
in the construction of living protoplasm out of inert food materials,
we can say nothing. This is altogether beyond our ken at present, and
doubtless will remain so, since it involves a chemical alteration, or
change, akin to that of bringing the dead to life. With the processes
of katabolism, however, we may hope for more satisfactory results; and,
indeed, to-day we have considerable information of value as to some of
the methods, at least, which are the cause of this phase of nutrition.
This knowledge, however, has been slow of attainment.

In the earlier years of the sixteenth century, when anatomy and
physiology were beginning to make progress, the savants of that day,
hampered as they were by grave misconceptions and by the lack of
any understanding of chemical phenomena, could not take advantage,
naturally, of the suggestion that as wood burns or oxidizes in the air
with liberation of heat, so might the food substances, absorbed by the
body, undergo oxidation in the tissues and thus give rise to animal
heat. Such suggestions were at that time as a closed book, and so we
find Vesalius, in 1543, teaching the Galenic doctrines in physiology
then prevalent. The conception of heat production, as it existed at
that time, may be inferred from the following quotation:[15] “The parts
of the food absorbed from the alimentary canal are carried by the
portal blood to the liver, and by the influence of that great organ
are converted into blood. The blood thus enriched by the food is by
the same great organ endued with the nutritive properties summed up
in the phrase ‘natural spirits.’ But blood thus endowed with natural
spirits is still crude blood, unfitted for the higher purposes of the
blood in the body. Carried from the liver by the vena cava to the right
side of the heart, some of it passes from the right ventricle through
innumerable invisible pores in the septum to the left ventricle. As the
heart expands it draws from the lungs through the vein-like artery air
into the left ventricle. And in that left cavity, the blood which has
come through the septum is mixed with the air thus drawn in, and by the
help of that heat, which is innate in the heart, which was placed there
as the source of the heat of the body by God in the beginning of life,
and which remains there until death, is imbued with further qualities,
is laden with ‘vital spirits,’ and so fitted for its higher duties. The
air thus drawn into the left heart by the pulmonary vein, at the same
time tempers the innate heat of the heart and prevents it from becoming
excessive.” In other words, heat was considered as a divine gift, and
as can readily be seen, there was an utter lack of appreciation of the
use of air in breathing. Even van Helmont, who lived in 1577–1644, and
was in a sense an alchemist, still gave credence to the spirits, viz.,
that the food absorbed from the stomach and intestine is in the liver
endued with natural spirits, while in the heart the natural spirits
are converted into vital spirits, and in the brain the vital spirits
are transformed into animal spirits.[16] Later, Malpighi discovered
the true structure of the lungs, and Borelli, in 1680, exposed the
erroneous views then prevalent regarding the purpose of breathing.
It is not true, says Borelli, that the use of breathing is to cool
the excessive heat of the heart or to ventilate the vital flame, but
we must believe that this great machinery of the lungs, with their
accompanying blood vessels, is for some grand purpose. In a long and
vigorous argument, he contends that the “air taken in by breathing is
the chief cause of the life of animals, far more essential than the
working of the heart and the circulation of the blood.” He quotes the
experiments of Boyle, who showed in 1660 “that even in a partial vacuum
brought about by his air pump, flame was extinguished and life soon
came to an end; the candle went out and the mouse or the sparrow died.”

  [15] Taken from Sir Michael Foster’s “Lectures on the History
  of Physiology during the Sixteenth, Seventeenth, and Eighteenth
  Centuries.” Cambridge, 1901, p. 12.

  [16] See Foster’s Lectures, p. 136.

At this time, and for long afterwards, the belief was prevalent
that the air taken up by the blood in the lungs was the air of the
atmosphere in its entirety. No one appears to have thought of the
possibility of only a part of the air being used, for at that time
there was no suspicion that air was a mixture of substances. Mayow,
however, in 1668, showed that it was not the whole air which was
employed for respiration, but a particular part only. At this time,
great attention was being given to a study of nitre or saltpetre; its
wonderful properties in combustion were being recognized, and Mayow,
who was a chemist of repute, claimed that it had its origin partly
in the air and partly in the earth. The air “which surrounds us, and
which, since by its tenuity escapes the sharpness of our eyes, seems
to those who think about it to be an empty space, is impregnated
with a certain universal salt, of a nitro-saline nature, that is to
say, with a vital, fiery, and in the highest degree fermentative
spirit,” to which the name of “igneo-aereus” was applied. Nitre was
shown to be composed of a _sal fixum_ or sal alkali,--potash as it
is now called,--and was obviously derived from the earth, while the
other part of nitre was made up of the _spiritus acidus_, or nitric
acid. For a time it was supposed that the whole of this _spiritus
acidus_ was contained in the atmosphere, but it was soon recognized
that this could not be the case, since nitric acid was found to be a
corrosive liquid, destructive to life and quite incapable of supporting
combustion. Hence, Mayow concluded that only a part of the acid
exists in the atmosphere, viz., that part which he termed _spiritus
nitro-aereus_. In combustion, there is something in the air which is
necessary for the burning of every flame, unless perchance igneo-aereal
particles should pre-exist in the thing to be burnt. These igneo-aereal
particles form “the more active and subtle part of air which is thus
necessary for combustion, exist in nitre and indeed constitute its
‘more active and fiery part.’” Mayow fully recognized that burning
and breathing involved in a measure the same process; both consisted
in the consumption of the igneo-aereal particles present in the air.
“If a small animal and a lighted candle be shut up in the same vessel,
the entrance into which of air from without be prevented, you will see
in a short time the candle go out, nor will the animal long survive
its funeral torch. Indeed, [says Mayow] I have found by observation
that an animal shut up in a flask together with a candle will continue
to breathe for not much more than half the time than it otherwise
would, that is, without the candle.” Something contained in the air,
necessary alike for supporting combustion and for sustaining life,
passes from the air into the blood. Mayow expressed his thoughts in
these words: “And indeed it is very probable that certain particles
of a nitro-saline nature, and those very subtle, nimble, and of very
great fermentative power, are separated from the air by the aid of the
lungs and introduced into the mass of the blood. And so necessary for
life of every kind is that aereal salt (constituent) that not even
plants can grow in earth the access of air to which is shut off. But
if that same earth be exposed to air and so forthwith impregnated with
that fecundating salt, it at once becomes fit again for growing.”[17]
Mayow fully appreciated the importance of his nitro-aereal particles in
the processes of life; he had indeed a fairly accurate conception of a
sound theory of animal heat; he saw that they were equally necessary
for burning, or combustion, and for respiration, and so was enabled to
draw a parallelism between the two processes; he pointed out that they
were essential for the ordinary activity of the muscles of the body,
that as muscle work was increased more particles from the air were
required; indeed, he clearly foresaw the need which the body had for
these igneo-aereal particles in all the chemical processes of life. And
thus was foreshadowed a conception of oxidation, a hundred years before
Priestley evolved his phlogiston theories and Lavoisier discovered
oxygen.

  [17] Quoted from Foster’s Lectures, p. 195.

The discoveries of Lavoisier, published in 1789, led to a clear
understanding of combustion as a process of oxidation, and paved the
way for a fuller knowledge of the part played by the oxygen of the
air in the chemical reactions going on in the animal body. Lavoisier
showed that the oxygen drawn into the lungs with the air breathed was
used in the body for the oxidation of certain substances, carbon being
transformed thereby into carbon dioxide, and hydrogen into water.
Further, he noted that these oxidations were carried forward on a
large scale, and he emphasized the importance of oxygen as being the
true cause of the varied decompositions taking place in the living
body. The larger the amount of oxygen inspired, the more extensive
the oxidation, and consequently the rate of respiration as modifying
the intake of oxygen served in his opinion as a regulator to control
the extent of the oxidative processes. He pointed out that a definite
relationship existed between the amount of work done by the body and
the oxygen consumed; greater muscular activity, lower temperature of
the surrounding air, the activities attending the digestive functions,
all seemed to be associated with a greater utilization of oxygen.
Oxidation was the pivot around which all the chemical reactions of the
body seemed to centre. Lavoisier, however, was not a physiologist, and
he was, quite naturally perhaps, led into some errors. For example,
he considered that the process of combustion or oxidation took place
in the lungs, certain fluids rich in carbon and hydrogen formed in
the different organs of the body being brought there for exposure to
the inspired oxygen. Further, his views implied a simple and complete
combustion, in which complex substances rich in carbon were directly
and completely oxidized to carbon dioxide and water, in much the same
manner as combustion occurs outside the body. Again, he assumed that
the amount of oxygen taken into the lungs determined the extent of
oxidation, just as the use of the bellows, by increasing the draft of
air, causes the fire to burn more brightly.

To Liebig (1842) the next great advance was due. This phenomenally
clear-minded man, while recognizing at their full value the fundamental
theories advanced by Lavoisier, saw and fully appreciated their
incompleteness, and he likewise understood their failure to explain
many of the phenomena of life more familiar to the physiological mind
than to that of a simple chemist like Lavoisier. Liebig had made a
special study of the chemical composition of foodstuffs, and likewise
of the tissues and organs of the body. He had, moreover, given great
attention to the decomposition products formed in the body, especially
the nitrogenous substances excreted through the kidneys, as well as the
carbon dioxide and water passed out through the lungs and skin. It was
not strange, therefore, that he should take exception to Lavoisier’s
view that oxidation in the body consisted in the combustion of a
fluid, rich in carbon and hydrogen, which was brought to the lungs.
On the contrary, Liebig contended that it was the organic compounds,
proteids, fats, and carbohydrates, that underwent oxidation, and not
necessarily in the lungs, but all over the body, wherever organs and
tissues were active. Especially noteworthy was the view advanced by
Liebig, and upheld for many years, that of these three classes of
compounds the proteids alone served for the construction of organized
tissues, like muscle, and that in the activity of this tissue, as
in muscle contraction or muscle work, the energy for the work was
derived solely from the breaking down or oxidation of this organized
proteid. On this ground he termed the proteid foodstuffs “plastic,” or
tissue-building foods. Liebig further pointed out that the substances
of the body have the power of combining with and holding on to the
inspired oxygen, and that fats and carbohydrates, _i. e._, the
non-nitrogenous compounds, easily undergo oxidation or combustion, and
thereby furnish the heat of the body. For this reason he termed the
corresponding foodstuffs “respiratory” foods. Proteids, on the other
hand, according to Liebig’s view, are capable of combustion only in
slight degree. The cause of the decomposition of proteid substances in
the body was to be traced solely to muscle work, _i. e._, the energy
of muscle contraction, or muscle work, was derived from the breaking
down of the proteids of the muscle tissue, and work was the stimulus
which brought about proteid decomposition. Non-nitrogenous substances
played no part in these reactions; muscle work was without influence
on these compounds, oxygen being the sole stimulus which led to their
combustion, and heat was the sole product of the combustion.

If Liebig’s theory is correct, that the proteids of the body are
decomposed only as the result or the accompaniment of muscle work, and
the proteids of the food are used up only as they take the place of the
organized proteid so metabolized, it follows that with a like degree of
muscular activity a given body will always decompose the same amount
of proteid. If excess of proteid food is taken, the surplus will be
stored in the tissues, or, in other words, the excretion of nitrogen
will not be influenced by the amount of proteid consumed in the food.
This was the line of argument made use of by various physiologists[18]
who were disposed to criticise Liebig’s view, and quite naturally the
question was soon made the subject of many experiments. It will suffice
here merely to say that many concordant results were obtained, showing
that an abundance of proteid food leads to an increase in the excretion
of nitrogen, muscle activity remaining at a constant level. Hence, as
Voit states, some other ground than muscle work must be sought as the
true cause of proteid katabolism. Consequently, we find this hypothesis
of Liebig replaced by the theory of “luxus consumption,” in which it
is maintained that while whatever proteid is used up by the work of
the muscle must be made good from the proteid of the food, any excess
of proteid absorbed from the intestinal canal is to be considered as
“luxus,” and like the non-nitrogenous foods may be burned up in the
blood, by the oxygen therein, without being previously organized.
Hence, we see suggested two causes for the decomposition of proteid in
the body, viz., the work of the muscle and the oxygen of the blood.
Further, as stated by C. Voit,[19] the nitrogen excretion of the hungry
or fasting animal affords, according to these views, a measure of the
extent to which tissue proteid must be broken down in the maintenance
of life, and of the amount of proteid food necessary to be consumed in
order to make good the loss; viz., the minimum proteid requirement.
Again, since any excess of proteid food beyond this minimal
requirement, according to the theory, is destined to be burned up in
the blood, or elsewhere, to furnish heat the same as non-nitrogenous
foods, it follows that the excess of proteid food can be replaced by
non-nitrogenous aliment.

  [18] See C. Voit: Hermann’s Handbuch der physiologie des
  Gesammt-Stoffwechsels. Band 6, Theil 1, p. 269, 1881.

  [19] Loc. cit., p. 270.

Oxidation, however, is the keynote in any explanation of the processes
of metabolism, whether nitrogenous or non-nitrogenous matter is
involved. Both alike undergo oxidation, but it is not simple oxidation
or combustion that we have to deal with. In the time of Lavoisier, as
already stated, it was thought that oxygen alone was the cause of the
decomposition going on in the body, but simply increasing the intake
of air or oxygen, as in quickened breathing or deeper inspiration,
does not increase correspondingly the rate of oxidation. In other
words, it is not a direct combination of oxygen with the carbon and
hydrogen of the foodstuffs, or tissue elements, that takes place in
the body, but rather a gradual, progressive decomposition of complex
organic compounds into simpler products; made possible, however, by the
agency of the oxygen carried from the lungs by the circulating blood.
It was demonstrated years ago that animals breathing pure oxygen do
not consume any more of the gas than when breathing ordinary air, and
likewise no more carbon dioxide is produced in the one case than in
the other. Fifty years ago, Liebig and other physiologists showed that
frogs’ muscle placed in an atmosphere free of oxygen could be made to
contract or do work for some considerable time, and with liberation
of heat. This fact implies a breaking down of muscle substance into
simpler bodies, but there is here no free oxygen to act as the inciting
cause; indeed, what actually occurs is a cleavage or splitting up
of substances in the muscle tissue, but at the expense of oxygen in
some form of combination in the muscle. This oxygen must have been
taken from the blood at some previous time and stored in the tissue
for future use. Again, as C. Voit has expressed it, if oxygen were
really the immediate cause of the decompositions taking place in the
organism, we should expect combustion to occur in harmony with the
well-known relationship of the three classes of organic foodstuffs to
oxygen. In other words, fats would undergo combustion most readily,
carbohydrates next, and lastly the nitrogenous or albuminous compounds.
In reality, however, proteid matter is decomposed in largest quantity;
a generous addition of proteid food is always accompanied by an
increased consumption of oxygen. Yet oxygen is not the inciting cause
of the proteid decomposition, as is seen from the fact that in muscle
work, where the intake of oxygen is greatly increased, there is no
noticeable change in the amount of proteid material broken down.
Plainly, in the body we have to deal not with a direct oxidation of
the complex compounds of the tissues or of the food, but rather with
a gradual cleavage of these higher compounds into simpler substances,
these latter undergoing progressively a still further breaking down
with intake of oxygen. To repeat, oxygen is not the _cause_ of the
decompositions within the body, but the extent of the breaking down of
the tissue or food material is the determining factor in the amount
of oxygen taken on and used up. The products of decomposition contain
more oxygen than the original substances undergoing the breaking down
process, which means that oxygen is taken from the blood and used in
the physiological combustion that is going on. It is not, however,
strictly a combustion process; it is more complicated and more gradual
than ordinary combustion, involving first of all a series of what may
be termed oxidative cleavages, in which large molecules are gradually,
step by step, broken down into simpler molecules, and these latter then
oxidized to still simpler forms. Hence, we find the oxidative changes
preceded by a variety of alterations in which oxygen may take no part
whatever; such as hydrolytic cleavage, where the elements of water are
taken on as a necessary step in the cleavage process; dissociation of
a simple sort, as when a large molecule breaks up directly into smaller
molecules, etc.

These statements by no means detract from the importance of oxygen in
the katabolic processes of the body, but it is physiological oxidation
that we have to do with, and not simple combustion. Oxygen is not the
direct cause of the transformations taking place in the body. As one
looks over the history of progress in our knowledge of nutrition from
the time of Lavoisier to the present, it is easy to note the gradual
change of view regarding oxidation in the living organism. Step by
step, it has been demonstrated that there are many factors involved in
this breaking down of complex substances; that while oxygen is an ever
present requirement, there are other equally important factors to be
taken into account. The contrast between the older views and those now
current is clearly shown by the difference in attitude regarding the
_place_ in the body where oxidation occurs. Thus, in the earlier days,
when the view was gradually gaining ground that nutritional changes
were mainly the result of oxidation, and that the oxygen drawn into the
lungs in inspiration was a primary factor, then, as we have seen, the
lungs were considered as the laboratory where the transformation takes
place. This view, however, was soon exploded, and next we find the
blood, the lymph, and other fluids, but especially the blood, looked on
as the locality where oxidation occurs. This was indeed quite a natural
view to hold, since the blood is the carrier of oxygen, but we now
know, in harmony with the fact that the breaking down of complex food
material is a complicated process, involving various kinds of chemical
change, that these katabolic processes are not located in any one
place, but occur all over the body wherever there are active tissues.
As has been previously stated, the human body is a “nation” of cells,
all of which are more or less active, and it is in these miniature
laboratories mainly that oxidation and all the other nutritional
changes coincident to life take place. Muscle tissue and nerve tissue,
the large secreting glands, such as the liver, stomach, and pancreas,
all are the seat of oxidative and other changes which we class under
the broad term of nutritional. To these cells, therefore, we must
look for an explanation of the causes of oxidation, and the other
transformations of a kindred nature that take place in the body.

In our brief survey of digestion, and of the methods there followed
for the proper utilization of the organic foodstuffs, it was seen
that the unorganized ferments or enzymes are the active agents in
accomplishing the breaking down of proteids, and the less profound
alteration of fats and carbohydrates. Is it not possible that the
tissues of the body are likewise supplied with enzymes of various
types, and that upon these powerful agents rests the responsibility
for the different kinds of decomposition, oxidation and other changes,
that take place in the body? Some years ago much interest was aroused
by the observation that certain glands in the body, if simply warmed
at body temperature with water, in the presence of some germicidal
agent sufficient to prevent putrefactive changes, underwent what is
now termed autodigestion, _i. e._, a process of self-digestion, with
formation of various products, notably such as would naturally result
from the breaking down of proteid material by ordinary proteolytic
enzymes. This would seem to imply the presence in the glands of
a proteid-splitting enzyme, the products formed being proteoses,
peptones, amino-acids, etc., just such products as result from the
action of trypsin. To-day, we know that practically all tissues and
organs can, under suitable conditions, undergo autolysis, and in many
instances the enzymes themselves can be separated from the tissues by
appropriate treatment. Liver, muscle, lymph glands, spleen, kidneys,
lungs, thymus, etc., all contain what are very appropriately called
intracellular enzymes. These enzymes are of various kinds. Especially
conspicuous are the hydrolytic, proteid-splitting enzymes, which
behave in a manner quite similar to, if not identical with, that of
the digestive enzymes of the gastro-intestinal tract, _i. e._, pepsin,
trypsin, and erepsin. Further, there are other hydrolytic cleavages
taking place in tissue cells, such as the cleavage of fats, due as we
now know to intracellular enzymes of the lipase type, and by which
neutral fats are split apart into glycerin and fatty acid. Again, there
are in many organs intracellular enzymes which act upon the complex
nucleoproteids of the tissue, causing them to break apart into proteid
and nucleic acid, the latter being further broken down by other enzymes
with liberation of the contained nuclein or purin bases. Many other
chemical reactions are brought about by specific enzymes of various
kinds, present in the cells of particular glandular organs. Thus,
intracellular enzymes have been found, as in the liver, which are able
to transform amino-acids into amides, and still others capable of
splitting up amides.

Equally important, and even more suggestive, are the data which have
been collected recently regarding oxidative processes in the tissues
of the body. Specific ferments, known as oxidases, are found widely
distributed in many organs and tissues, and it is difficult to escape
the conclusion that as intracellular enzymes they have an important
part to play in some, at least, of the transformations characteristic
of tissue katabolism.[20] As a single example, mention may be made of
aldehydase, which accomplishes the oxidation of substances having the
structure of aldehydes into corresponding acids. Ferments or enzymes
of this class are found in the liver, spleen, salivary glands, lungs,
brain, kidneys, etc., and they may well be considered as important
agents in the chemical transformations going on in the tissues of
the body. It would take us too far afield to enter into a detailed
consideration of these intracellular enzymes; it must suffice to
emphasize the general fact that in all the tissues and organs of the
body there are present a large number of enzymes of different types,
endowed with different lines of activity, and consequently capable
of accomplishing a great variety of results in metabolism. Oxidation
may still be a dominant feature in nutrition, oxidative changes may
characterize more or less every tissue and organ in the body, but the
processes are subtle and are not to be defined in harmony with simple
chemical or physical laws. The living cell, with its intracellular
enzymes, is the guiding and controlling power by which the processes of
katabolism are regulated in harmony with the needs of the body. Complex
organic matter is broken down step by step in the various tissues, with
gradual liberation of the contained energy; processes of hydrolytic
cleavage alternate with processes of oxidation, the molecules acted
upon growing smaller with each downward step, until at last the final
end-products are reached, viz., carbon dioxide, water, and urea, which
the body eliminates through various channels as true physiological
waste-products.

  [20] See M. Jacoby: Ueber die Bedeutung der intracellulären Fermente
  für die Physiologie und Pathologie. Ergebnisse der Physiologie,
  Erster Jahrgang, 1. Abtheilung, p. 230.

It will be advisable for us to consider briefly some of these
intermediary products of tissue metabolism, since in any discussion
of nutritive changes it is quite essential to have some understanding
of the chemical relationship existing between the various products
which result from the breaking down of proteid and other materials in
tissue katabolism. This is especially true of proteid material, since
in the gradual disintegration of this substance in tissue metabolism
many intermediary bodies are formed, which undoubtedly exercise some
physiological influence prior to their transformation into simpler
bodies, with ultimate formation of the final product, urea. As has
been pointed out so many times, the proteid foods are peculiar in that
they alone contain the necessary nitrogen, and in the peculiar form
able to meet the physiological requirements of the body. Variations in
the proteid intake are of necessity accompanied by variations in the
formation of nitrogenous intermediary products, and both quality and
quantity of these substances must be given due attention in any study
of nutrition. Further, it is only by an understanding of the general
or ground structure of proteids that we can hope to attain knowledge
of the processes going on in the different tissues and organs in
connection with metabolism, while a true appreciation of the chemical
peculiarities of the individual proteids will help to explain the
different nutritional value of vegetable as contrasted with animal
proteids.

Our understanding of the chemical structure of any organic substance
is based primarily upon a study of the decomposition products which
result from its breaking down, under the influence of various chemical
agencies. Simple proteid substances when acted upon by pancreatic
juice reinforced by the enzyme erepsin, or when boiled with dilute
acids, undergo hydrolytic cleavage with ultimate formation of a large
number of relatively simple bodies, mostly amino-acids, the chemical
structure of which throws some light upon the nature of the proteid.
Thus, in the pancreatic digestion of proteid in the intestine we may
adopt the following scheme as showing in a general way the progressive
transformation that occurs, understanding at the same time that like
transformations may be accomplished by corresponding intracellular
enzymes in the tissues and organs of the body; and further, that by the
long-continued action of hydrolytic agents there is a complete breaking
down into amino-acids and other simple products.

            Native Proteid
                 /\
                /  \
               /    \
   Protoproteose    Heteroproteose : Primary proteoses
         |                |
  Deuteroproteose  Deuteroproteose : Secondary proteoses
         |                |
      Peptone          Peptone
         |                |
    Amino-acids      Amino-acids

Among these end-products, or amino-acids, are leucin, tyrosin, aspartic
acid, glutaminic acid, glycocoll, arginin, lysin, histidin, and
likewise the peculiar aromatic body tryptophan. The chemical make-up
of these substances may be indicated by the following structural
formulæ, which, if even only partially understood, will suggest to the
non-chemical mind some idea of close chemical relationship:

         CH(NH_{2})COOH
        /                       CH(NH_{2})COOH
  CH_{2}                        |
        \                       CH_{2}-COOH
         CH_{2}-COOH

        Glutaminic acid          Aspartic acid

                          CH_{3}
  CH_{2}-NH_{2}                 \
  |                              CH-CH_{2}-CH(NH_{2})-COOH
  COOH                          /
                          CH_{3}

  Glycocoll                      Leucin

             OH
            /
  C_{6}H_{4}
            \
             CH_{2}-CH(NH_{2})COOH

            Tyrosin
                                   C·CH_{2}·CH·(NH_{2})·COOH
                                  / \\
                         C_{6}H_{4}  CH
                                  \  /
                                   NH

                               Tryptophan

  CH_{2}-NH               CH_{2}-NH_{2}         CH--N
  |        \              |                     ||   \\
  CH_{2}    C--NH_{2}     CH_{2}                ||    CHµ
  |        //             |                     ||   /
  CH_{2} NH               CH_{2}                C--NH
  |                       |                     |
  CH·NH_{2}               CH_{2}                CH_{2}
  |                       |                     |
  COOH                    CH-NH_{2}             CH-NH_{2}
                          |                     |
                          COOH                  COOH

  Arginin                 Lysin                 Histidin

In these various decomposition products there is apparent certain
definite lines of resemblance, on which is based one or more
suggestions regarding possible ways in which these chemical groups are
linked, or bound together, in the proteid molecule. Thus, there is
apparently present a complex or nucleus which may be indicated as

   |                  |
  HC-NH-CO-    also   HC-NH-C(NH)-
   |                  |

The proteid molecule is presumably built up of amino-acids variously
joined together, this synthesis being accomplished, doubtless, by the
condensation of different types of amino-acids, in which the first
of the above groups represents the more common method of union. We
may indeed conjecture that such methods of condensation take place in
the human body, in the epithelial cells of the intestine, and in the
tissues in general; and that by such methods, construction of proteid
is accomplished out of the various fragments split off by digestion,
etc. In a tentative way, the principle may be illustrated by the fusion
of leucin and glutaminic acid,--following Hofmeister’s suggestion,--in
which a still larger complex is formed:

       :                        :
  --CO-:-NH--CH--CO--NH--CH--CO-:-NH--
       :     |           |      :
          C_{4}H_{9}  (CH_{2})_{2}
                         |
                        CO·OH

          Leucin       Glutaminic acid

In this way, step by step, the proteid molecule is built up, and
naturally in katabolism the proteid breaks down along certain definite
lines of cleavage, with formation of katabolic products containing
those groups, or chemical nuclei, which characterize the different
proteid molecules. For it is to be clearly understood that there are
many different forms of proteid, perhaps superficially alike, but
possessed of physiological individuality. This is well illustrated
by the two primary proteoses formed in digestion. As will be
recalled, there are at first two proteoses produced, protoproteose
and heteroproteose. These are, superficially at least, not radically
unlike; they possess essentially the same percentage composition, but
when broken down by vigorous chemical methods they show a totally
different make-up. In other words, at the very beginning of digestion
there is a splitting up of the proteid into two parts, which have
quite a different chemical structure, as is clearly indicated by the
difference in the character and amount of the decomposition products
yielded by hydrolytic cleavage. Thus, heteroalbumose as derived from
blood-fibrin contains 39 per cent of its total nitrogen in basic form,
_i. e._, in a form which goes over into the basic bodies, arginin,
lysin, and histidin, etc. On the other hand, protoalbumose from the
same source yields hardly 25 per cent of basic nitrogen. Further,
heteroalbumose yields only a very small amount of tyrosin, while
protoalbumose gives on decomposition a large amount of this substance.
Again, heteroalbumose furnishes a large yield of leucin and glycocoll,
while protoalbumose gives no glycocoll and only a little leucin.
Obviously, these two proteoses have an inner structure quite divergent
one from the other, and owing to this fact they must play a quite
different rôle in metabolism.

Even greater differences in inner chemical structure are found among
native proteids. By way of illustration, we may take egg-albumin,
the casein of cow’s milk, gliadin of wheat, and the edestin of hemp
seed. These are all typical proteids; they are all useful as food, but
they are radically different in their inner chemical structure, as is
clearly indicated by the following data,[21] which show the percentage
yield of the different amino-acids and ammonia:

  [21] These data were furnished the writer by Dr. Thomas B. Osborne,
  and represent in large measure the results of his own chemical work.

  +--------+-------+--------+----------+--------+------+---------+--------+
  |        |       |        |Glutaminic|        |      |         |        |
  |        |Leucin.|Tyrosin.|   Acid.  |Arginin.|Lysin.|Histidin.|Ammonia.|
  +--------+-------+--------+----------+--------+------+---------+--------+
  |Egg-    |       |        |          |        |      |         |        |
  | albumin|  6.1  |  1.1   |    9.0   |   ...  | ...  |   ...   |  1.6   |
  |Casein  | 10.5  |  4.5   |   10.7   |   4.8  | 5.8  |   2.6   |  1.9   |
  |Gliadin |  5.7  |  1.2   |   37.3   |   3.2  |  0   |   0.6   |  5.1   |
  |Edestin | 19.9  |  2.7   |   14.0   |  14.2  | 1.6  |   2.2   |  2.3   |
  +--------+-------+--------+----------+--------+------+---------+--------+

These are not mere technical differences, but they represent
divergences of structure which cannot help counting as material factors
in nutritional processes. Especially noticeable is the large yield of
glutaminic acid from wheat proteid, as contrasted with the proteid
(casein) of animal origin. As a rule, glutaminic acid forms a larger
proportion of the decomposition products of vegetable than of animal
proteids. Similarly, arginin is present in much larger proportion
in most vegetable proteids than in most animal proteids. While many
other data more or less trustworthy might be added, these figures
will suffice to emphasize the main point under discussion, viz., that
individual proteids show marked variation in the amount of the several
amino-acids which serve as corner-stones or nuclei in the building
up of the molecule, and consequently they must yield correspondingly
different katabolic products when serving the body as food.

Turning now to another phase of tissue metabolism, we may consider
briefly the nucleoproteids and their characteristic decomposition
products; bodies which are widely distributed as cleavage products
formed in the disintegration of most cell protoplasm, and having
special interest in nutrition because of their chemical relationship
to that well-known substance, uric acid. Nucleoproteids of some type
are found in all cells; consequently they are present in all tissues,
in all glandular organs, and their widespread distribution constitutes
evidence of their great physiological importance. Nucleoproteids are
compound substances made up of some form of proteid and nucleic acid.
By simple hydrolysis with dilute mineral acids they are broken down
into proteid, phosphoric acid, and one or more bodies known as nuclein
bases. Of these latter substances, there are four well-defined bodies,
viz., adenin, hypoxanthin, guanin, and xanthin, which from their
peculiar chemical constitution are known as “purin bases.” In the
body, there is present in many cells a peculiar intracellular enzyme
termed _nuclease_, which has the power of liberating these purin bases
from their combination as a component part of tissue nucleoproteids,
or of the contained nucleic acid. In autolysis or self-digestion of
many glands, such as the spleen, thymus, etc., this chemical reaction
is easily induced by action of the contained nuclease. Further, the
liberated purin bases then undergo change because of the presence of
certain deamidizing enzymes, and as a result guanin is transformed into
xanthin, and adenin is converted into hypoxanthin. These ferments are
true intracellular enzymes, and are termed respectively _guanase_ and
_adenase_. The real essence of the reaction they accomplish is clearly
indicated by the following formulæ, which likewise show the chemical
nature and relationship of the four substances:

        HN--CO                   HN--CO
         |  |                    |   |
  H_{2}N·C  C--NH    + H_{2}O =  CO  C--NH   + NH_{3}
         || ||  \                ||  ||  \
         || ||   CH              ||  ||   CH
         || ||  //               ||  ||  //
         N--C--N                 HN--C--N

         Guanin                   Xanthin

   N==C·NH_{2}              HN--CO
   |  |                      |  |
  HC  C--NH     + H_{2}O =  HC  C--NH   + NH_{3}
  ||  ||  \                 ||  ||  \
  ||  ||   CH               ||  ||   CH
  ||  ||  //                ||  ||  //
   N--C--N                   N--C--N

    Adenin                  Hypoxanthin

These two enzymes are typical hydrolyzing enzymes, but it is to be
noted that there is not only a taking on of water with a retention of
the oxygen, but there is also a giving off of ammonia, by which the
transformation is made possible. Adenin is known as an amino-purin
and guanin as an amino-oxypurin, while hypoxanthin is an oxypurin and
xanthin a dioxypurin. In other words, the two intracellular enzymes
are able to transform the two amino-purins into the corresponding
oxypurins; _i. e._, the enzymes are deamidizing ferments, liberating
the NH_{2} group of the adenin and guanin and thus forming two new
compounds. These reactions, though more or less technical, are
emphasized in this way not merely because they illustrate the action
of intracellular enzymes in intermediary metabolism, thus affording a
striking example of the gradual changes that take place in ordinary
katabolic processes, but especially because they throw light upon the
production of another substance common in body metabolism, viz., uric
acid. It has long been known that nucleoproteids, nucleins, and other
compounds containing these purin radicles, when taken as food, cause
at once an increased output of uric acid, and it has been clearly
recognized that in some way this latter substance, as a product of
metabolism, must come from the transformation of nuclein bases. To-day,
we understand that in many tissues, as in the liver, spleen, lungs,
and muscle, there is present a peculiar oxidizing ferment, an oxidase,
by the action of which hypoxanthin can be converted into xanthin, and
the latter directly oxidized to uric acid. This conversion into uric
acid is purely a process of oxidation, brought about by a typical
intracellular oxidase, known specifically as “xanthin oxidase,” the
reaction involved being as follows:

  HN--CO             HN--CO
   |  |               |  |
   CO C--NH   + O =   CO C--NH
   |  ||  \           |  ||  \
   |  ||   CH         |  ||   CO
   |  ||  //          |  ||  /
  HN--C--N           HN--C--NH

   Xanthin            Uric acid

From these several reactions, it is clear how various intracellular
enzymes working one after the other are able gradually to evolve uric
acid from tissue nucleoproteids. Further, it is to be noted that there
is another tissue oxidase--contained principally in the kidneys,
muscle, and liver--which has the power of oxidizing and thus destroying
uric acid, with formation, among other substances, of urea. Remembering
that urea has the following chemical constitution

    NH_{2}
   /
  C==O
   \
    NH_{2}

it is easy to see, by comparison of the formulæ, how uric acid might
easily yield two molecules of urea through simple oxidation. In this
way, excess of uric acid produced in the body can be converted into
urea, and in this harmless form be excreted from the system.

Finally, reference should be made here to several other products of
tissue metabolism, products of the breaking down of proteid matter in
the body, since they are liable to prove of interest to us in other
connections. Thus creatin, abundant in the muscle and other places; the
related substance creatinin, present in the urine; methyl guanidin,
a decomposition product of creatin; and urea, all call for a word of
description. The chemical relationship of these bodies is clearly
indicated by the following formulæ:

    NH_{2}                    NH-------
   /                         /          \
  C==NH                     C==NH        CO
   \                         \          /
    N(CH_{3})CH_{2}COOH       N(CH_{3})CH_{2}

       Creatin                Creatinin

    NH_{2}                    NH_{2}
   /                         /
  C==NH                     C==O
   \                         \
    NH(CH_{3})                NH_{2}

   Methyl guanidin             Urea

Creatinin is chemically the anhydride of creatin, _i. e._, it can be
formed from creatin by the simple extraction of one molecule of water,
H_{2}O. Creatin, by hydrolytic cleavage, will break down into one
molecule of urea and one molecule of sarcosin or methyl glycocoll, as
shown in the following equation:

    NH_{2}                                                NH_{2}
   /                             CH_{2}·NH·(CH_{3})      /
  C==NH             + H_{2}O =   |                   +  C==O
   \                             COOH                    \
    N (CH_{3}) CH_{2}COOH                                 NH_{2}

       Creatin                   Sarcosin                 Urea

Methyl guanidin is a decomposition product of creatin, while guanidin,
as can be seen from the formula, is like urea, excepting that the
group NH replaces the oxygen of urea. These simple statements will
suffice for our present purpose, viz., to indicate the more or less
close chemical relationships existing between many of these nitrogenous
decomposition products resulting from proteid katabolism; also to
suggest how by slight chemical alteration one decomposition product
may be resolved into another related substance in the processes of
katabolism. Our conception of the processes involved in proteid
katabolism is that of a series of progressive chemical decompositions,
in which intracellular enzymes play the all-important part. The
intermediary products formed are definite bodies because of the
specific nature of the active enzymes, and, secondly, because of the
chemical nature of the substances acted upon. In other words, oxidation
in the animal body takes the shape of a series of well-defined chemical
reactions, in which chemical constitution and specific enzyme action
are the predetermining cause. In the absence of the particular chemical
groups, the oxidase is unable to bring about oxidation, or, given the
proper compound or mother substance in the absence of the specific
oxidase, there is no oxidation. Hence, oxidation in the animal body
is not the result of simple combustion, but, on the contrary, it
consists of a series of orderly chemical processes, each one of which
is presided over by an intracellular enzyme, specific in its nature,
in that it is capable of acting only upon substances having a certain
definite constitution, and leading invariably to a certain definite
result. The processes which years ago were considered as due to the
peculiar vital properties of the tissue cells, and which were supposed
to be entirely dependent upon their morphological and functional
integrity, are now seen to be due primarily to a great variety of
enzymes, manufactured indeed by the living cells, but capable of
manifesting their activity even when free from the influence of the
living protoplasm. The varied processes of tissue katabolism are the
result of orderly and progressive chemical changes, in which cleavage,
hydrolysis, reduction, oxidation, deamidization, etc., alternate with
each other under the influence of specific enzymes, where chemical
constitution and the structural make-up of the various molecules are
determining factors in the changes produced.




CHAPTER III

THE BALANCE OF NUTRITION

  TOPICS: Body equilibrium. Nitrogen equilibrium. Carbon equilibrium.
  Loss of nitrogen during fasting. Influence of previous diet on loss
  of nitrogen in fasting. Output of carbon during fasting. Influence
  of pure proteid diet on output of nitrogen. Influence of fat on
  proteid metabolism. Effect of carbohydrate on nitrogen metabolism.
  Storing up of proteid by the body. Transformation of energy in the
  body. Respiration calorimeter. Basal energy exchange of the body.
  Circumstances influencing energy exchange. Effect of food on heat
  production. Respiratory quotient and its significance. Influence of
  muscle work on energy exchange. Elimination of carbon dioxide during
  work and with different diets. Effect of excessive muscular work
  on energy exchange. Oxygen consumption under different conditions.
  Output of matter and energy subject to great variation. Body
  equilibrium and approximate nitrogen balance to be expected in health.


Man, strictly speaking, is always in a condition of unequilibrium. If
placed upon a large and sensitive pair of scales with the opposite
side exactly counterpoised, he will be found to lose weight constantly
until water or food are taken, when the losses of an hour or two may
be made good, or perchance more than balanced. The human body is a
maelstrom of chemical changes; chemical decompositions are taking place
continuously at the expense of the proteids, fats, and carbohydrates
of the tissues and of the food, the stored-up energy of these organic
compounds being thereby transformed into the active or “kinetic”
forms of heat and motion; while carbon dioxide, water, urea, and
some few other nitrogenous substances are being continually formed as
the normal waste products of these tissue changes, and constantly or
intermittently excreted. In other words, the body is in a perpetual
condition of chemical oscillation, constantly consuming its own
substance, rejecting the waste products which result, and giving off
energy in the several forms characteristic of living beings. The
condition of the body plainly depends upon the relation which it is
able to maintain between the income and the expenditure of matter
and energy. If the income equals the output, the body is kept in a
condition approaching equilibrium; if the intake exceeds the outgo, the
body adds to its capital of matter and energy; while if the expenditure
is greater than the income, the accumulated capital is drawn upon;
and this, if continued indefinitely, results in a drain upon the bank
which must eventually end in disaster. It is comparatively easy,
however, for man to maintain his body in a condition of equilibrium
from day to day; _i. e._, the losses of the morning can be made good
at luncheon, or the expenditures of an entire day counterbalanced by
a corresponding addition to capital the following day, in which case
the body may be said to be in balance. It is necessary, however, to
discriminate between body equilibrium, meaning thereby the maintenance
from day to day of a constant body-weight, and nitrogen equilibrium,
or carbon equilibrium. In the latter cases, what is meant is that the
intake of nitrogen, or of carbon, exactly equals the output of these
two elements. It is quite possible, however, to have a condition of
nitrogen equilibrium without the body being in a state of balance, as
when the outgo of carbon exceeds the intake of carbon, or when there is
an increased output of water.

As a rule, it may be stated that when a man puts out less carbon and
less nitrogen than he takes in he must be gaining in weight; the only
exception being the possible case of an increased excretion of water,
which might more than counterbalance the gain. On the other hand, if
he gives off more carbon and more nitrogen than he takes in, the body
must lose in weight. Where the output of carbon is beyond the amount of
carbon ingested, the lost carbon represents a drain upon body fat. In
a reversal of this condition, _i. e._, where the carbon taken in is in
excess of the outgo, the body is gaining in fat. Theoretically, gain
or loss of carbon may mean gain or loss of either carbohydrate or fat,
but practically stored-up carbon generally stands for accumulated fat;
and, correspondingly, loss of carbon represents a withdrawal from the
store of adipose tissue, since glycogen and sugar from a quantitative
standpoint figure only slightly in these metabolic processes. When the
body excretes more nitrogen than is taken in during a given period,
there is only one interpretation possible, viz., that the body is
losing proteid or flesh. If, on the other hand, the nitrogen import
exceeds the outgo, then the body must be gaining flesh. Here, again,
there is the theoretical possibility that gain or loss of nitrogen
might represent increase or decrease of proteid in some glandular
organ, or even in the blood; but practically it is the relatively
bulky muscle tissue, with its high content of proteid matter, that is
most subject to change in metabolism. Finally, it is easy to see how,
knowing the percentage of nitrogen in proteid and the percentage of
carbon in fat, one can calculate from the nitrogen and carbon lost or
gained the amounts of proteid or fat added to the capital stock, or
withdrawn from the store of nutritive material.

When there is no income, as in fasting, the body loses rapidly, living
during the hunger period upon its store of energy-containing material.
Many careful observations have been made upon people who have fasted
for long periods, some as long as thirty days, the income consisting
solely of water. The following figures[22] show the daily excretion of
nitrogen in several notable cases:

  [22] Taken from Landergren: Untersuchungen über die Eiweissumsetzung
  des Menschen. Skandinavisches Archiv für Physiologie, Band 14, p.
  112; and from A. Magnus-Levy: v. Noorden’s Handbuch der Pathologie
  des Stoffwechsels, 1906, p. 312.

  +-----------------+-------------+-------------+-------------+
  |                 | Breithaupt. |   Cetti.    |   Succi.    |
  | Day of Fasting. | 59.9 Kilos. | 56.5 Kilos. | 62.4 Kilos. |
  +-----------------+-------------+-------------+-------------+
  |                 |    grams    |    grams    |    grams    |
  |        0        |     13.0    |     13.5    |     16.2    |
  |        1        |     10.0    |     13.6    |     13.8    |
  |        2        |      9.9    |     12.6    |     11.0    |
  |        3        |     13.3    |     13.1    |     13.9    |
  |        4        |     12.8    |     12.4    |     12.8    |
  |        5        |     11.0    |     10.7    |     12.8    |
  |        6        |      9.9    |     10.1    |     10.1    |
  |        7        |      ...    |     10.9    |      9.4    |
  |        8        |      ...    |      8.9    |      8.4    |
  |        9        |      ...    |     10.8    |      7.8    |
  |       10        |      ...    |      9.5    |      6.7    |
  +-----------------+-------------+-------------+-------------+

In Succi’s case, the fasting was continued for thirty days. The daily
average loss of nitrogen from the 11th to the 15th day was 5.8 grams;
from the 16th to the 20th day, 5.3 grams; from the 20th to the 25th
day, 4.7 grams; and from the 26th to the 30th day, 5.3 grams. A daily
loss of 5.3 grams of nitrogen means a breaking down, or using up, of 33
grams of proteid, or a little more than one ounce. On the sixth day of
fasting, all three of these subjects showed essentially the same daily
loss of nitrogen; viz., 10 grams, which implies a using up of 62.5
grams of proteid material. We must not be led astray by these figures,
however, or draw too hasty conclusions therefrom regarding the
requirements of the body for proteid food. Noting the close agreement
in the nitrogen output of the three subjects on the sixth day, combined
with the fact that their body-weight was essentially the same, we
might infer that 62.5 grams of proteid matter represents the amount of
nitrogenous food necessary to maintain nitrogen equilibrium and keep
the body in a condition of balance. Such a conclusion, however, would
be quite erroneous for several reasons. First, a man fasting, if he was
in an ordinary condition of nutrition prior to the fast, has in his
tissues a large store of fat. It is considered that in fasting only
about 10–12 per cent of the total energy of the body is derived from
tissue proteid; the major part comes from the fat stored up. When there
is no income to make good the loss, the body must naturally draw upon
its own store. A certain amount of proteid must be used up daily, but
in addition there are the energy requirements to be considered. These
are met mainly by fat and carbohydrate, and so long as fat endures
proteid will be drawn upon only, or mainly, to meet the nitrogen
requirement; but if the fat gives out, then proteid must be used in
larger quantity, as a source of energy. Hence in fasting, the daily
loss of nitrogen will be governed largely by the condition of the body
as regards fat. Thus, Munk has reported the case of a well-nourished
and fat person, suffering from disease of the brain, who gave off daily
in the later stages of starvation only one-third the amount of nitrogen
voided by Cetti, who had been poorly nourished. Obviously, in fasting,
as soon as the adipose tissue of the body has been largely used up,
there will be an increase in the amount of tissue proteid consumed,
since under such conditions the heat of the body and the energy of
muscular work (work of the heart and involuntary muscles) must come
from the decomposition of proteid. In harmony with this statement, it
is frequently observed that in cases of starvation there comes toward
the end a sudden and marked increase in the output of nitrogen.

Secondly, the elimination of nitrogen during the earlier days of
fasting is governed in large measure by the character and extent of
the diet on the days just preceding the fast. This is well illustrated
by some experiments conducted by C. Voit on a dog. In the first series
of experiments, the dog received daily 2500 grams of meat prior to
fasting; in the second series, 1500 grams of meat were fed daily before
the fast; while in the third series, a mixed diet relatively poor
in proteid was given. The following figures[23] show the amounts of
proteid used up by the dog (calculated from the nitrogen excreted) each
day of the fasting period, under the different conditions:

  [23] Expressed in this form from Voit’s figures by A. Magnus-Levy.
  Loc. cit., p. 311.

  +------------------+---------------+----------------+---------------+
  |                  | First Series. | Second Series. | Third Series. |
  +------------------+---------------+----------------+---------------+
  |                  |     grams     |      grams     |     grams     |
  |First fasting day |      175      |       77       |      40       |
  |Second   "     "  |       72      |       54       |      33       |
  |Third    "     "  |       56      |       46       |      30       |
  |Fourth   "     "  |       50      |       53       |      36       |
  |Fifth    "     "  |       36      |       43       |      35       |
  |Sixth    "     "  |       39      |       37       |      37       |
  +------------------+---------------+----------------+---------------+

We see very clearly in these experiments the effects of the large
quantities of proteid fed on the destruction of proteid in the early
days of fasting. When the body is rich in proteid from food previously
taken, the metabolism of nitrogenous matter is very large at first,
as in the first series of experiments. Indeed, in this series, even
on the fifth day of fasting, the amount of proteid metabolized was
larger than on the second day of the third series. We have here a
forcible illustration of the physiological axiom that excess of proteid
matter in the tissues, or in the blood, stimulates proteid metabolism;
and it affords convincing proof of the contention that in the first
days of fasting the output of nitrogen, or the amount of proteid used
up, will depend in large measure upon the proteid condition of the
body at the time of the fast. Equally noticeable is the fact that
there comes a time--the sixth day in the above experiment--when the
nitrogen output reaches a common level, irrespective of the previous
proteid condition of the body. Further, it is easy to see that the
greater loss of nitrogen, _i. e._, the large breaking down of proteid
during the first few days of fasting, in those cases where proteid
food has been freely taken, suggests the existence in the tissues of
two forms of proteid. We may term them, following the nomenclature of
Voit, as circulating and morphotic, or tissue, proteid; or, we may
designate them as labile and stable forms of proteid. In other words,
following the usually accepted view, this circulating or labile proteid
represents reserve or surplus material which is easily decomposed
and hence rapidly gotten rid of, while the stable proteid is more
slowly oxidized, and its metabolism may be taken as representing more
nearly the real necessities of the body. However this may be, it is
plainly manifest that the nitrogen output, meaning the metabolism of
proteid matter, during hunger or fasting is modified by a variety of
circumstances, notably the previous nutritive condition of the body as
regards both fat and proteid. It is hardly necessary to add that the
amount of muscular work performed is another factor of importance in
this connection. Fat in the body represents inert material stored up
mainly for nutritive purposes; hence, in hunger it is used largely, and
serves to protect more important tissues. Thus, experiments have shown
that in long periods of fasting, adipose tissue may be consumed to the
extent of 97 per cent of the total amount present, while the heart and
nervous tissue will not lose over 3 per cent of their tissue substance.
The influence of tissue fat upon the consumption of proteid during
hunger can thus be fully appreciated.

The output of carbon during fasting may be illustrated by the following
experiment[24] made upon a young man, the nitrogen data being included
for comparison, and likewise the intake of food, in terms of nitrogen
and carbon, preceding the fast and for two days following the fast. The
fasting was of five days’ duration.

  [24] Taken from Johansson, Landergren, Sondén, and Tiegerstedt:
  Beiträge zur Kenntniss des Stoffwechsels beim hungernden Menschen.
  Skandinavisches Archiv für Physiologie, Band 7, p. 29.

  +------+--------------+---------------------+------------------------+
  |      |              |       Intake.       |         Output.        |
  | Day. | Body-weight. +---------+-----------+------------+-----------+
  |      |              | Carbon. | Nitrogen. | Carbon.[25]| Nitrogen. |
  +------+--------------+---------+-----------+------------+-----------+
  |      |    kilos     |  grams  |   grams   |    grams   |   grams   |
  |  2   |    67.4      |  438.7  |   30.96   |    303.4   |   25.81   |
  |  3   |    66.9      |    0    |     0     |    197.6   |   12.17   |
  |  4   |    65.7      |    0    |     0     |    188.8   |   12.85   |
  |  5   |    64.8      |    0    |     0     |    183.2   |   13.61   |
  |  6   |    63.9      |    0    |     0     |    180.8   |   13.69   |
  |  7   |    63.1      |    0    |     0     |    176.2   |   11.47   |
  |  8   |    63.9      |  439.9  |   35.65   |    270.5   |   26.83   |
  |  9   |    65.5      |  391.7  |   23.68   |    258.8   |   19.46   |
  +------+--------------+---------+-----------+------------+-----------+

  [25] The carbon output represents the total carbon of the expired
  air, urine, and excrement.

On the non-fasting days, the intake consisted of an ordinary food
mixture of proteids, fats, and carbohydrates, with a small addition
of alcohol. The point to be emphasized here, however, is that the
carbon-content was more than sufficient to meet the needs of the body.
Thus, it will be observed that on all three of the days when food was
taken, the income of carbon was far in excess of the output. In other
words, on the day preceding the beginning of the fast the body stored
up 135 grams of carbon, and on the day following the fast the body
retained 169 grams of carbon to help make good the loss. Similarly,
the amount of proteid food taken in on the day prior to the fast was
considerably in excess of the needs of the body, 5.1 grams of nitrogen
equivalent to 31.8 grams of proteid being stored for future use.
Plainly, the man was not in either carbon or nitrogen balance prior
to the fast, but was taking far more food than the needs of the body
called for. This fact may be emphasized by noting that the total fuel
value of the daily food, plus the fuel value of the alcohol, amounted
on an average to about 4200 large calories, while the fuel value of the
material metabolized on the feeding days averaged only 2500 calories.
Looking at the figures showing the output of carbon, as well as of
nitrogen, during the fasting days, it is to be seen that in the early
days of fasting, the metabolism of the body tends to remain at a fairly
constant level, especially when figured per kilogram of body-weight.

To fully appreciate what takes place in a man of the above body-weight
fasting for five days (though living on a large excess of food prior to
the fast), the daily losses of carbon and nitrogen may be translated
into terms of fat and proteid. If it is assumed that the total carbon,
aside from what necessarily belongs to the proteid indicated by the
nitrogen figures, comes from the oxidation of fat, it is easy to
compute the amounts of fat and proteid metabolized, or destroyed, each
day of the fasting period. These are shown in the following table:

  +------+--------------+--------------+
  | Day. |   Proteid    |     Fat      |
  |      | metabolized. | metabolized. |
  +------+--------------+--------------+
  |      |    grams     |    grams     |
  |  3   |     76.1     |    206.1     |
  |  4   |     80.3     |    191.6     |
  |  5   |     85.1     |    181.2     |
  |  6   |     85.6     |    177.6     |
  |  7   |     71.7     |    181.2     |
  +------+--------------+--------------+

Finally, if from these figures we calculate the fuel value of the
proteid and fat oxidized per day, it is possible to gain a fairly clear
conception of the part played by these two classes of tissue material
during fasting, in furnishing the heat of the body and the energy for
muscular motion, etc.

  +------+---------------+---------------+-------------+
  |      | Fuel Value of | Fuel Value of |    Total    |
  | Day. |  the Proteid  |   the Fat     | Fuel Value. |
  |      |  metabolized. |  metabolized. |             |
  +------+---------------+---------------+-------------+
  |      |    calories   |    calories   |  calories   |
  |  3   |      303      |      1916     |    2220     |
  |  4   |      320      |      1781     |    2102     |
  |  5   |      339      |      1684     |    2024     |
  |  6   |      341      |      1651     |    1992     |
  |  7   |      286      |      1684     |    1970     |
  +------+---------------+---------------+-------------+

These somewhat general statements, with the illustrations given, will
serve in a brief way to emphasize some of the essential features of
metabolism in the fasting individual; where there is no income of
energy-containing material, and where the body must draw entirely
upon its store of accumulated fat and proteid to keep the machinery
in motion, maintain body temperature, and do the tasks of every-day
life. When it is remembered that persons have fasted for periods of
thirty days or longer without succumbing, it is evident that the body
of the well-nourished man has a large reserve of nutritive material,
which can be drawn upon in cases of emergency. At the same time, the
facts presented show us that in the early days of fasting the actual
amounts of tissue proteid and body fat consumed are not large. In
Cetti’s case, on the sixth day of fasting the metabolized nitrogen
amounted to 10 grams, which implies a loss of 62.5 grams of proteid.
At this rate of loss, one pound of dry proteid matter in the form of
tissue proteid would meet the wants of a man of 130 pounds body-weight
for seven and a half days, provided of course there was a reasonable
stock of fat to help satisfy the energy requirements. Finally, we may
again emphasize the fact that the loss of nitrogen in the fasting man
is by no means a measure of the minimal proteid requirement. By feeding
fat, or carbohydrate, or both, the output of nitrogen can be materially
diminished, although naturally we cannot establish a nitrogen balance
by so doing, since the income is free from nitrogen; but we can
postpone for a time the approach of nitrogen starvation.

We may next profitably consider the effect of a pure proteid diet--such
as lean meat free from fat--on the output of nitrogen. In studying this
problem, we at once meet with several important and surprising facts.
First, we are led to see that, strange as it may seem, every addition
of proteid to the diet results in an increased excretion of nitrogen.
In other words, increase of proteid income is followed at once by an
increase in the metabolism of proteid, with a corresponding outgo of
nitrogen. The hungry or fasting man with his income entirely cut off,
and consequently suffering from a heavy drain upon his capital stock,
would be expected, when suddenly supplied with fresh capital in the
form of meat or other kind of proteid food, to hold on firmly to this
all-important foodstuff; but such is not the case. It is impossible,
for example, to establish nitrogen equilibrium by an income of proteid
equal to what the individual during fasting is found to metabolize.
As stated by another, “It is one of the cardinal laws of proteid
metabolism that the store of nitrogenous substances in the body is not
increased by, or not in proportion to, an increase in the nitrogen
intake.” The principle is well illustrated in the fasting experiment
just described. On the fifth day of fasting, the nitrogen output
amounted to 11.4 grams. On the day following, the man took 35.6 grams
of nitrogen in the form of proteid, while the excretion of nitrogen for
that day rose to 26.8 grams. In other words, although deprived of all
proteid income for five days, and during that period drawing entirely
upon his proteid capital, the man was wholly unable to avail himself of
the proteid so abundantly supplied at the close of the fast and make
good the losses of the preceding days; only a small proportion of the
proteid income could be retained. If a dog fed on a definite quantity
of meat suddenly has his proteid income increased, there is at once an
acceleration of proteid metabolism, and a corresponding increase in the
output of nitrogen. Addition of still more proteid to his income is
followed by an accumulation of a portion of the proteid; but this tends
to decrease gradually, while there is a corresponding daily increase
in the excretion of nitrogen. In this manner, there finally results a
condition of nitrogenous equilibrium or nitrogen balance.

Again, an animal brought into nitrogen equilibrium by excessive
proteid feeding, if suddenly given a small amount of meat per day,
tends to put out nitrogen from its own tissues. This tissue loss,
however, decreases slowly, and eventually the animal is quite likely
to re-establish nitrogen equilibrium at a lower level. There is, in
other words, a strong tendency for the body to pass into a condition
of nitrogen balance under different conditions of proteid feeding,
even after a long period of nitrogen loss and with an abundance of
proteid in the intake. The starving body, as we have seen, cannot make
use of all the nitrogen fed, although we can well conceive its great
need for all the proteid available. A certain amount of the proteid
fed, or its contained nitrogen, is at once passed out of the body and
lost, even though the organism be gasping, as it were, for proteid to
make good the drain incidental to long fasting. A recent writer[26]
has suggested that some explanation for these anomalies may be found
in the supposition “that a long succession of generations in the past,
which have lived from choice or necessity on a diet rich in proteids,
have handed down to us, as an inheritance, a constitution in which
arrangements exist for the removal of nitrogen from a considerable
part of this proteid. The fact that the amount of proteid taken is
re-adjusted to suit the actual needs of the body, though it makes
these arrangements unnecessary, will not necessarily remove them. The
denitrifying enzyme, which has been trained to keep guard over the
entrances by which nitrogenous substances are admitted into the body,
will continue to levy its toll of nitrogen, even when the amount of
proteid presented to it is no more than the tissues which it serves
actually require.”

  [26] Leathes: Problems in Animal Metabolism. Philadelphia, 1906, p.
  157.

As an illustration of how the body behaves with a low nitrogen intake
followed by a sudden increase in the income of proteid, some data from
an experiment performed by Sivén[27] on himself may be cited:

  [27] Sivén: Zur Kenntniss des Stoffwechsels beim erwachsenen
  Menschen, mit besonderer Berücksichtigung des Eiweissbedarfs.
  Skandinavisches Archiv für Physiologie, Band 11, p. 308.

  +--------+--------------+-------------+-----------+----------+
  |        |              | Nitrogen of | Nitrogen  | Nitrogen |
  |  Date. | Body-weight. |  the Food.  | excreted. | Balance. |
  +--------+--------------+-------------+-----------+----------+
  |        |    kilos     |    grams    |   grams   |   grams  |
  | Nov. 6 |     65.4     |     2.69    |    8.31   |   -5.62  |
  |      7 |     65.4     |     2.69    |    5.37   |   -2.68  |
  |      8 |     65.1     |     2.69    |    5.71   |   -3.02  |
  |      9 |     65.3     |     2.69    |    4.88   |   -2.19  |
  |     10 |     65.0     |     2.69    |    4.32   |   -1.63  |
  |     11 |     64.9     |     2.69    |    4.25   |   -1.56  |
  |     12 |     64.9     |     2.69    |    4.47   |   -1.78  |
  |     13 |     64.6     |     2.96    |    4.88   |   -1.92  |
  |     14 |     64.4     |     2.96    |    4.30   |   -1.44  |
  |     15 |     64.3     |     2.96    |    4.75   |   -1.79  |
  |     16 |     64.4     |     2.96    |    4.36   |   -1.40  |
  |     17 |     64.4     |     2.96    |    4.13   |   -1.17  |
  |     18 |     64.4     |     2.96    |    4.35   |   -1.39  |
  |     19 |     64.4     |     2.96    |    4.32   |   -1.36  |
  |     20 |     64.4     |     2.96    |    4.22   |   -1.26  |
  |     21 |     64.0     |     2.96    |    4.06   |   -1.10  |
  |        |              |             |           +----------+
  |        |              |             |           |  -31.31  |
  |        |              |             |           |          |
  |     22 |     64.1     |     4.02    |    4.22   |   -0.20  |
  |     23 |     64.4     |     4.02    |    4.35   |   -0.33  |
  |     24 |     64.4     |     4.02    |    4.21   |   -0.19  |
  |     25 |     64.4     |     4.02    |    4.40   |   -0.38  |
  |        |              |             |           +----------+
  |        |              |             |           |   -1.10  |
  |        |              |             |           |          |
  |     26 |     64.2     |     8.24    |    6.56   |   +1.68  |
  |     27 |     64.4     |    13.45    |    8.67   |   +4.78  |
  |     28 |     64.4     |    13.66    |   10.54   |   +3.12  |
  |     29 |     64.0     |    13.45    |   11.10   |   +2.35  |
  |     30 |     64.2     |    13.24    |   12.83   |   +0.41  |
  | Dec. 1 |     64.2     |    13.24    |   11.70   |   +1.54  |
  |      2 |     63.9     |    12.61    |   12.00   |   +0.61  |
  |        |              |             |           +----------+
  |        |              |             |           |  +14.49  |
  |        |              |             |           |          |
  |      3 |     64.0     |    22.93    |   16.24   |   +6.69  |
  |      4 |     63.9     |    22.41    |   21.47   |   +0.94  |
  |      5 |     63.9     |    22.41    |   23.10   |   -0.69  |
  |      6 |     63.6     |    23.35    |   23.12   |   +0.23  |
  |      7 |     63.9     |    23.04    |   22.82   |   +0.22  |
  |      8 |     63.8     |    22.62    |   22.86   |   -0.24  |
  |        |              |             |           +----------|
  |        |              |             |           |   +6.15  |
  +--------+--------------+-------------+-----------+----------+

I have ventured to give these data in some detail, because of their
exceeding great interest in several directions aside from the point
under discussion. Confining our attention to the nitrogen exchange, it
is to be observed that for a period of two weeks Sivén lived on less
than 3 grams of nitrogen per day, and without any excessive intake
of carbohydrate or fat. During this time, the body naturally was in
a condition of minus balance as regards nitrogen, the output being
considerably larger than the income. The total amount of nitrogen lost
in the period, 31 grams, corresponds to a breaking down of 193 grams of
tissue proteid, or over one-third of a pound. On increasing the income
of nitrogen to 4 grams per day, the nitrogen loss still continued,
though at a much lower rate; indeed, the body is seen to approach very
closely to a condition of nitrogen equilibrium. Still further increase
of the nitrogen income to 13 grams per day was followed at once by a
slight accumulation of proteid, and the body showed a decided plus
balance of nitrogen, as on November 27. This, however, is seen to
decrease gradually with a corresponding daily increase in the outgo of
nitrogen, until on December 2 the body was once more practically in
nitrogenous equilibrium. On again increasing the nitrogen income, to 23
grams per day, the same process was repeated, although in this case the
body more quickly approached a condition of nitrogen balance.

We see in these data striking confirmation of the statement that the
nitrogen outgo tends to keep pace with the income of nitrogen, the
body always striving to maintain a condition of nitrogen equilibrium.
Consequently, the fasting man having lost largely of his store of
proteid can replace the latter only slowly, even though he eats
abundantly of proteid food. Thus, Sivén in the week ending December 2,
though taking over 13 grams of nitrogen a day, retained in his body
only 14.5 grams of nitrogen during the entire seven days; while in the
six days following, with a daily intake of 23 grams of nitrogen, he
gained only about 8 grams additional. The human body does not readily
store up proteid, and this is true no matter how greatly the tissues
are in need of replenishment.

If the daily income is reinforced by the addition of carbohydrate or
fat, there is observed a decided influence on the outgo of nitrogen;
the rate or extent of proteid metabolism is at once modified, fat and
carbohydrate both having a direct saving effect on proteid. Neither fat
nor carbohydrate can prevent the katabolism of proteid, but they can
and do decrease it, and thus serve as proteid-sparers. In the fasting
body, or where there is only an intake of proteid, the latter material,
except for the fat contained in the tissues, must serve the double
purpose of meeting the specific nitrogen requirements of the body and
furnishing the requisite energy. The energy requirements, however, can
be met more advantageously by either of the non-nitrogenous foodstuffs,
and just so far as they are oxidized, so far will there be a saving of
proteid. Herein lies the philosophy of a mixed diet, with its natural
intermingling of proteid, fat, and carbohydrate. For the same reason,
the body of a man rich in fat will in fasting lose far less proteid
per day than the lean man; or, if fed with a given amount of proteid
food, the fat man may attain nitrogen equilibrium, or even store up a
little proteid, while on the same diet the lean man will lose proteid.
Further, if a man is in nitrogen balance with a given amount of proteid
food, the addition of fat or carbohydrate to the diet will permit of a
reduction in the amount of proteid necessary to maintain nitrogenous
equilibrium. Fat, however, when added to food, does not always protect
proteid to the extent possibly suggested by the preceding statements.
The following data from oft-quoted experiments by Voit[28] on dogs will
serve to illustrate:

  [28] C. Voit: Hermann’s Handbuch der Physiologie des
  Gesammtstoffwechsels, Band 6, p. 130.

  +-----------------------++-----------------------------+
  |          Food.        ||            Flesh.           |
  +-----------+-----------++--------------+--------------+
  |   Meat.   |    Fat.   || Metabolized. | On the Body. |
  +-----------+-----------++--------------+--------------+
  |   grams   |   grams   ||    grams     |    grams     |
  |    1500   |     0     ||     1512     |     -12      |
  |    1500   |    150    ||     1474     |     +26      |
  |           |           ||              |              |
  |     500   |     0     ||      556     |     -56      |
  |     500   |    100    ||      520     |     -20      |
  +-----------+-----------++--------------+--------------+

It is to be observed that in both of these experiments the fairly large
addition of fat results in a saving of proteid, but the sparing effect
in the first experiment amounts to only 38 grams of proteid for the
150 grams of fat added. In the second experiment, however, there is
a saving of 36 grams of proteid, although only 100 grams of fat were
fed. The radical point of difference in the two experiments is the
amount of proteid ingested. Proteid food stimulates proteid metabolism;
it likewise accelerates the metabolism of non-nitrogenous matter,
consequently the sparing or protecting effect of fat on proteid is
most conspicuous when the intake of proteid is relatively small. Only
under such conditions, does fat protect in large degree the consumption
of proteid in the body. In the ordinary, daily, dietary of man, with
its great variety of food materials and with its proteid-content not
exceeding 125 grams, fat is apt to be a conspicuous element, and under
such conditions its sparing effect on proteid metabolism is most
marked. Further, it must not be forgotten, as Voit originally pointed
out, that the adipose tissue of the body acts like the food-fat, and
consequently the proteid-sparing effect of the former may be added to
that of the latter.

The addition of carbohydrate to a meat diet produces at once a saving
in the decomposition of proteid, as shown in the following figures,
covering an experiment of two days:

     Meat.         Sugar.     Proteid metabolized.
  500 grams.     200 grams.        502 grams.
  500              0               564

Without the sugar, there was a minus balance of 64 grams of proteid,
but addition of the carbohydrate caused practically a saving of all
of this, with establishment of essentially a nitrogen balance. The
sparing of proteid by carbohydrate is greater than by fats, a fact
of considerable dietetic importance which is well illustrated by the
following experiments (on dogs) taken from Voit:

  +-------------------------------++-------------------------------------+
  |            Food.              ||               Flesh.                |
  +-------+-----------------------++--------------+----------------------+
  | Meat. | Non-nitrogenous Food. || Metabolized. | Balance of the Body. |
  +-------+-----------------------++--------------+----------------------+
  | grams |         grams         ||    grams     |        grams         |
  |  500  |        250 Fat        ||      558     |          -58         |
  |  500  |        300 Sugar      ||      466     |          +34         |
  |  500  |        200 Sugar      ||      505     |           -5         |
  |  800  |        250 Starch     ||      745     |          +55         |
  |  800  |        200 Fat        ||      773     |          +27         |
  | 2000  |    200–300 Starch     ||     1792     |         +208         |
  | 2000  |        250 Fat        ||     1883     |         +117         |
  +-------+-----------------------++--------------+----------------------+

In considering the results of this experiment, it must be remembered
that the calorific or fuel value of fat as compared with carbohydrate
is as 9.3 : 4.1; in other words, fats have a fuel value of more than
twice that of carbohydrates. In spite of this fact, it is clearly
evident that carbohydrates as a class--for the different sugars and
starches act alike in this respect--are far more efficient than fats
in saving proteid. Thus, with an income of 500 grams of meat and 250
grams of fat, the body of the animal lost 58 grams of proteid, while
with a like amount of meat and 300 grams of sugar the body not only
saved the 58 grams, but in addition stored 34 grams of proteid, showing
a plus balance to that extent. The sparing of proteid by carbohydrate
amounts on an average, according to Voit, to 9 per cent--in the highest
cases to 15 per cent--of the proteid given, while the saving produced
by fat averages only 7 per cent. Further, increasing quantities of
carbohydrates in the food diminish the rate of proteid metabolism much
more regularly and constantly than increasing quantities of fat. We
may attribute this difference in action, in a measure at least, to the
greater ease in oxidation and utilization of the carbohydrate. In any
event, starches and sugars are most valuable adjuncts to the daily
diet, because of this marked proteid-saving power, while their fuel
value adds just so much to the total energy intake.

A more striking illustration of the action of carbohydrate in sparing
proteid is seen in experiments on man, where the nitrogen intake is
reduced to a minimum, so as to constitute a condition of specific
nitrogen-hunger. In such a case, increasing amounts of carbohydrate
added to the intake reduce enormously the using up of tissue proteid.
The following experiment with a young man 22 years old and 71.3 kilos
body-weight, reported by Landergren,[29] affords good evidence of the
extent to which this proteid sparing power may manifest itself.

  [29] Landergren: Untersuchungen über die Eiweissumsetzung des
  Menschen, Skandinavisches Archiv für Physiologie, Band 14, p. 114.

We see here the nitrogen consumption fall to the exceedingly low level
of 3.34 grams per day, or 0.047 gram per kilo of body-weight. To
appreciate the full significance of this drop in the extent of proteid
metabolism, we may recall that Succi, with a body-weight of only 62.4
kilos, on the seventh day of fasting excreted 9.4 grams of nitrogen,
corresponding to a metabolism of 58.7 grams of tissue proteid. In
other words, with an intake of only 5.6 grams of proteid, the addition
of 908 grams of carbohydrate, with a total fuel value of 3745 calories,
reduced the consumption of tissue proteid to 20.8 grams. The same
individual, if fasting, would undoubtedly have used up at least 70
grams of tissue proteid.

  +----+------------------------------------------++---------++------------+
  |    |                Intake.                   || Output. ||            |
  |Day.+--------+-----+--------+--------+---------++---------++  Proteid   |
  |    |Proteid.| Fat.| Carbo- |Alcohol.|Calories.||Nitrogen ||metabolized.|
  |    |        |     |hydrate.|        |         ||of Urine.||            |
  +----+--------+-----+--------+--------+---------++---------++------------+
  |    | grams  |grams|  grams | grams  |         ||  grams  ||   grams    |
  | 1  |  35.2  | 6.1 |   507  |  26.6  |  2465.9 ||  12.16  ||    76.0    |
  | 2  |  28.7  | 4.7 |   787  |  26.6  |  3574.3 ||   8.37  ||    52.3    |
  | 3  |  28.8  | 4.7 |   841  |  26.6  |  3796.1 ||   5.02  ||    31.3    |
  | 4  |  28.3  | 4.9 |   839  |  13.3  |  3690.5 ||   4.50  ||    28.1    |
  | 5  |   5.4  |  .. |   898  |  ....  |  3703.9 ||   4.01  ||    25.0    |
  | 6  |   6.0  |  .. |   931  |  ....  |  3841.7 ||   3.36  ||    21.0    |
  | 7  |   5.6  |  .. |   908  |  ....  |  3745.8 ||   3.34  ||    20.8    |
  +----+--------+-----+--------+--------+---------++---------++------------+

It is evident from what has been said that both of these
non-nitrogenous foods, fat and carbohydrate, play a very important
part in nutrition, because of their ability to maintain in a measure
the integrity of tissue proteid. When we recall that a diet of pure
proteid, such as meat or eggs, must be excessive in quantity in order
to meet the energy requirements of the body, and that the stimulating
action of proteid food serves to whip up body metabolism, we can
appreciate at full measure the great physiological economy which
results from the addition of carbohydrate and fat to the daily diet.
The establishment of nitrogenous equilibrium is made possible at a much
lower level by the judicious addition of these two non-nitrogenous
foodstuffs. The same principle may be illustrated in another way, viz.,
by noting the effect on tissue proteid of withdrawal of a portion of
the fat or carbohydrate of the intake, in the case of a person nearly
or quite in nitrogen balance. The following experiment[30] affords a
good example of what will occur under such treatment:

  [30] An experiment by Miura, quoted from A. Magnus-Levy in v.
  Noorden’s Handbuch der Pathologie des Stoffwechsels, 1906, p. 331.

  +-------+---------------------------------------++---------+----------+
  |       |            Income.                    ||         |Balance of|
  |       +---------+-----+-------------+---------+|Output of| Nitrogen |
  |       |Nitrogen.| Fat.|Carbohydrate.|Calories.||Nitrogen.| in Body. |
  +-------+---------+-----+-------------+---------++---------+----------+
  |Av. of |  grams  |grams|    grams    |         ||  grams  |          |
  |3 days | 15.782  |40.47|    289.6    |   1955  ||  14.927 |  +0.862  |
  |Nov. 30| 15.782  |40.34|    177.3    |   1493  ||  14.959 |  +0.830  |
  |Dec.  1| 15.782  |40.34|    177.3    |   1493  ||  17.546 |  -1.757  |
  |      2| 15.782  |40.34|    177.3    |   1493  ||  18.452 |  -2.663  |
  +-------+---------+-----+-------------+---------++---------+----------+
  |                   Average of the last two days              -2.210  |
  +---------------------------------------------------------------------+

Starting with the body in a condition of plus nitrogen balance, _i.
e._, with a mixed diet more than sufficient to maintain the tissue
proteid intact, the reduction of the fuel value of the food from 1955
to 1493 calories by cutting off 112 grams of carbohydrate per day was
followed by a gradual, but marked, increase in the output of nitrogen;
indicating thereby the extent to which the body proteid was then drawn
upon to make good the loss of energy-containing income. The body showed
at the close of the experiment a minus nitrogen balance averaging
2.2 grams per day, or a loss of 13.8 grams of tissue proteid, which
would obviously have continued, under the above conditions, until the
body was exhausted. In other words, the 112 grams of carbohydrate,
if added to the diet on December 3 and the following days, would
have quickly saved the daily loss of 2.4 grams of nitrogen, and thus
changed the drain of tissue proteid to an actual gain, with consequent
establishment of a growing plus balance.

It is obvious from what has been stated, that in man the body can
accomplish a storing of proteid only when the intake is reinforced by
substantial additions of fat or carbohydrate. It is plainly a matter of
great physiological importance that the body should be able to increase
at times its reserve of proteid. This, however, cannot apparently be
accomplished on a large scale under ordinary conditions. Any storing
up of nutritive material in excess, whether it be proteid or fat,
necessarily involves overfeeding, _i. e._, the taking of an amount of
food beyond the capacity of the body to metabolize at the time. Fat,
as we know, may be stored in large quantities, and it is in cases of
overfeeding with non-nitrogenous foods that we find accumulation of
fat most marked. Overfeeding with proteid, however, does not lead to
corresponding results, owing primarily to the peculiar physiological
properties of proteid; its general stimulating effect on metabolism,
the tendency of the body to establish nitrogenous equilibrium at
different levels, and the fact emphasized by von Noorden that flesh
deposition is primarily a function of the specific energy of developing
cells. In other words, the protoplasmic cells of the body are more
important factors in the storing or holding on to proteid than an
excess of proteid-containing food.

It is generally considered as a settled fact, that in man it is
impossible to accomplish any large permanent storing or deposition of
flesh by overfeeding. Similarly, it is understood that the muscular
strength of man cannot be greatly increased by an excessive intake of
food. The only conditions under which there is ordinarily any marked
and permanent flesh deposition are such as are connected with the
regenerative energy of living cells. Thus, as von Noorden has stated,
an accumulation or storing of tissue proteid is seen especially in the
growing body, where new cells are being rapidly constructed; also in
the adult where growth may have ceased, but where increased muscular
work has resulted in an hypertrophy or enlargement of the muscular
tissue; and lastly in those cases where, owing to previous insufficient
food or to the wasting away of the body incidental to disease, the
proteid content of the tissues has been more or less diminished, and
consequently an abundance of proteid food is called for and duly
utilized to make good the loss. In some oft-quoted experiments by
Krug, conducted on himself, it was observed that with an abundant food
intake, sufficient to furnish 2590 calories per day (44 calories per
kilo of body-weight), a condition approaching nitrogenous equilibrium
was easily maintained. On then increasing the fuel value of the food
to 4300 calories (71 calories per kilo of body-weight) by addition
of fat and carbohydrate, there was during a period of fifteen days a
sparing of 49.5 grams of nitrogen or 309 grams of proteid, which would
correspond to about 1450 grams, or three pounds, of fresh muscle. It
is to be noted, however, that of this excess of calories added to the
intake only 5 per cent was made use of for flesh deposit, the remaining
95 per cent going to make fat.

Again, we may call attention to the well-known fact that in feeding
animals for food, while fat may be laid on in large amounts, flesh
cannot be so increased by overfeeding. In this matter, however, race
and individuality count for considerable. Thus, there is on record a
more recent series of experiments conducted by Dapper[31] on himself
which shows some remarkable results. Starting with a daily diet not
excessive in amount, he was able by an addition of only 80 grams of
starch to accomplish a laying up of 3.32 grams of nitrogen per day for
a period of twelve days, or a total gain of 39.8 grams of nitrogen,
equal to 248 grams of proteid. It may be said that the gain of proteid
or flesh here for the twelve days was no greater than in the preceding
case (fifteen days), but the difference lies in the fact that Krug
accomplished his gain by increasing the daily intake from 2590 to 4300
calories, an amount which he found too large to be eaten with comfort,
while the later investigator raised the fuel value of his daily food
from 2930 to only 3250 calories. As the experiments by Dapper contain
other points of interest bearing on the question before us, we may
advantageously consider them somewhat in detail. The following table
gives the more important results:

  [31] Max Dapper: Ueber Fleischmast beim Menschen. Inaug. Disser.
  Marburg, 1902.

  +----+-----+--------------+-------------------+--------+------------------+
  |No. |     |              | Food Composition. |        |                  |
  |of  |Dura-|   Character  +---------+---------+Nitrogen|Maxima and Minima |
  |Exp.|tion.|    of Food.  |Nitrogen.|Calories.|Balance.|of Nitrogen-gain. |
  +----+-----+--------------+---------+---------+--------+------------------+
  |    |days |              |  grams  |         |  grams |     grams        |
  | 1  |  6  |Ordinary mixed|  20.25  |   2930  |  +2.18 |+3.2 on 4th day.  |
  |    |     |  diet        |         |         |        |+1.5 on 6th day.  |
  |    |     |              |         |         |        |                  |
  | 2  | 12  |Ditto + 80    |  20.09  |   3250  |  +3.32 |+4.75 on 2d day.  |
  |    |     | grams starch |         |         |        |+4.65 on 12th day.|
  |    |     |              |         |         |        |+2.30 on 8th day. |
  |    |     |              |         |         |        |                  |
  | 3  |  9  |Ditto + 80    |  24.58  |   3400  |  +2.55 |+5.98 on 1st day. |
  |    |     | grams starch,|         |         |        |+4.73 on 2d day.  |
  |    |     | + 40 grams   |         |         |        |+0.50 on 6th day. |
  |    |     | plasmon      |         |         |        |+1.60 on 9th day. |
  +----+-----+--------------+---------+---------+--------+------------------+

As we look at these results, the nitrogen gain for the first and
second days of the third experiment and the first day of the second
experiment may well attract our attention, since they show an
astonishing laying by of proteid, or gain of flesh, under the influence
of a comparatively small increase in the fuel value of the food. A
gain of 5.98 grams of nitrogen means 37.3 grams of proteid, or more
than an ounce; by no means an inconsiderable addition for one day to
the store of tissue proteid. In the third experiment, where plasmon
(dried, milk proteid) was added to the diet, there is to be noted a
gradual falling off in the proteid-sparing power, which may perhaps
be interpreted as implying that the body was practically saturated
with proteid, and that owing to this fact the body was unable to
continue its laying hold of nitrogen. In the entire period of 21
days, however, the body had succeeded in accumulating a store of 62.8
grams of nitrogen, or 392 grams of proteid, and this without adding
very largely to the intake of non-nitrogenous matter. This experiment
affords a striking illustration of the ability of the body to “fatten
on nitrogen,” but it is very doubtful if such results can generally
be obtained. Lüthje,[32] however, has reported a large retention of
nitrogen on a diet containing 50 grams of nitrogen daily, with a fuel
value of 4000 calories. It is more than probable that there existed
in these particular cases some personal peculiarity or idiosyncrasy
which favored the proteid-sparing power. The personal coefficient of
nutrition is not to be ignored; it shows itself in many ways, and the
above results are to be counted among those that are exceptional and
not the rule. In the words of Magnus-Levy, “a given diet with Cassius
may lead to different results than with Anthony.”

  [32] Zeitschrift für klinische Medizin, Band 44, p. 22.

For the study of many questions in nutrition, it becomes necessary to
determine accurately the transformations of energy within the body
as contrasted with the transformation of matter; the total income
and outgo of energy, measured in terms of heat, are to be compared
one with the other and a balance struck. Further, in studying the
metabolism of carbohydrate and fat it is necessary to determine the
output of gaseous products through the lungs and skin; to estimate the
excretion of carbon dioxide and water, and the intake of oxygen. For
these purposes, a special form of apparatus known as a respiration
calorimeter is employed. The double name is indicative of the twofold
character of the apparatus, viz., a suitably constructed chamber so
arranged as to permit of measuring at the same time the respiratory
products and the energy given off from the body. The form of apparatus
best known to-day, and with which exceedingly satisfactory work has
been done, is the Atwater-Rosa apparatus, as modified by Benedict. It
consists essentially of a respiration chamber, in reality an air-tight,
constant-temperature room (with walls of sheet metal, outside of which
are two concentric coverings of wood completely surrounding it, with
generous air spaces between), sufficiently large to admit of a man
living in it for a week or more at a time. Connected with the chamber
is a great variety of complex apparatus for maintaining and analyzing
the supply of oxygen, determining the amount of carbon dioxide and
of water, etc., etc. As an apparatus for measuring heat, the chamber
may be described as “a constant-temperature, continuous-flow water
calorimeter, so devised and manipulated that gain or loss of heat
through the walls of the chamber is prevented, and the heat generated
within the chamber cannot escape in any other way than that provided
for carrying it away and measuring it.”[33]

  [33] For an account of the respiration calorimeter and the great
  diversity of apparatus accessory thereto, together with a description
  of the methods of measurement, analysis, etc., see Publication No.
  42, Carnegie Institution of Washington, “A Respiration Calorimeter
  with Appliances for the Direct Determination of Oxygen.” By W. O.
  Atwater and F. G. Benedict.

In illustration of the efficiency of an apparatus of this description,
and of the close agreement obtainable by direct calorimetric
measurement with the estimated energy, as figured from the materials
oxidized in the body, we may quote the following data from Dr.
Benedict’s report, referred to in the footnote. The subject was a young
man who had been fasting for five days. The experiment deals with the
metabolism on the first day after the fast, when a diet composed mainly
of milk was made use of, containing 53.31 grams of proteid, 211.87
grams of fat, and 75.41 grams of carbohydrate. The following table
shows the results of the experiment:

  +--------------------------+--------+---------+---------+--------+-----------------+
  |   Heat of Combustion of  |   (d)  |   (e)   |   (f)   |        |  Heat Measured  |
  |    Food and Excreta as   | Avail- | Total   |Estimated|  Heat  |   Greater or    |
  |    Determined by Bomb    |  able  | Energy  | Energy  |Measured|   Less than     |
  |       Calorimeter.       | Energy |from Body|  from   |   by   |   Estimated.    |
  +--------+--------+--------+  from  |Material | Material|Respira-+--------+--------+
  |  (a)   |  (b)   |   (c)  |  Food. |Gained or| Oxidized|  tion  |        |        |
  | Food.  | Excre- | Urine. | a-(b+c)|Lost.[34]|  in the | Calor- | Amount.|Propor- |
  |        |  ment. |        |        |         |  Body.  | imeter.|        | tion.  |
  |        |        |        |        |         |   d-e   |        |        |        |
  +--------+--------+--------+--------+---------+---------+--------+--------+--------+
  |calories|calories|calories|calories|calories | calories|calories|calories|per cent|
  | 2569   |  149   |  103   |  2317  |  +229   |   2088  |  2113  |  +25   |  +1.2  |
  +--------+--------+--------+--------+---------+---------+--------+--------+--------+

  [34] In the experiment, the body lost 29.16 grams of proteid = 165
  calories, but gained fat and glycogen = 393 calories. Hence, there
  were 229 calories gained from body material.

As is seen from the above figures, the total fuel value of the food
was 2569 calories. The fuel value of the unoxidized portion of the
food contained in the excreta was 149 + 103 calories, leaving as the
available energy of the food 2317 calories. This must be further
corrected by the fact, mentioned in the footnote, that a portion of
the food was stored as fat and glycogen, while the body lost at the
same time a small amount of proteid. Making the necessary correction
for these causes, we find 2088 calories as the energy from material
oxidized in the body. The actual output of energy as measured by the
calorimeter was 2113 calories, only 1.2 per cent greater than the
estimated amount.

By aid of the respiration calorimeter, many important questions in
nutrition can be more or less accurately answered, especially such as
relate to the total energy requirements of the body. The law of the
conservation of energy obtains in the human body as elsewhere, and if
we can measure with accuracy the total heat output, with any energy
liberated in the form of work, and at the same time determine the total
excretion of carbon dioxide, water, nitrogen, etc., together with the
intake of oxygen, it becomes not only possible to ascertain the energy
requirements of the body under different conditions, but, aided by
data obtainable through study of the exchange of matter, we can draw
important conclusions concerning the sources of the energy, _i. e._,
whether from proteid, fat, or carbohydrate.

It is obvious that a man asleep, or lying quietly at rest, in the
calorimeter, especially when he has been without food for some hours,
furnishes suitable conditions for ascertaining the minimal energy
requirements of the body. Under such conditions, bodily activity and
heat output are at their lowest, and we are thus afforded the means of
determining what is frequently called the basal energy exchange of the
body. The following table taken from Magnus-Levy, and embodying results
from many sources, shows the heat production during sleep, calculated
for 24 hours, of various individuals of different body-weight and of
different body surface.

I venture to present these individual results, rather than make a
general statement simply, because it is important to recognize the
fact that the basal energy exchange differs according to body-weight,
extent of body surface, and the condition of the body. In the table,
the results are arranged in the order of body-weight, and it is
plain to see that the absolute energy exchange is greater with heavy
persons than with light, yet the energy exchange does not increase
in proportion to increase of body-weight. With a man of 83 kilos
body-weight, the basal exchange is only 30–40 per cent higher than
in a man of 43 kilos body-weight. In other words, the man of small
body-weight has, per kilo, a much higher basal exchange than the
heavier man. The energy exchange is more closely proportional to the
extent of body surface than to weight.

  +-------------+----------------+--------------+
  | Body-weight | Total Calories | Calories per |
  |   of the    | for 24 Hours.  |   Kilo of    |
  | Individual. |                | Body-weight. |
  +-------------+----------------+--------------+
  |    kilos    |                |              |
  |    43.2     |      1333      |     30.9     |
  |    48.0     |      1214      |     25.3     |
  |    50.0     |      1315      |     25.9     |
  |    53.0     |      1527      |     28.8     |
  |    55.0     |      1590      |     28.9     |
  |    56.5     |      1519      |     26.8     |
  |    57.2     |      1560      |     27.3     |
  |    58.0     |      1510      |     26.0     |
  |    62.5     |      1431      |     22.9     |
  |    63.0     |      1418      |     22.5     |
  |    63.0     |      1492      |     23.7     |
  |    64.0     |      1656?     |     25.8     |
  |    64.9     |      1475      |     22.7     |
  |    65.0     |      1498      |     23.0     |
  |    65.0     |      1445      |     22.2     |
  |    67.5     |      1608      |     23.8     |
  |    67.5     |      1621      |     24.0     |
  |    70.0     |      1661      |     23.7     |
  |    70.0     |      1620      |     23.1     |
  |    71.2     |      1787      |     25.1     |
  |    72.6     |      1550      |     21.3     |
  |    72.7     |      1657      |     22.8     |
  |    73.0     |      1584      |     21.7     |
  |    73.0     |      1630      |     22.4     |
  |    75.6     |      1670      |     22.1     |
  |    82.0     |      1556      |     19.0     |
  |    82.7     |      2030?     |     24.5     |
  |    83.5     |      1670      |     20.0     |
  |    88.3     |      2019?     |     22.9     |
  |    90.4     |      1773      |     19.6     |
  +-------------+----------------+--------------+

As Richet has expressed it, the basal energy exchange is inversely
proportional to the body-weight and directly proportional to the body
surface. This is in harmony with the view advanced by v. Hösslin, “that
all the important physiological activities of the body, including
of course its internal work and the consequent heat production, are
substantially proportional to the two-thirds power of its volume, and
that since the external surface bears the same ratio to the volume,
a proportionality necessarily exists between heat production and
surface.”[35]

  [35] See Armsby: Principles of Animal Nutrition, p. 368.

There are, however, many circumstances that modify, or influence,
energy exchange. Thus, the taking of food, with all the attendant
processes of digestion, assimilation, etc., involves an expenditure of
energy not inconsiderable. This has been experimentally demonstrated
on man by several investigators. With fatty food, Magnus-Levy found
that his subject lying upon a couch, as completely at rest as possible,
produced in the 24 hours 1547 calories when 94 grams of fat were eaten,
and 1582 calories when 195 grams of fat were consumed. The increase of
heat production over the basal energy exchange was 10 and 58 calories
respectively. With a mixed diet, where proteid food is a conspicuous
element, the increase in heat production is much more marked. Thus,
in some experiments reported from Sweden the following data were
obtained:[36]

  [36] Taken from Armsby: Principles of Animal Nutrition, p. 383.

  +---------+---------------------+------------------+
  |   Day.  | Energy of the Food. | Heat Production. |
  +---------+---------------------+------------------+
  |         |      calories       |     calories     |
  | First   |        4141         |       ....       |
  | Second  |        4277         |       2705       |
  | Third   |          0          |       2220       |
  | Fourth  |          0          |       2102       |
  | Fifth   |          0          |       2024       |
  | Sixth   |          0          |       1992       |
  | Seventh |          0          |       1970       |
  | Eighth  |        4355         |       2436       |
  | Ninth   |        3946         |       2410       |
  +---------+---------------------+------------------+

We see here an increase of 495 calories per day in heat production,
due to metabolism of the food ingested. In other words, with a basal
energy exchange of 2022 calories, the average of the five fasting days,
energy equivalent to 495 calories was expended in taking care of the
ingested food. It should be added, however, that the daily ration here
was somewhat excessive, 4193 calories being considerably in excess
of the requirements of the body. Finally, it should be stated that of
the several classes of foods, proteids cause the greatest increase in
metabolism and fats the least.

In studying heat production in the body under varying conditions,
one of the important aids in drawing conclusions as to the character
of the body material burned up is the respiratory quotient. This is
the relationship, or ratio, of the oxygen absorbed to the oxygen of
the carbon dioxide eliminated, viz., CO_{2}/O_{2}. Carbohydrates
(C_{6}H_{12}O_{6}, C_{12}H_{22}O_{11}) all contain hydrogen and oxygen
in the proportion to form water, H_{2}O, and in their oxidation they
need of oxygen only such quantity as will suffice to oxidize the carbon
(C) of the sugar to carbon dioxide (CO_{2}). Carbohydrates, starch
and sugars, have a respiratory quotient of 1.00. Fat, on the other
hand, has a respiratory quotient of 0.7, and proteid, 0.8. Hence, it
is easy to see that the respiratory quotient will approach nearer to
unity as the quantity of carbohydrate burned in the body is increased.
Similarly, the respiratory quotient will grow smaller the larger the
amount of fat burned up. Practically, we never find a respiratory
quotient of 1.0 or 0.7, because there is always some oxidation of
proteid in the body. If, by way of illustration, we assume that the
energy of the body under given conditions comes from proteid to the
extent of 15 per cent, while the remaining 85 per cent is derived from
the oxidation of carbohydrate, the respiratory quotient will be 0.971.
If, however, the 85 per cent of energy comes from fat, the respiratory
quotient will change to 0.722. In the resting body, as in the early
morning hours, after a night’s sleep and before food is taken, the
respiratory quotient is generally in the neighborhood of 0.8. When,
however, as sometimes happens, the quotient at this time of day
approaches 0.9, it must be assumed that sugar is being burned in the
body, presumably from carbohydrate still circulating from the previous
day’s intake.

As can easily be seen, any special drain upon either fat or
carbohydrate in the processes of the body will be indicated at once
by a corresponding change in the respiratory quotient. This we shall
have occasion to notice later on, in considering the source of the
energy of muscle contraction. Further, the respiratory quotient will
naturally change in harmony with transformations in the body which
involve alterations in oxygen-content, without the oxygen of the
inspired air being necessarily involved; as in the formation of a
substance poor in oxygen, such as fat, from a substance rich in oxygen,
such as carbohydrate. Moreover, the reversal of this reaction, as
in the formation of sugar from proteid with a taking on of oxygen,
will produce a corresponding effect upon the respiratory quotient.
As Magnus-Levy has clearly pointed out, in the formation of fat from
carbohydrate, carbon dioxide is produced in large amount without the
oxygen of the inspired air being involved at all. In such a change,
100 grams of starch will yield about 42 grams of fat, while at the
same time 45 grams of carbon dioxide will be produced. This might
cause the respiratory quotient to rise as high as 1.38. Again, in
the formation of sugar from proteid, the respiratory quotient may
sink very decidedly, the changes involved being accompanied by a
taking on of oxygen from the air, without, however, any corresponding
increase of carbon dioxide in the expired air. Assuming a manufacture
of 60 grams of dextrose from 100 grams of proteid, _i. e._, from the
non-nitrogenous moiety of the proteid molecule, a respiratory quotient
of 0.613 would be possible. Thus, a diabetic patient, living upon a
carbohydrate-free diet, consuming only proteid and fat, may show a
respiratory quotient of 0.613–0.707. These illustrations will suffice
to show how chemical alterations taking place in the body, involving
transformations of proteid, fat, and carbohydrate of the tissues
and of the food, may produce alterations in the respiratory quotient
without necessarily being directly connected with intake of oxygen or
output of carbon dioxide through the lungs; and how, conversely, the
respiratory quotient becomes a factor of great significance in throwing
light upon the character of the nutritive changes taking place in the
body.

Among the various conditions that influence the energy exchange of
the body, muscle work stands out as the most conspicuous. It needs no
argument to convince one that all forms of muscular activity involve
liberation of the energy stored up in the tissues of the body; and
consequently that all work accomplished means chemical decomposition,
in which complex molecules are broken down into simple ones with
liberation of the contained energy, the energy exchange being
proportional to the amount of work done. As we have seen, the basal
energy exchange of the normal individual is ascertained by studying his
heat production while at rest--best during sleep--without food, when
involuntary muscle activity and heat production are at their lowest.
The maximum energy exchange is seen in the individual at hard muscular
work. Heat production is then at its highest, as can be ascertained
by direct calorimetric observation; or, by studying the output of
excretory products, which measure the extent of the oxidative processes
from which comes the energy for the accomplishment of the work. As
an illustration of the general effect of muscular work on the energy
exchange of the body, we may cite a summary of some results reported by
Atwater and Benedict,[37] the figures given being average results, from
several individuals, and covering different periods of time. Though
not strictly comparable in all details, they are sufficiently so to
illustrate the main principle.

  [37] Atwater and Benedict: Experiments on the Metabolism of Matter
  and Energy in the Human Body 1900–1902. Bulletin No. 136, Office of
  Experiment Stations, U. S. Department of Agriculture, 1903, p. 141.


HEAT GIVEN OFF BY BODY, INCLUDING FOR WORK EXPERIMENTS THE HEAT
EQUIVALENT OF THE EXTERNAL MUSCULAR WORK.

  +----------------+--------+---------------------------------------+--------+
  |                |  Total |            Rates per Hour.            |        |
  |                | Amount +-------------------+-------------------+ Average|
  |    Kind of     | of Heat|    Day Periods.   |   Night Periods.  |   for  |
  |  Experiment.   | in 24  |                   |                   |   24   |
  |                | Hours. +---------+---------+---------+---------+ Hours. |
  |                |        |7 A.M. to|1 P.M. to|7 P.M. to|1 A.M. to|        |
  |                |        | 1 P.M.  | 7 P.M.  | 1 A.M.  | 7 A.M.  |        |
  +----------------+--------+---------+---------+---------+---------+--------+
  |                |calories| calories| calories| calories| calories|calories|
  |Rest experiments|  2262  |  106.3  |  104.4  |   98.3  |   67.9  |  94.3  |
  +----------------+--------+---------+---------+---------+---------+--------+
  |Work experiments|}       |         |         |         |         |        |
  | Heat eliminated|} 4225  |  231.7  |  235.6  |  118.1  |   78.4  | 166.6  |
  |                |        |         |         |         |         |        |
  |Heat equivalent |}       |         |         |         |         |        |
  | of external    |}  451  |   58.5  |   56.8  |   ...   |    ...  |   ...  |
  | muscular work  |}       |         |         |         |         |        |
  +----------------+--------+---------+---------+---------+---------+--------+
  |     Total      |  4676  |  290.2  |  292.4  |  118.1  |   78.4  | 194.8  |
  +----------------+--------+---------+---------+---------+---------+--------+

The work done in these experiments was on a stationary bicycle in the
calorimeter, and the heat equivalent was calculated from measurements
made by an ergometer attached to the bicycle. We are not concerned here
with details, but simply with the general question of the influence of
muscular work upon the energy exchange of the body. We note that the
work of the day periods, 7 A. M. to 7 P. M., resulted, in the several
cases brought together under the average figures, in an increased heat
production amounting to more than 100 per cent. Further, we observe
that in the body, as in all machines, only a fraction of the energy
liberated by the accelerated chemical decomposition, or oxidation,
was manifested as mechanical work, the larger part by far being heat
eliminated and lost. Thus, Zuntz has found that, in man, about 35 per
cent of the extra energy of the food used in connection with external
muscular work is available for that work. This, however, shows a
noticeably higher degree of efficiency than is generally obtainable
by the best steam or oil engines. Lastly, attention may be called to
the fact that after the work of the day was finished at 7 P. M., the
next period of six hours still showed an accelerated metabolism, as
contrasted with what took place during absolute rest.

As bearing upon the exchange of matter in the body in connection with
muscular work, and as showing the relationship which exists here
between energy exchange and exchange of matter, we may quote a few
data relating to the elimination of carbon dioxide; remembering that
this substance represents particularly the final oxidation product in
the body of carbonaceous materials, such as fat and carbohydrate. The
following data, taken from Atwater and Benedict,[38] being results of
experiments upon the subject “J. C. W.,” are of value as showing the
variations in output of carbon dioxide that may be expected under the
conditions described:

  [38] Loc. cit., pp. 130 and 131.

  +------------------+---------+---------+-----------+---------+----------+
  |                  |         |         |           |         |  Extra   |
  |                  | Rest Ex-| Rest Ex-|  Work Ex- | Work Ex-|  Severe  |
  |     Period.      |periments|periments| periments |periments|   Work   |
  |                  | without |  with   |with Carbo-|  with   |Experiment|
  |                  |  Food.  |  Food.  |  hydrate  |Fat Diet.|   with   |
  |                  |         |         |   Diet.   |         |Fat Diet. |
  +------------------+---------+---------+-----------+---------+----------+
  |                  |  grams  |  grams  |   grams   |  grams  |   grams  |
  | 7 A.M. to 1 P.M. |  189.6  |  230.4  |   694.0   |  642.3  |   907.0  |
  | 1 P.M. to 7 P.M. |  172.6  |  232.0  |   705.6   |  634.8  |   821.3  |
  | 7 P.M. to 1 A.M. |  167.2  |  196.6  |   260.1   |  230.3  |   842.7  |
  | 1 A.M. to 7 A.M. |  146.7  |  153.1  |   161.1   |  157.6  |   502.6  |
  +------------------+---------+---------+-----------+---------+----------+
  |Total for 24 hours|  676.1  |  812.1  |  1820.8   | 1665.0  |  3073.6  |
  +------------------+---------+---------+-----------+---------+----------+

In considering these figures bearing on the output of carbon dioxide
under the conditions specified, we note at once a correspondence with
the total energy exchange, as indicated in the preceding table. As
previously stated, we are at present dealing simply with generalities,
and the important point to be observed here is that muscular work--7 A.
M. to 7 P. M.--in the work experiments, increases enormously the output
of carbon dioxide. We see clearly emphasized a connection between the
total energy exchange of the body, as expressed in calories or heat
units, and the oxidation of carbonaceous material, of which carbon
dioxide is the natural oxidation product. We note that on the cessation
of work--7 P. M. to 7 A. M.--the output of carbon dioxide tends to drop
back to the level characteristic of the corresponding period in rest,
with or without food. In the experiment with “extra severe muscular
work,” the results are different simply because here the subject
worked sixteen hours, necessitating a portion of the work being done
at night-time. Finally, it should be mentioned that the differences in
output of carbon dioxide in these experiments are somewhat greater than
in many experiments of this type, although all show the same general
characteristics. This may be explained, as stated by the authors from
whom the data are taken, “by the fact that J. C. W. was a larger and
heavier man than any of the others; that the differences in diet were
wider, and that the amounts of external muscular work were larger in
these experiments than in those with the other subjects.”

If we pass from experiments of this type, conducted in a calorimeter,
to those cases where competitive trials of endurance are held by
trained athletes, _i. e._, where external muscular activity is pushed
to the extreme limit, we then see even more strikingly displayed the
effect of work in increasing the energy exchange of the body. One of
the best illustrations of this type of experiment is to be found in the
observations made in connection with the six-day bicycle race held in
New York City, at the Madison Square Garden, in December, 1898.[39]
The observations in question were made upon three of the athletes,
one of whom withdrew early in the fourth day, while the others
continued until the close of the race--142 consecutive hours--winning
the first and fourth places, respectively. The following table gives
the computation of energy of the material metabolized, exclusive of
body-fat lost:

  [39] See W. O. Atwater and H. C. Sherman: The effect of severe
  and prolonged muscular work on food consumption, digestion, and
  metabolism. Bulletin No. 98, Office of Experiment Stations, U. S.
  Department of Agriculture.

  +------------+-------------+--------------+-------------+
  |  Subject.  | Duration of | Total Energy | Average per |
  |            | Experiment. | Metabolized. |    Day.     |
  +------------+-------------+--------------+-------------+
  |            |    days     |   calories   |  calories   |
  | Miller     |     6       |     28917    |    4820     |
  | Albert     |     6       |     36441    |    6074     |
  | Pilkington |     3       |     13301    |    4464     |
  +------------+-------------+--------------+-------------+

Miller, the winner of the race, who averaged a daily energy exchange
of 4820 calories, rode 2007 miles during the week, and finished the
race without physical or mental weakness resulting from the fatigue and
strain. During the first five days, he rode about 21 hours a day and
slept only 1 hour. Albert, who weighed a few pounds less than Miller,
covered 1822 miles in 109 hours, with an average daily exchange of
6074 calories. We may add a table (on the following page) showing the
balance of income and outgo of nitrogen in these three subjects, as
being of general interest in this connection. The figures given are
averages per day.

  +----------+-----+---------------------------------+-------------------+
  |          |Dura-|         Income in Food.         |     Nitrogen.     |
  | Subject. |tion +-----+-----+------+--------+-----+------+------+-----+
  |          | of  |     |     |Carbo-|        |     |      |  In  |     |
  |          |Exp. | Pro | Fat.| hy-  |  Fuel  | In  | In   |Excre-|Loss.|
  |          |     |teid.|     |drate.| Value. |Food.|Urine.|ment. |     |
  +----------+-----+-----+-----+------+--------+-----+------+------+-----+
  |          |days |grams|grams|grams |calories|grams|grams |grams |grams|
  |Miller    |  6  | 169 | 181 | 585  |  4770  | 29.4| 36.2 | 1.8  | 8.6 |
  |Albert    |  6  | 179 | 198 | 559  |  6095  | 29.1| 33.7 | 2.5  | 7.1 |
  |Pilkington|  3  | 211 | 178 | 509  |  4610  | 36.0| 38.9 | 2.2  | 5.1 |
  +----------+-----+-----+-----+------+--------+-----+------+------+-----+

The special significance of these data, as bearing upon the topic
under discussion, is that apparently all three of the subjects were
drawing in a measure upon their body material. As stated by Atwater
and Sherman, Pilkington lost per day 5.1 grams of nitrogen; that is
to say, the total nitrogen excreted exceeded the total nitrogen of
the food by 5.1 grams per day, corresponding to 33 grams of proteid,
which must have been drawn from the supply in the body. If we assume
that lean flesh contains 25 per cent of proteid, this would mean about
4-3/4 ounces per day. The other two subjects, Miller and Albert, lost
from the body per day 8.6 grams and 7.1 grams respectively of nitrogen,
which would imply a loss of about 54 grams and 44 grams of body proteid
respectively, or 8 ounces and 6-1/4 ounces of lean flesh per day. It is
evident, therefore, that none of the three subjects consumed sufficient
food to avoid loss of body proteid, under the existing conditions of
muscular activity. Indeed, it may be noted in Miller’s case that the
average fuel value of the food per day was 4770 calories, while the
average expenditure of energy per day was 4820 calories. We should
naturally expect, however, that any small deficiency in fuel value
would be made good by a call upon body fat. “Why the body should use
its own substance under such circumstances is a question which at
present cannot be satisfactorily answered. The fact that such was the
case, each of the contestants who finished the race consuming during
the period body protein equivalent to 2 or 3 pounds of lean flesh, and
that no injury resulted therefrom, would seem to indicate that these
men had stores of protein which could be metabolized to aid in meeting
the demands put upon the body by the severe exertion, without robbing
any of the working parts, and at the same time relieving the system
of a part of the labor of digestion. Possibly, the ability to carry
such a store of available protein is one of the factors which make for
physical endurance.”[40] This possibility we shall have occasion to
discuss in another connection. At present, the facts presented are to
be accepted as accentuating the general law that the energy exchange
of the body, everything else being equal, is increased proportionally
to increase in the extent of external muscular activity. It may be
noted that Albert, who did considerably less work than Miller, showed
a much larger exchange of energy than the latter athlete. This,
however, is to be connected with the fact that his fuel intake was 1300
calories larger per day than Miller’s; in other words, the conditions
were not equal. This fact also calls to mind the observations of
Schnyder,[41] who, studying the relationship between muscular activity
and the production of carbon dioxide, maintained that the quantity of
this excretory product formed depends less upon the amount of work
accomplished than upon the intensity of the exertion; efficiency in
muscular work varying greatly with the condition of the subject, and
his familiarity with the particular task involved.

  [40] Atwater and Sherman. Loc. cit., p. 51.

  [41] L. Schnyder: Muskelkraft und Gaswechsel. Zeitschrift für
  Biologie, Band 33, p. 289.

From what has been said, it is obvious that oxygen consumption, as
well as output of carbon dioxide, must vary enormously with variations
in the muscular activity of the body. The one important factor
influencing the quantities of oxygen and carbon dioxide exchanged in
the lungs, _i. e._, the extent of the respiratory interchange, is
muscular activity; and since, as we have seen, carbonaceous material
is the substance mainly oxidized in muscle work, it follows, as
carbon dioxide is excreted principally through the lungs, that the
respiratory interchange becomes in good measure an indicator of the
extent of chemical decomposition incidental to external work. If we
recall that man, on an average, at each inspiration draws in about 500
cubic centimeters of air (30 cubic inches), and that for the 24 hours
he averages 15 breaths a minute, it is easy to see that in one minute
the average man will inspire 7.5 litres of air, or 450 litres an hour,
with a total of 10,800 litres for the entire day, which is equivalent
to about 380 cubic feet. This would be a volume of air just filling a
room 7-1/3 feet in length, width, and height. Inspired air loses to the
body 4.78 volumes per cent of oxygen, while expired air contains an
excess of 4.34 volumes per cent of carbon dioxide. In muscular work,
respiration is increased in frequency and in depth. The volume of air
exchanged in the lungs during severe labor may be increased sevenfold,
while oxygen consumption and carbon dioxide excretion are frequently
increased 7–10 times. The following figures, being values for one
minute, show the effect on oxygen consumption of walking on a level and
climbing, the subject being a man of 55.5 kilos body-weight:[42]

  [42] G. Katzenstein: Ueber die Einwirkung der Muskelthätigkeit auf
  den Stoffverbrauch des Menschen. Pflüger’s Archiv für die gesammte
  Physiologie, Band 49, p. 330. Also Magnus-Levy: v. Noorden’s Handbuch
  der Pathologie der Stoffwechsel, p. 233.

  +------------------+----------------------------------------+-----------+
  |                  |Oxygen Consumption in Cubic Centimeters.|           |
  |                  +---------+------------------------------+           |
  |                  |         |     After Deducting Value    |           |
  |  Form of Work.   |         |           for Rest.          |Respiratory|
  |                  |  Total. +-----------+------------------+ Quotient. |
  |                  |         |           |     For Each     |           |
  |                  |         |  Total.   |  Kilo of Moving  |           |
  |                  |         |           |     Weight.      |           |
  +------------------+---------+-----------+------------------+-----------+
  |Standing at rest  |  263.75 |   ....    |       ....       |   0.801   |
  |Walking on a level|  763.00 |  499.25   |       8.990      |   0.805   |
  |Climbing          | 1253.20 |  989.45   |      17.819      |   0.801   |
  +------------------+---------+-----------+------------------+-----------+

Remembering that these figures represent the oxygen consumption for
only one minute of time, it is easy to see the striking effect of
moderate and vigorous exercise on respiratory interchange. Simply
walking along a level suffices to increase the consumption of oxygen
threefold over what occurs when the body stands at rest. When the more
vigorous exercise attendant on lifting the body up a steep incline
is attempted, most striking is the great increase in the amount of
oxygen consumed. We thus see another forcible illustration of the
influence of muscular activity upon the exchange of matter in the
body, and a further confirmation of the statement, so many times made,
that oxidation--especially the oxidation of fats and carbohydrates by
which large quantities of heat are set free, easily convertible into
mechanical energy--is a primary factor in the metabolic processes, by
which the machinery of the living man is able to work so efficiently.

Finally, we cannot avoid the conclusion that the outgoings of the body,
in the form of matter and energy, are subject to great variation,
incidental to the degree of activity of the day or hour. The ordinary
vicissitudes of life, bringing days of physical inaction, followed
perhaps by periods of unusual activity; changes in climatic conditions,
with their influence upon heat production in the body; alterations in
the character and amount of the daily dietary, etc.,--all seemingly
combine as natural obstacles to the maintenance of a true nutritive
balance. Outgo, however, must be met by adequate amounts of proper
intake if there is to be an approach toward a balance of nutrition.
In some way the normal, healthy man does maintain, approximately at
least, a condition of balance; not necessarily for every hour or for
every day, but the intake and outgo if measured for a definite period,
not too short, say for a week or two, will be found to approach each
other very closely. Body equilibrium and approximate nitrogen balance
may be reasonably looked for, as well as a balance of total energy, in
the case of a healthy man leading a life which conforms to ordinary
physiological requirements. The man who, on the other hand, consciously
or unconsciously, continues an intake way beyond the outgo, whose daily
income of nitrogen and total fuel value far exceeds the requirements
of his body, obviously lives with an accumulating plus balance, which
ordinarily shows itself in increasing body-weight and with a storing
away of fat.

Equally conspicuous is the effect of an inadequate income of proper
nutriment; a food supply which persistently fails to furnish the
available nitrogen and total energy value called for by the body under
the conditions prevailing, will inevitably result in a minus balance,
which, if continued too long, must of necessity tax the body’s store
to the danger limit. At the same time, the well-nourished individual,
without being unduly burdened by a bulky store of energy-containing
material, is always supplied with a sufficient surplus to meet all
rational demands, when from any cause the intake fails, for brief
periods of time, to be commensurate with the needs of the body. It is
reasonable to believe, however, that in the maintenance of good health,
and the preservation of a high degree of efficiency, the body should be
kept in a condition approaching a true nutritive balance.




CHAPTER IV

SOURCE OF THE ENERGY OF MUSCLE WORK, WITH SOME THEORIES OF PROTEID
METABOLISM

  TOPICS: Relation of muscle work to energy exchange. Views of Liebig.
  Experimental evidence. Relation of nitrogen excretion to muscle
  work. Significance of the respiratory quotient in determining
  nature of the material oxidized. Fats and carbohydrates as source
  of energy by muscles. Utilization of proteid as a source of energy.
  Formation of carbohydrate from proteid. Significance of proteid
  metabolism. Theories of Carl Voit. Morphotic proteid. Circulating
  proteid. General conception of proteid metabolism on the basis of
  Voit’s theories. Pflüger’s views of proteid metabolism. Rapidity of
  elimination of food nitrogen. Methods by which nitrogen is split off
  from proteid. Theories of Folin. Significance of creatinin and of the
  percentage distribution of excreted nitrogen. Endogenous or tissue
  metabolism. Exogenous or intermediate metabolism. Needs of the body
  for proteid food possibly satisfied by quantity sufficient to meet
  the demands of tissue or endogenous metabolism. Bearings of Folin’s
  views on current theories and general facts of proteid metabolism.
  Large proteid reserve and voluminous exogenous metabolism probably
  not needed. Importance of feeding experiments in determining the true
  value of different views.


As we have already seen, every form of muscular activity begets an
increase in the energy exchange of the body. Between the two extremes
of absolute rest and excessive muscular exertion, we find differences
of 2000 calories or more per day as representing the degree of chemical
decomposition corresponding to the particular state of muscular
activity. The work of the involuntary muscles, such as have to do with
peristalsis, respiration, rhythmical beat of the heart, etc., is a
relatively constant factor, though naturally subject to some variation,
as has been pointed out in other connections. External muscular
activity, however, is the one factor above all others that modifies the
rate of energy exchange. A little longer walk, a heavier load to carry,
a steeper hill to climb, any increase great or small in the activity
of the muscles of the body, means a corresponding increase in chemical
decomposition, with increased output of the ordinary products of tissue
oxidation. The material so consumed, or oxidized, must be made good
to hold the body in equilibrium; the supplies drawn upon are to be
replaced, if the tissues of the body are to be kept in a proper state
of efficiency.

What is the nature of the material used up in connection with muscle
work? As can readily be seen, this is an important question, for on its
answer depends, in some measure at least, the character of the proper
intake, or food, to be supplied in order to make good the loss. If the
energy of mechanical work, the energy of muscle contraction, comes from
the breaking down of proteid matter alone, then obviously excessive
muscular work would need to be accompanied, or followed, by a generous
supply of proteid food. If, on the other hand, external work means
liberation of energy solely from non-nitrogenous materials, then it
is equally clear that fats and carbohydrates are the proper foods to
offset the drain incidental to vigorous muscular action.

The views of Liebig, briefly referred to in a previous chapter, held
sway over physiologists for many years. His dictum that proteid foods
were true plastic foods, entering into the structure of the tissues
of the body, and that they alone were the real sources of muscular
energy, met for a time with no opposition. It was not until the advent
of a more critical spirit, accompanied by a fuller appreciation of
the necessity of experimental evidence, that physiologists began to
test with scientific accuracy the validity of the current views. It
is worthy of note that long prior to this time, even before oxygen
was discovered, the far-sighted and resourceful John Mayow, in his
work with the various “spirits” of the body and their relation to
respiration, etc., evolved the view that muscular power has its origin
in the combustion of fat brought to the muscles by the blood and burned
there by aid of a gas or “spirit” taken from the air by the lungs, and
likewise carried to the muscles by the circulating blood. Considering
the time when Mayow lived and the dearth of true scientific knowledge
as we measure it to-day, his hypothesis was a wonderful forestalling of
present views.

It is quite obvious that the views of Liebig, if true, admit of easy
proof; since, if the energy of muscular power comes from the breaking
down of proteid, there should be a certain parallelism between the
output of nitrogen from the body and the amount of muscular work
accomplished, everything else being equal. As stated in a previous
chapter, such study of this question as was made soon disclosed the
fact that the one element above all others that seemed to influence the
output of nitrogen was the intake of proteid food. Thus, the English
investigators, Lawes and Gilbert, found by experimenting with animals
that when the latter were kept under uniform conditions of muscular
work, the amount of nitrogen excreted ran parallel with the intake
of nitrogen. Further, in the early experiments of Voit, the results
obtained clearly showed that variations in the amount of work performed
were practically without influence on the excretion of nitrogenous
waste products.

The experiment, however, that came as a death blow to the theories of
Liebig was that of Fick and Wislicenus,[43] who in 1865 made an ascent
of the Faulhorn, 6500 feet high, using a diet wholly non-nitrogenous.
From the nitrogen excreted they were able, of course, to calculate the
amount of proteid oxidized in the body during the period of work, and
found that the proteid consumed could not have furnished, at the most,
more than one-half the energy required to lift the weights of their
bodies to the top of the high peak. Further, they observed that neither
during the work period, nor immediately after, was there any noticeable
increase in the excretion of nitrogen. Obviously, as they state, the
oxidation of proteid matter in the body cannot be the exclusive source
of the energy of muscular contraction, since the measurable amount
of external work performed in the ascent of the mountain was far
greater than the equivalent of the energy capable of being furnished
by the proteid actually burned. To which may be added the fact that
considerable energy, not measurable in their experiment, must have
been employed in the work of the involuntary muscles of the body; thus
increasing by so much the difference between the muscular work actually
accomplished and the available energy from proteid consumed. It is true
that minor criticisms regarding certain details of the experiment can
be offered to-day, such as the fact that the men were, in a measure,
in a state of “nitrogen starvation,” etc., but these criticisms do
not in any degree militate against the main thesis that the energy of
muscular contraction does not come exclusively from the consumption or
breaking down of proteid, either of food or tissue. Vigorous and even
severe muscular work does not necessarily increase the decomposition
of proteid material. Dogs made to run in large treadmills, with the
same diet as on resting days, were found to excrete practically no
more nitrogen than during the days of rest. Occasionally, however,
in some one experiment the output of nitrogen would show an increase
over the output on resting days. Further, experiments made with horses
led to essentially the same result, except that greater increase in
the excretion of nitrogen was observed than with dogs. This increase
in nitrogen output, however, as a concomitant of increased muscular
activity, could be prevented by adding to the amount of carbohydrate
food.

  [43] See Gesammelte Schriften von Adolf Fick. Ueber die Entstehung
  der Muskelkraft. Band 2, p. 85. Würzburg, 1903.

While experiments of this nature, on man and animals, all tended to
show little or no increase in the excretion of nitrogen, as a result
of muscle work; and likewise no increase in the output of sulphur and
phosphorus, thus strengthening the view that muscular energy is not the
result of proteid disintegration, there was observed marked increase
in the consumption of oxygen, and in the excretion of carbon dioxide.
Non-nitrogenous matter was thus at once suggested as the material
with which muscle chiefly does its work. There is to-day no question
of the general truth of this statement, yet there are other aspects
of the problem to be considered before we can lay it aside. Pflüger,
working with dogs, and Argutinsky, experimenting on himself by arduous
mountain climbing, reached conclusions seemingly quite opposed to what
has just been said. Their results, however, admit of quite a different
interpretation from what they were disposed to attach to them. Thus,
Pflüger[44] would go back to the old view that all muscle work is at
the expense of proteid material, because lean dogs fed mainly, or
entirely, on meat and made to do an excessive amount of work were found
by him to excrete nitrogen somewhat in proportion to the amount of work
done. Argutinsky,[45] likewise, in his mountain climbing carried to the
point of fatigue, and with a high proteid intake likewise, saw in the
increased output of nitrogen a suggestion of the same idea. In reality,
however, their results merely prove that, under some circumstances,
proteid may serve as the chief source of muscular energy; as when the
body is poor in fat and carbohydrate, or when the intake consists
solely of proteid matter. In other words, muscular work may result in
an increased excretion of nitrogen when the work is very severe, and
there is not a corresponding increase in the fats or carbohydrates
(fuel ingredients) of the food. In the words of Bunge,[46] “we might
assume _à priori_, on teleological grounds, that in the performance
of its most important functions the organism is to a certain extent
independent of the quality of its food. As long as non-nitrogenous
food is supplied in adequate quantity or is stored up in the tissues,
muscular work is chiefly maintained from this store. When it is gone
the proteids are attacked.”

  [44] Pflüger: Die Quelle der Muskelkraft. Pflüger’s Archiv für die
  gesammte Physiologie, Band 50, p. 98.

  [45] Argutinsky: Muskelarbeit und Stickstoffumsatz. Ibid., Band 46,
  p. 552.

  [46] Bunge: Textbook of Physiological and Pathological Chemistry.
  Second English Edition, 1902, p. 352.

There is no question that the energy of muscular contraction can come
from all three classes of organic foodstuffs. Voluntary muscular
movement is under the control of the nervous system, and when the
stimulus is applied the muscle is bound to contract, provided of
course there is sufficient energy-containing material present to
furnish the means. Muscle tissue, like other tissues and organs, has
a certain power of adaptability, by which it is able to do its work,
even though it is not adequately supplied with its preferred nutrient.
While proteid is plainly not the material from which the energy of
muscular contraction is ordinarily derived, it is equally evident
that in emergency, as when the usual store of carbohydrate and fat is
wanting, proteid can be drawn upon, and in such cases vigorous work
may be attended with increased nitrogen output. In harmony with this
statement, we find on record in recent years many experiments, both
with man and animals, where severe muscular labor is accompanied by an
excretion of nitrogen beyond what occurs on days of rest; but by simply
adding to the intake of non-nitrogenous food this increased outgo of
nitrogen is at once checked. With moderate work, the nitrogen outgo
is rarely influenced; it is only when the work becomes excessive, or
the store of non-nitrogenous reserve is small and the intake of the
latter food is limited, that proteid matter is drawn upon to supply the
required energy.

Recalling what has been said regarding the significance of the
respiratory quotient, it is obvious that we have here a means of
acquiring information as to the character of the material that is
burned up in the body during muscular work. Increased metabolism
of carbohydrate will necessarily result in raising the respiratory
quotient, and if the latter food material alone is involved the
respiratory quotient must naturally approach 1.0. Zuntz, however, has
clearly shown that vigorous muscular activity does not materially
change the respiratory quotient; except in cases of very severe work,
where the oxygen-supply of the muscles is interfered with. Indeed, the
muscles may be made to do work sufficient to increase the consumption
of oxygen threefold or more, without any change in the respiratory
quotient being observed. And as there is frequently no change whatever
in the output of nitrogen under these conditions, it follows that the
energy of the muscle work must have come from the decomposition of
non-nitrogenous material. If carbohydrates alone were involved, the
respiratory quotient would obviously undergo change. Since, however,
this remains practically stationary, we are led to the conclusion that
fat must be involved in large degree, in addition to carbohydrate.

In this connection, it is a significant fact that with _fasting_
animals, where the store of carbohydrate material is more or less used
up, severe muscle work may be accomplished without any appreciable
increase in nitrogen output, thus showing that proteid material is not
involved and clearly pointing to fat as the source of the muscular
energy. Thus, in an experiment referred to by Leathes, a dog on the
sixth and seventh day of starvation was made to do work in a treadmill
equivalent to climbing to a height of 1400 meters, yet the output
of nitrogen was increased from six to only six and a half grams.
Obviously, not much of the energy of this muscle work could have come
from the breaking down of proteid, but it must have been derived mainly
from the oxidation of fat. There is abundant evidence that fat can be
used as a source of energy by muscles, as well as carbohydrates and
proteids, and there is every reason for believing that the yield of
work for a given amount of chemical energy in the form of fat is as
good as in the case of either of the other two substances. In fact, the
observations of Zuntz show that fat can be used just as economically
by the body for muscle work as either carbohydrates or proteid. Thus,
in one experiment,[47] he determined the oxygen-consumption and
respiratory quotient in a man resting and working on three different
diets--one principally fat, one principally carbohydrate, and the other
principally proteid--and found that slightly less oxygen and energy
were required to do work on the fat diet than on the others. This is
clearly shown in the following table:

  [47] Quoted from Leathes: Problems in Animal Metabolism, p. 100.

  +------------+------------------+------------------+------+----------------+
  |            |                  |                  |      |      Per       |
  |            |      Resting.    |      Working.    |Kilo- | Kilogram-meter |
  |            |                  |                  |gram- |    of Work.    |
  |   Diet     +--------+---------+--------+---------+meters+------+---------+
  |Principally.| Oxygen |Respira- | Oxygen |Respira- |  of  |Oxygen|Calories.|
  |            |Used per|  tory   |Used per|  tory   | Work | Used.|         |
  |            | Minute.|Quotient.| Minute.|Quotient.| Done.|      |         |
  +------------+--------+---------+--------+---------+------+------+---------+
  |            |  c.c.  |         |  c.c.  |         |      | c.c. |         |
  |Fat         |  319   |  0.72   |  1029  |   0.72  |  354 | 2.01 |   9.39  |
  |Carbohydrate|  277   |  0.90   |  1029  |   0.90  |  346 | 2.17 |  10.41  |
  |Proteid     |  306   |  0.80   |  1127  |   0.80  |  345 | 2.38 |  11.35  |
  +------------+--------+---------+--------+---------+------+------+---------+

From these data, we see that per kilogram-meter of work less energy
was required and less oxygen consumed with fat than with either of the
other two foodstuffs; but practically, fat and carbohydrate as sources
of muscle energy have about the same value.

Much stress is ordinarily laid upon the importance of a large intake
of proteid food whenever the body is called upon to perform severe, or
long-continued, muscular work; but in view of what has been stated it
may be questioned whether there is any real physiological justification
for such custom. The pedestrian Weston,[48] who in 1884 walked 50 miles
a day for 100 consecutive days, was found by Blyth during a period of
five days to consume in his food 37.2 grams of nitrogen a day, while he
excreted only 35.3 grams, leaving a balance of 1.9 grams of nitrogen
per day apparently stored in the body. His daily food during this
period was composed of 262 grams of proteid, 64.6 grams of fat, and 799
grams of carbohydrate, with an estimated fuel value of 4850 calories.
Yet he performed this large amount of work daily, and still laid by a
certain amount of proteid on a ration, the energy value of which would
not ordinarily be considered high for the muscular work to be done.
Fourteen years prior to this, Weston, while in New York, was carefully
studied by Dr. Flint during a period of 15 days, on 5 of which he
walked a total of 317 miles. His diet was essentially a proteid diet,
consisting principally of beef extract, oatmeal gruel, and raw eggs.
Nitrogen intake and output were carefully compared during the days of
rest and during the days of work, with the results tabulated.

  [48] This and the following account of Weston are taken from Bulletin
  No. 98, U. S. Department of Agriculture, Office of Experiment
  Stations. The effect of severe and prolonged muscular work on food
  consumption, digestion, and metabolism. By W. O. Atwater and H. C.
  Sherman, p. 13.

  +--------------+----------------+--------+--------------------------+
  |              |                |        |         Nitrogen.        |
  |              |                |        +-----+------+------+------+
  |              |                |Duration| In  | In   |  In  |Gain +|
  |   Period.    |  Occupation.   |of Test.|Food.|Urine.|Excre-| or   |
  |              |                |        |     |      |ment. |Loss -|
  +--------------+----------------+--------+-----+------+------+------+
  |              |                |  days  |grams|grams |grams |grams |
  |Fore period   |Comparative rest|    5   | 22.0| 18.7 | 1.4  | +1.9 |
  |              |                |        |     |      |      |      |
  |Working period|Walking 62 miles|{       |     |      |      |      |
  |              |  per day       |{   5   | 13.2| 21.6 | 1.6  |-10.0 |
  |              |                |        |     |      |      |      |
  |After period  |Rest            |    5   | 28.6| 22.0 | 2.2  | +4.4 |
  +--------------+----------------+--------+-----+------+------+------+

In this case it will be noted that the daily ration was comparatively
small, and, further, that during the working period the subject
consumed much less proteid than on the resting days. Moreover, when
we remember that the total energy value of his diet must have been
quite small, it is not at all strange that in the laborious task of
walking 62 miles a day he should have temporarily drawn upon his store
of body proteid to the extent of 62.5 grams, or 10 grams of nitrogen a
day. Such experiences, however, do not by any means constitute proof
that in excessive muscular work there is need for the consumption of
correspondingly increased quantities of proteid food, or that the
energy of muscular work comes preferably from the breaking down of
proteid material. Carbohydrate and fat unquestionably take precedence
over proteid in this respect, and we may accept as settled the view
that in all practical ways carbohydrate and fat stand on an equal
footing as sources of muscular energy. Less clear, perhaps, is the
question as to how these two radically different types of organic
material are utilized by the muscle. It has been a favorite belief
among some physiologists that the contracting muscle makes use of
only one substance as the direct source of its energy, and that this
substance is the sugar dextrose. This view would seemingly imply that
fat and proteid must undergo alteration prior to their utilization
by the muscle; that, possibly, the carbon of the fat and proteid
is transformed into sugar before the muscle can make use of it. So
far as fat is concerned, this view is not supported by the facts
available, since experiments show that the heat and energy liberated
in the utilization of a given amount of fat in muscle work are in
harmony with the energy value of the fat; in other words, the fat is
apparently burned, or oxidized, directly, without undergoing previous
transformation into any form of carbohydrate; or, if transformation
does occur, under some conditions, it must take place within the muscle
and without loss of energy. The practical significance of these facts
is at once apparent, for if fat, in order to be available as a source
of muscle energy, must first undergo conversion into sugar, it would
be far more economical from a physiological standpoint to replace the
fat of the diet with carbohydrate in any attempt to provide suitable
nourishment for the working muscle. We may safely conclude, however,
that fat and carbohydrate, as previously suggested, are in reality both
capable of direct metabolism by the muscular tissue, and that each is
of value as a source of muscular energy in proportion to its heat of
combustion, yielding substantially the same proportion of its potential
energy in the form of mechanical work.

Regarding the utilization of proteid as a source of energy by the
muscle, there are many grounds for believing that here the body
has to deal with certain alterations, before the proteid can be
made available. We may indeed conjecture the transformation of a
non-nitrogenous portion of the proteid molecule into carbohydrate, as
a necessary step in its utilization for muscle work. It is certainly
true that in the ordinary katabolic processes, through which proteid
passes, there is a tendency for the nitrogen-containing portion to
be quickly split off and eliminated, leaving a carbonaceous residue
which may represent as much as 80 per cent of the total energy of the
original proteid. This so-called carbon moiety of the proteid molecule
is apparently much less rapidly oxidized than the nitrogenous portion,
and may indeed be temporarily stored in the body, in the form of fat or
carbohydrate.[49] We have very convincing proof that the carbohydrate
glycogen can be formed from proteid. Thus, the feeding of proteid to
warm-blooded animals may be accompanied by an accumulation of glycogen
in the liver. This is interpreted as meaning that in the cleavage
of proteid by digestion the various nitrogenous products formed are
somewhere, probably in the liver, still further acted upon; the
contained nitrogen with some of the carbon being converted into urea,
while the non-nitrogenous residue is transformed into glycogen, or
sugar. That some such change takes place, or, more specifically, that
carbohydrate does result from proteid is more strikingly shown in human
beings suffering with diabetes. In severe forms of this disease, all
carbohydrate food consumed is rapidly eliminated through the kidneys in
the form of sugar, the body having lost the power of burning sugar. If
such a person is placed upon a diet composed exclusively of proteid,
sugar still continues to be excreted, and there is observed a certain
definite relationship between the nitrogen output and the excretion of
sugar, thus implying that they have a common origin.

  [49] See Leo Langstein: Die Kohlehydratbildung aus Eiweiss.
  Ergebnisse der Physiologie, Band 3, Erster Theil, p. 456.

  See also, Lüthje: Zur Frage der Zuckerbildung aus Eiweiss. Archiv für
  d. gesammte Physiologie, Band 106, p. 160.

Further, there are certain drugs, such as phloridzin, which, when
introduced into the circulation, set up a severe diabetes and
glycosuria. Dogs treated in this way, fed solely on proteid or even
starved for some time, will continue to excrete sugar, and as in the
previous instance, there is observed a certain definite ratio between
the nitrogen output and the elimination of sugar; thus leading to
the conclusion that both arise from the destruction of the proteid
molecule. Careful study of this ratio of dextrose to nitrogen has led
Lusk to the conclusion that full 58 per cent of the proteid may undergo
conversion into sugar in the body. Hence, it is easy to see how in
muscle work, when proteid is the sole source of the energy of muscular
contraction, the work accomplished may still result from the direct
oxidation of carbohydrate material, indirectly derived from the proteid
molecule. It requires no argument, however, to convince one that such a
procedure for the normal individual is less economical physiologically
than a direct utilization of carbohydrate and fat, introduced as such
and duly incorporated with the muscle substance. Consequently, in the
nourishment of the body for vigorous muscular work, there is reason
in a diet which shall provide an abundance of carbohydrate and fat;
proteid being added thereto only in amounts sufficient to meet the
ordinary requirements of the body for nitrogen and to furnish, it may
be, proper pabulum for the development of fresh muscle fibres, where,
as in training, effort is being made to strengthen the muscle tissue
and so enable it to do more work. Increase in proteid food may help to
make new tissue, but the source of the energy of muscle work is to be
found mainly in the breaking down of the non-nitrogenous materials,
carbohydrate and fat.

In view of these facts, we may advantageously consider next the real
significance of the proteid metabolism of the body. As we have seen, a
meal rich in proteid leads at once--within a few hours--to an excretion
of urea equivalent to full 50 per cent of the nitrogen of the ingested
proteid, while a few hours later finds practically all of the nitrogen
of the intake eliminated from the body. Further, it is to be remembered
that in a general way this occurs no matter what the condition of the
body may be at the time and no matter how large or small the amount of
proteid consumed. In other words, there is practically no appreciable
storing of nitrogen or proteid for future needs,--at least none that is
proportional to the increase in nitrogen intake, even though the body
be in a condition approximating to nitrogen starvation. Moreover, it
is to be recalled that the increased proteid metabolism attendant on
increased intake of proteid food is accompanied by an acceleration of
the metabolism of non-nitrogenous matter; thus resulting in a stirring
up of tissue change, with consequent oxidation and loss of a certain
proportion of accumulated fat and carbohydrate. Coincident with this
increased excretion of nitrogen, the output of carbon dioxide is
likewise increased somewhat, due as is believed mainly to increased
metabolism of the involuntary muscle fibres of the gastro-intestinal
tract, by action of which the accelerated peristalsis so characteristic
of food intake is accomplished. Further, the increased output of
carbon dioxide, under these conditions, is to be attributed also to
the greater activity of the digestive and excretory organs, naturally
stimulated to greater functional power by the presence of proteid
foods and their decomposition products. Still, as stated by Leathes,
“the two main end-products of proteid metabolism, urea and carbonic
acid, are, to a great extent, produced independently of each other,
and the reactions which result in the discharge of the nitrogen are
not those in which energy is set free, work done, and carbonic acid
produced.” In other words, there is suggested what we have already
referred to, viz., that in proteid metabolism a nitrogenous portion of
the proteid molecule is quickly split off and gotten rid of, while the
non-nitrogenous part may be reserved for future oxidation, serving as
a source of muscle energy or for other purposes. This being so, it is
plain that “proteid metabolism in so far as it is concerned with the
evolution of energy, proteid metabolism in its exothermic stages, may
be almost entirely non-nitrogenous metabolism” (Leathes).

Is there any advantage to the body, however, in this carbonaceous
residue of the proteid molecule over simple carbohydrate and fat?
Can the processes of the body be accomplished more economically,
or more advantageously, with a daily diet so constructed that the
tissues and organs must depend mainly upon this carbon moiety of
the proteid molecule for their energy-yielding material? It has been
one of the physiological dogmas of the past, that the tissues and
organs of the body, or rather their constituent cells, preferred to
use proteid for all their needs whenever it was available. If proteid
were wanting, either because of insufficient intake, or because of
excessive activity, then the tissue cells would draw upon their store
of non-nitrogenous material. Food proteid and tissue proteid, however,
were the materials preferred by the organism, so ran the argument, and
the large and incessant output of nitrogen which accompanied the intake
of proteid was accepted as proof of the general truth of this idea.
We might well question wherein lies the great advantage to the body
in this continual excretion of nitrogen; whether the loss of energy
in handling and removing the nitrogenous portion of the necessarily
large proteid intake, in order to render available the non-nitrogenous
part of the molecule, might not more than compensate for the supposed
gain? But the truly astonishing fact that the output of nitrogen runs
parallel with the intake of proteid, that the body cannot store up
nitrogen to any large extent, has been taken as conclusive evidence
that the organism prefers to use proteid for all of its requirements.
Truly, we might just as well argue that this significant rise in
the excretion of nitrogen after partaking of a proteid meal is an
indication that the body has no need of this excess of nitrogen; that
it is indeed a possible source of danger, since the system strives
vigorously to rid itself of the surplus, and that the energy-needs of
the body can be much more advantageously and economically met from fat
and carbohydrate than from the carbonaceous residue resulting from the
disruption of the proteid molecule.

In discussing these questions, we shall need to refer to several of
the current theories concerning proteid metabolism, notably, the
theories of Voit, Pflüger, and Folin. In 1867 Carl Voit,[50] of
Munich, advanced the view that the proteid material of the body exists
in two distinct forms, viz., as “morphotic” or “organized” proteid,
representing proteid which has actually become a part of the living
units of the body, _i. e._, an integral part of the living tissues;
and “circulating” proteid, or that which exists in the internal meshes
of the tissue, or in the surrounding lymph and circulating blood.
The real point of distinction here is that while one portion of the
body proteid is raised to the higher plane of living matter, _i. e._,
becomes a component part of the living protoplasm, another and perhaps
larger portion is outside of the morphological framework of the tissue,
constituting a sort of internal medium which bathes the living cells,
and acts as middleman between the blood and lymph on the one side and
the living cells on the other. According to Voit’s view, it is this
circulating proteid that undergoes metabolism; the proteid of the food
after digestion and absorption being carried to the different tissues
and organs, and then, without becoming an integral part of the living
protoplasm of the cells, it is broken down under the influence of
the latter. Obviously, small numbers of tissue cells are constantly
dying, their proteid matter passing into solution, where it likewise
undergoes metabolism. In other words, according to Voit, the great
bulk of the proteid undergoing katabolism is the circulating proteid,
derived more or less directly from the food, and which at no time has
been a part of the tissue framework; while a smaller, but more constant
amount, represents the breaking down of tissue cells. This conception
of proteid metabolism is akin to our conception of morphological and
physiological destruction. In the words of Foster: “We know that an
epithelial cell, as notably in the case of the skin, may be bodily
cast off and its place filled by a new cell; and probably a similar
disappearance of and renewal of histological units takes place in all
the tissues of the body to a variable extent. But in the adult body
these histological transformations are, in the cases of most of the
tissues, slow and infrequent. A muscle, for instance, may suffer very
considerable wasting and recover from that wasting without any loss or
renewal of its elementary fibres. And it is obvious that the metabolism
of which we are now speaking does not involve any such shifting of
histological units. On the other hand, we find these histological
units, the muscle fibre or the gland cell, for instance, living on
their internal medium, the blood, or rather on the lymph, which is
the middleman between themselves and the actual blood flowing in the
vascular channels.”

  [50] See Voit: Hermann’s Handbuch der Physiologie, Band 6, p. 301.

Voit claims that the proteid dissolved in the fluids of the body
is more easily decomposable than that which exists combined in
organized form, or as more or less insoluble tissue proteid; and it
is this soluble and circulating form which, under the influence of
the living cells, undergoes destruction or metabolism. We know, as
has been previously stated, that oxidation does not take place to
any extent in the circulating blood, and similarly there is every
reason for believing that proteid metabolism does not occur in this
menstrum. Metabolism is limited mainly to the active tissues of the
body, but according to the present conception of the matter it does
not occur at the expense of the proteid of the living cells, but
involves material contained in the fluids bathing the cells; _i. e._,
it is not the organized proteid that undergoes metabolism, but the
proteid circulating in and about the internal meshes of the cells and
tissues, the living cell being the active agent in controlling the
process. Further, this view lessens the difficulty of understanding
the elimination of nitrogen after a meal rich in proteid. If it was
necessary to assume that all the proteid of our daily food is built
up into living protoplasm before katabolism occurs, it would be
exceedingly difficult to explain the sudden and rapid elimination of
nitrogen which follows the ingestion of proteid. For example, we can
hardly imagine that merely eating an excess of proteid food will lead
to an actual breaking down of the living framework of the tissues,
equivalent to the amount of nitrogen which the body at once eliminates.
Voit’s theory, on the other hand, supposes a twofold origin of the
nitrogen excreted; one part, the larger and variable portion, comes
from the direct metabolism of the circulating proteid, being the
immediate result of the ingested food and varying in amount with the
quantity of proteid food consumed; the other, smaller and less variable
in amount, has its origin in the metabolism of the true tissue proteid,
or the actual living framework of the body.

In a fasting animal, the tissues and organs of the body still contain
a large proportion of proteid matter, yet only a small fraction of
this proteid is eliminated each day, hardly 1 per cent. If, however,
proteid is absorbed from the intestine, proteid metabolism is at once
increased, and the excretion of nitrogen may be fifteen times greater
than during hunger. In other words, the extent of proteid metabolism
is not at all proportional to the total amount of proteid contained
in the body as a whole, but runs parallel in a general way with the
quantity of proteid absorbed from the intestine. Obviously, the newly
absorbed proteid is quite different in nature from the proteid which
in much larger amounts is deposited throughout the body, since it is
not organized and is so much more easily decomposable (Voit). This is
the circulating proteid of the body; it exists in solution, and it is a
significant fact that, according to Voit, the chemical transformations
that characterize proteid katabolism occur only in solution. The
organized proteid, on the other hand, is in a state of suspension,
and its katabolism, which is relatively very small, is preceded by
solution of the proteid in the fluids of the tissue, after which
its further breaking down is assumed to be the same as that of the
circulating proteid. This latter view is a fundamental part of the Voit
theory; in long-continued fasting, for example, the living protoplasm
of the various tissues and organs is of necessity drawn upon for the
nourishment of the more vital parts of the body, such as the brain,
spinal cord, etc., consequently the organized proteid is gradually
dissolved and then decomposed, after it has become liquefied and has
thus lost its organized structure.

In this conception of proteid metabolism, we picture the different
organs and tissues of the body as being permeated by a fluid which
carries variable amounts of nutritive material, the quantity of the
latter determining in a way the extent of the proteid katabolism which
shall take place. As the proteid of the food passes into the blood and
lymph, the fluids bathing the cells are correspondingly enriched, and
as a result, proteid katabolism is accelerated in parallel degree.
During hunger, on the other hand, the organized proteid of the tissue
cells is gradually liquefied and passes out into the current of the
circulating fluids. As before stated, the organized proteid as such is
never decomposed; it must first enter into solution, and then under
the influence of the living cells it undergoes disruption in the same
manner as the circulating proteid. It is thus evident that the tissue
cells and the circulating fluids permeating them bear an ever changing
relationship to each other. Excess of circulating proteid will be
attended by increased katabolism, while at the same time there may be
some accumulation of proteid in the cells, and indeed some conversion
into organized proteid. During fasting, hunger, or with an insufficient
intake of proteid food, the current will naturally be in the opposite
direction, and organized proteid will slowly, but surely, be drawn
upon.

Again, we may ask in view of these facts, of what real use to the
body is this large katabolism of circulating proteid? We can easily
understand the need of proteid to supply the loss incidental to the
breaking down of organized or true tissue proteid, but this we are led
to believe is very small in amount. Is there any real need for proteid
beyond this requirement? The physiological fuel value of proteid is
no greater than that of carbohydrate and considerably less than half
that of fat, consequently there is on the surface no apparent reason
why proteid should be used for its energy value in preference to the
non-nitrogenous foodstuffs. Further, as we have seen, the energy of
muscle work comes mainly, at least, from the breaking down of fat and
carbohydrate; proteid, in the case of the well-nourished individual,
ordinarily playing no part in this important line of energy exchange.
Lastly, in the katabolism of proteid there is the large proportion
of nitrogenous matter to be split off and disposed of before the
carbon moiety of the molecule can be rendered available. Here, we have
involved not only a loss of energy, but in addition a certain amount of
what appears to be useless labor thrown upon the liver, kidneys, and
other organs. Is there any wonder that the thoughtful physiologist,
looking at the facts and theories presented by the Voit conception
of proteid katabolism, should ask wherein lies the value to the body
of this high rate of metabolism of circulating proteid, a rate of
metabolism which is seemingly governed primarily by the amount of
proteid food ingested?

Turning next to Pflüger’s[51] views regarding proteid katabolism, we
find a totally different outlook. Here, the supposition prevails that
the plasma of the blood and lymph, with its contained proteid, is the
food of the organs or their cells, but that before this food material
can undergo katabolism it must first be absorbed by the cell and built
up into the living protoplasm of the tissue. In other words, according
to the views expressed by Pflüger, katabolism must be preceded by
organization of the proteid. Expressed in still different language,
the proteid material circulating in blood and lymph must be eaten up
by the hungry cells and, by appropriate anabolic processes, made an
integral part of the living protoplasm before disassimilation can
occur. Further, according to Pflüger’s conception of these processes,
there is a radical difference in the chemical nature of living
protoplasm as compared with that of the so-called circulating proteid.
The latter is looked upon as being comparatively stable, resisting
oxidation in high degree, and hence not prone to undergo metabolism.
Living protoplasm, on the other hand, is characterized by instability,
suffering oxidation with the greatest ease, and hence readily broken
down in the ordinary processes of katabolism. Assuming for the moment
the correctness of this theory, we see at a glance that all disruption
of proteid matter in the body must be preceded by the upbuilding of the
proteid into living protoplasm. There can be no destruction of proteid
until the latter has been raised to the high plane of living matter.
The dead, inert circulating proteid can serve simply as food for the
living cells, and cannot undergo katabolism until it has been built up
into the organized structure of the tissue or organ. Even though we
grant that a small proportion of proteid may suffer katabolism without
previous organization, it does not materially modify the general trend
of the argument that, according to Pflüger’s hypothesis, proteid
katabolism is essentially a process involving the disruption of living
protoplasm.

  [51] Eduard Pflüger: Ueber einige Gesetze des Eiweissstoffwechsels
  (mit besonderer Berücksichtigung der Lehre vom sogenannten
  “circulirenden Eiweiss”). Archiv f. d. gesammte Physiologie, Band 54,
  p. 333.

Consider what this means in the light of facts already presented.
Remembering that the one factor above all others influencing the rate
of proteid katabolism is the amount of proteid food taken in, and that
the output of nitrogen, no matter what the previous condition of the
body or the amount of proteid food ingested, runs more or less parallel
with the consumption of proteid, we are forced to the conclusion, in
accepting this hypothesis, that there must be superhuman activity in
the building up of living protoplasm, only to be followed, however, by
its immediate and more or less complete breaking down. Further, think
of the daily or periodical fluctuation in the construction of bioplasm,
coincident with variations in the amount of proteid food consumed, and
the corresponding destruction of bioplasm as indicated by the daily
output of nitrogen. Imagine, if you will, the concrete case of a man
of 70 kilos body-weight eating a daily ration containing 125 grams
of proteid, the nitrogen equivalent of which is practically excreted
within twenty-four hours, and are we not wise in hesitating to believe
that all of that proteid has been so quickly built up into living or
organized tissue only to be immediately broken down and thrown out of
the body? Think of the enormous activity implied in the manufacture
of this bioplasm in the time allotted, and for what? Apparently, so
that it can be broken down again. But such energy as is liberated in
the breaking-down process might be derived far more economically by
simple destruction of the proteid, as contained in the meshes of the
tissue elements, without assuming a preliminary conversion into living
protoplasm. Obviously, we have here a theory which does not help us in
arriving at any very satisfactory conception of proteid metabolism.
The facts which Pflüger and his followers bring forward in support of
the theory are not very convincing, or at least not sufficiently so to
carry conviction in the face of a natural disinclination to believe in
the necessity of such a profound anabolic process, merely as a prelude
to the speedy destruction of the finished product. Finally, we may add
that if all proteid katabolized in the body must first be raised to the
high level of living protoplasm before the final disruption can occur,
it may be prudent to keep the daily intake of this foodstuff down to a
level somewhat commensurate with the real needs of the body.

As has been stated many times in the course of this presentation,
the most striking feature of proteid metabolism is the rapidity with
which large quantities of proteid consumed as food are broken down,
and the contained nitrogen eliminated from the body as urea. A few
hours will suffice to accomplish the more or less complete destruction
of food proteid; and any theory of proteid metabolism, to be at all
satisfactory, must explain this peculiar phenomenon. According to
recent investigations, it seems probable that some, at least, of the
cleavage products of proteid formed during intestinal digestion are
not built up into new proteid, but are at once eliminated mainly in
the form of urea, without becoming a part of either the so-called
circulating proteid, or the living protoplasm of the body. It will be
recalled that under the influence of the digestive enzymes, trypsin
and erepsin, proteid foodstuffs may be broken down while undergoing
intestinal digestion into monamino- and diamino-acids, such as
leucin, tyrosin, arginin, lysin, etc. A certain proportion of these
comparatively simple substances may be directly absorbed by the
portal circulation and carried to the liver, where they may undergo
conversion into urea. In this way, some portion of the nitrogen of
the ingested food may be quickly eliminated from the system. As has
been stated in another connection, we are not sure at present how
far proteid decomposition of the kind indicated takes place normally
in the body. We merely know that there are present in the intestine,
enzymes capable of splitting up proteid into these small fragments, and
that substances of this type when made to circulate through the liver
are transformed into urea. These facts, coupled with the well-known
tendency of the nitrogen of proteid food to appear in the excretions
a few hours after the food in question has been consumed, naturally
suggests a direct breaking down of proteid along the lines indicated,
with a possible retention of a carbonaceous residue (nitrogen-free)
for subsequent oxidation, as a source of energy for heat or work.
Obviously, all of the proteid food cannot behave in this manner, for
if such were the case there would be no proteid available for making
good the normal waste incidental to tissue changes. Either a certain
amount of proteid escapes this profound alteration produced by the
proteolytic enzymes in question, or else a certain proportion of these
simple decomposition products is synthesized in the intestine, or in
the tissues of the body, to form new proteid for the regeneration of
cell protoplasm. However this may be, we have presented in this view
a plausible explanation of the prompt appearance of food nitrogen in
the excretions, and without compelling belief in a theory, such as
Pflüger’s, which taxes one’s credulity to the utmost. To be sure,
as a prominent writer on physiology has recently said, such a view
stands opposed to our conceptions of the importance of proteid food;
but it seems possible, in the light of accumulating knowledge, that
our conceptions of the part played by proteid foods in the nutrition
of man have not been strictly logical, or quite in accord with true
physiological reasoning.

Again, in this connection, we may ask the question, why is it that the
body provides such an effective method for the speedy breaking down
of proteid food and the prompt elimination of the contained nitrogen?
Whatever the means made use of by the organism in accomplishing this,
the result is the same; the nitrogen of the ingested food is, in large
measure, quickly gotten rid of. We clearly recognize the all-important
position of proteid foods in the nutrition of the body, but there
appears a certain inconsistency in this prompt removal of the
nitrogen-containing portion of the proteid molecule. The nitrogenous
part of the proteid food is, physiologically considered, the
all-important part. It is the only source of nitrogen available to the
system, and yet apparently the larger proportion of this nitrogenous
material is not utilized in any recognizable way, but is eliminated as
quickly as possible. Is it not within the limits of possibility that
these methods, whatever may be the exact mechanism involved, are merely
a means of getting rid of a surplus of proteid for which the body has
no real need? This question I shall try to answer later on in another
connection, but we may advantageously keep this possibility in mind
while we are discussing these theories of proteid metabolism.

It is obvious, in the light of present knowledge, that there must
be a certain amount of true tissue proteid broken down each day,
independent of that larger metabolism coincident with the intake of
proteid food. However much this more voluminous proteid katabolism may
fluctuate, owing to variations in the intake of proteid, and whatever
the significance of this latter phase of metabolism, it is self-evident
that there must be a steady, constant metabolism, upon which the life
of the various tissues and organs of the body depends, and by which
the proteid integrity of the tissue cells is maintained. This implies
a certain degree of true tissue change, in which definite amounts of
proteid material are broken down and the resultant loss made good from
the proteid intake. No matter what specific name be applied to this
form of proteid katabolism, its existence is clearly recognized. It is
obviously a form of metabolism distinct, and probably quite different,
from that form, more variable in extent, which is associated with the
intake of proteid food. Plainly, if there is truth in these statements,
there should be some data available by means of which these two lines
of proteid katabolism can be more or less sharply differentiated.

Thanks especially to the work of Folin,[52] these data are now
apparently at hand, and the facts which he has accumulated with
painstaking care seem destined to throw additional light upon our
conception of proteid metabolism. It will be remembered that in the
breaking down of proteid, the great bulk of its contained nitrogen is
eliminated in the form of urea. In addition, a certain smaller amount
of nitrogen is excreted in the forms of creatinin and uric acid. As
we have seen, the total output of nitrogen, which measures the extent
to which proteid is decomposed in the body, varies with the intake of
proteid food; but it is found that the proportion of nitrogen excreted
in the forms of urea and uric acid varies with the extent of the
metabolism. In other words, quantitative changes in the daily proteid
katabolism are accompanied by pronounced changes in the distribution of
the excreted nitrogen. Let us take a single illustration from Folin’s
results; the case of a healthy man who on one day--July 13--consumed
a proteid-rich diet, and on the other day--July 20--was living on a
diet containing only about 1 gram of nitrogen. The composition of
the excretion through the kidneys on these two days is shown in the
following table:

  [52] Otto Folin: Laws Governing the Chemical Composition of Urine.
  American Journal of Physiology, vol. 13, p. 66. A theory of Protein
  Metabolism. Ibid., vol. 13, p. 117.

  +--------------------+-----------------------+---------------------+
  |                    |        July 13.       |       July 20.      |
  +--------------------+-----------------------+---------------------+
  | Volume of urine    |  1170 c.c.            |  385 c.c.           |
  | Total nitrogen     |  16.80 grams          |  3.60 grams         |
  | Urea-nitrogen      |  14.70   "   = 87.5%  |  2.20   "  = 61.7%  |
  | Uric acid-nitrogen |   0.18   "   =  1.1%  |  0.09   "  =  2.5%  |
  | Creatinin-nitrogen |   0.58   "   =  3.6%  |  0.60   "  = 17.2%  |
  +--------------------+-----------------------+---------------------+

Here we see, as would be expected, that on the high proteid diet, there
was a large excretion of total nitrogen and of urea; while on the low
proteid diet, nitrogen and urea were correspondingly diminished. The
point to attract our attention, however, is the marked difference in
the percentage of urea-nitrogen in the two cases; a difference which
amounts to about 26 per cent. A similar difference is to be noted in
the percentage of uric acid-nitrogen. Lastly, it is to be observed that
in spite of the great difference in the extent of metabolism on the
two days--an excretion of 16.8 grams of nitrogen, as contrasted with
3.6 grams--the _amount_ of creatinin-nitrogen is essentially the same.
Folin finds that these peculiarities in the percentage distribution
of excreted nitrogen hold good in all cases where there is this wide
divergence in the amount of proteid katabolized, and, further, that
there is a gradual and regular transition from the one extreme to the
other. He sees in these results evidence that there are in the body
two forms of proteid katabolism, essentially independent and quite
different. One kind is extremely variable in quantity, while the other
tends to remain constant. The variable form has its own particular
kind of waste products, of which urea is the chief. The constant
katabolism, on the other hand, is largely represented by creatinin
and to a lesser degree by uric acid. The more the total katabolism is
reduced, the more prominent become creatinin and uric acid, products
of the constant katabolism; while urea, as chief representative of the
variable katabolism, becomes less conspicuous. Folin suggests the term
_endogenous_ or _tissue_ metabolism for the constant variety, while the
variable form he would name _exogenous_ or _intermediate_ metabolism.

In these suggestions we have not theory only, but a number of very
important facts which plainly must have some significance. Take,
for example, the excretion of creatinin. It is a characteristic
nitrogenous waste product, but its elimination from the body is wholly
independent of quantitative changes in the total amount of nitrogen
excreted. In other words, the amount of creatinin eliminated is a
constant quantity for a given individual under ordinary conditions,
no matter how great the variation in the amount of proteid food,
provided no meat is eaten. Meat must be avoided in testing this point,
since meat contains a certain amount of creatin, or other components,
which would be excreted as creatinin. Further, it is found that every
individual has his own specific creatinin excretion, which fact again
emphasizes the idea that this substance is a product of true tissue
katabolism, having no connection with that variable metabolism, of
which urea is the striking representative. These are facts which cannot
be ignored. They are well established by the careful observations of
Folin, and they are confirmed by a large number of observations made in
our own laboratory. Turn now to that other, more conspicuous, product
of proteid katabolism, urea. With a so-called average proteid intake,
about 88–90 per cent of the excreted nitrogen will be in the form of
urea, but, as Folin states, “with every decided diminution in the
quantity of total nitrogen eliminated, there is a pronounced reduction
in the per cent of that nitrogen represented by urea. When the daily
total nitrogen elimination has been reduced to 3 grams or 4 grams,
about 60 per cent of it only is in the form of urea.” Here, we have the
chief product of exogenous metabolism, a substance quite distinct from
creatinin, just as the process by which it originates is likewise quite
distinct.

Exogenous metabolism is plainly a process of quite a different order
from that of endogenous, or tissue metabolism. The latter involves
oxidation, while the former consists essentially of a series of
hydrolytic cleavages which result in a rapid elimination of the
proteid-nitrogen as urea. In this conception of exogenous katabolism,
we have essentially the same viewpoint as was previously taken in
attempting to explain how excess of proteid food can be so quickly
decomposed, and its nitrogen removed from the body. Whether the
hydrolytic cleavage is accomplished solely by trypsin and erepsin,
whether it takes place only in the intestine and in the liver,
or whether other glands and tissues are involved, is at present
immaterial; the essential point is that we have in the body a variety
of proteid katabolism, quite different from true tissue katabolism, the
extent of which is dependent primarily upon the amount of proteid food
consumed. The process involved is one which aims at the rapid removal
of the proteid-nitrogen as urea; without incorporation of the absorbed
proteid, or its decomposition products, either as an integral or
adherent part of the tissue proteid. Hydrolytic cleavage is eminently
fitted to accomplish this with the least expenditure of energy, while
the carbonaceous residue of the proteid thus freed from nitrogen can be
transformed into carbohydrate, or directly oxidized as the needs of the
body demand.

As one considers these views so admirably worked out by Folin, the
question naturally arises, if the real demands of the body for proteid
food will not be adequately met by the quantity necessary to satisfy
the true tissue metabolism? We may well believe, with Folin, that “only
a small amount of proteid, namely, that necessary for the endogenous
metabolism, is needed. The greater part of the proteid furnished with
so-called standard diets, like Voit’s, _i. e._, that part representing
the exogenous metabolism, is not needed; or, to be more specific, its
nitrogen is not needed. The organism has developed special facilities
for getting rid of such excess of nitrogen, so as to get the use of
the carbonaceous part of the proteid containing it.” In endogenous
metabolism, we have a steady, constant process quite independent of
the amount of proteid food, and absolutely indispensable for the
maintenance of life. So far as we know at present, its representative
creatinin is, for a given individual, the same in amount during
fasting as when a rich, meat-free, proteid diet is taken. The one
factor that seemingly determines the amount of creatinin eliminated is
the weight of the individual, or more exactly the weight of the true
tissue elements of the body, as distinct from fat or adipose tissue.
Endogenous or tissue katabolism obviously calls for a certain quantity
of proteid to maintain equilibrium, but this is small in amount as
compared with the usual intake of proteid foods. The average man, with
his ordinary dietetic habits, consumes more nitrogen than the body can
possibly make use of. The excess is not stored up, “because the actual
need of nitrogen is so small that an excess is always furnished with
the food, except, of course, in carefully planned experiments” (Folin).

We have seen at what low levels of proteid intake, nitrogen equilibrium
can be established, and we may well have faith in the conception of an
endogenous proteid katabolism which involves only minimal quantities of
proteid. Further, we have observed the constant tendency of the body
to maintain a condition of nitrogenous equilibrium, even with varying
income, and how slow the body is to lay by nitrogen on a rich proteid
diet, even when long deprived of proteid food; a fact difficult of
explanation except on the assumption that the real need of the body for
nitrogen is small, and that the tissues habitually carry a relatively
large reserve of nitrogenous material. We may assume with Folin that
“all the living protoplasm in the animal organism is suspended in a
fluid very rich in proteid, and on account of the habitual use of more
nitrogenous food than the tissues can use as proteid the organism
is ordinarily in possession of approximately the maximum amount of
reserved proteid in solution that it can advantageously retain.
When the supply of food proteid is stopped, the excess of reserve
proteid inside the organism is still sufficient to cause a rather
large destruction of proteid during the first day or two of proteid
starvation, and after that the proteid katabolism is very small,
provided sufficient non-nitrogenous food is available. But even then,
and for many days thereafter, the protoplasm of the tissues has still
an abundant supply of dissolved proteid, and the normal activity of
such tissues as the muscles is not at all impaired or diminished. When
30 grams or 40 grams of nitrogen have been lost by an average-sized
man during a week or more of abstinence from nitrogenous food the
living muscle tissues are still well supplied with all the proteid
they can use. That this is so, is indicated on the one hand by the
unchanged creatinin elimination, and on the other by the fact that one
experiences no feeling of unusual fatigue or of inability to do one’s
customary work. Because the organism at the end of such an experiment
still has an abundance of available proteid in the nutritive fluids,
it is at once seemingly wasteful with nitrogen when a return is made
to nitrogenous food. This is why it only gradually, and only under
the prolonged pressure of an excessive supply of food-proteid again
acquires its original maximum store of this reserve material.”

We may reasonably suppose that the reserve of proteid present in the
body is contained in the fluid media, and not as a part of the living
protoplasm. Further, we are apparently justified in the belief that
the sole form of proteid katabolism which is vitally important for the
welfare of the body is the endogenous katabolism. This must be provided
for adequately and indeed liberally, and in addition there should be
sufficient intake to keep up an abundant supply of reserve proteid, but
beyond these necessities there would seem to be no legitimate demand
for additional proteid. The voluminous exogenous proteid katabolism so
conspicuous in most individuals would seem to have no justification
in fact, or in physiological reasoning. What good, for example, can
be accomplished by this constant splitting off of nitrogen, with its
subsequent speedy removal from the body? The organism can neither use
it nor store it up, and why therefore should this daily burden of an
excessive and accelerated proteid katabolism be borne? As we have seen,
the energy of muscle work is derived mainly, and can come wholly, from
the breaking down of non-nitrogenous materials, fats and carbohydrates.
The very fact that an intake of say 120 grams of proteid is followed
at once by the removal of the larger part of the contained nitrogen,
as a result of the exogenous katabolism of the body, would seemingly
warrant the view that the proteid so decomposed might advantageously be
replaced by a corresponding amount of carbohydrate. In muscle work, as
in heat production, carbohydrate and fat are the materials burned up,
or oxidized. Proteid, on the other hand, is not so oxidized, at least
not the nitrogen-containing portion of the molecule.

There are apparent only two possible reasons for assuming a need on
the part of the body for the high exogenous katabolism of proteid so
commonly observed. The one is that the carbonaceous residue left after
the cleavage of nitrogen from the proteid molecule is better adapted
for the needs of the body than either carbohydrate or fat. Although
this does not seem very probable, it is of course a possibility and
merits consideration. Feeding experiments, with a comparatively small
proteid intake, continued over a sufficient length of time, would show
conclusively how much weight should be attached to this hypothesis. The
other possibility is that the body may derive some advantage from the
presence, in the tissues and fluids, of the varied nitrogenous cleavage
products split off from proteid so abundantly in exogenous katabolism.
These substances are mainly amino-acids on their way to urea, and there
is no apparent reason why they should be of service to the organism.
Still, the processes going on in the tissues and organs of the body are
intricate and not wholly understood, and we can conceive of some useful
function of which as yet we have no knowledge. In the construction of
tissue proteid, for example, as in a possible synthesis out of the
fragments formed by hydrolytic cleavage, it is not impossible that
certain corner-stones are needed, and that in order to obtain these
there must be a more or less wasteful breaking down of food-proteid.
However improbable this may seem, it, like the preceding hypothesis,
can be tested in a way by adequate feeding experiments, which shall
determine the effect on the body of a low proteid intake continued over
a long period of time. On the other hand, it is equally plausible, and
for some reasons more probable, to assume that this excessive exogenous
katabolism may be in a measure prejudicial to the best interests of the
body; that the many nitrogenous fragments formed in the efforts of the
organism to prevent undue accumulation of reserve proteid may in the
long run do as much harm as good.

Further, there is reason in the question whether the continual carrying
of excessive amounts of nitrogen reserves in the shape of soluble
proteid in the blood and lymph, and in the meshes of tissue and cell
protoplasm, is advantageous for the maintenance of the highest degree
of efficiency? We all recognize that an excessive accumulation of fat
is distinctly disadvantageous to the welfare of the body, and there
is, physiologically speaking, equally good ground for considering that
the storage of unorganized proteid in amounts beyond all possible
requirements of the body may be equally undesirable. Because less
tangible to the eye, the accumulation of unnecessary proteid is not
so easily recognizable, but this fact does not diminish the possible
danger which such accumulation may constitute. It must be granted,
however, that we are dealing here with hypotheses and not facts, but
though hypothetical the suggestions made are of sufficient moment to
merit attention and experimental study. In a later chapter, we shall
have occasion to present some facts bearing on these questions.

In the meantime, we may lay due stress upon the significance of these
views regarding proteid katabolism. We must accept as settled the
general idea that there are two distinct forms of proteid katabolism
within the body; one form representing the decay of tissue or cell
protoplasm, small in amount, with its own particular decomposition
products, and absolutely essential for the continuance of life.
The other form, the so-called exogenous katabolism, runs a totally
different course with distinctive side-products and end-products; it
is variable in extent, in harmony with variations in proteid intake,
and subject to the suspicion that at the level ordinarily maintained
it constitutes a menace to the preservation of that high degree of
efficiency which is an attribute of good health.




CHAPTER V

DIETARY HABITS AND TRUE FOOD REQUIREMENTS

  TOPICS: Dietetic customs of mankind. Origin of dietary standards.
  True food requirements. Arguments based on custom and habit.
  Relationship between food consumption and prosperity. Erroneous
  ideas regarding nutrition. Commercial success and national wealth
  not the result of liberal dietary habits. Instinct and craving not
  wise guides to follow in choice and quantity of food. Physiological
  requirements and dietary standards not to be based on habits and
  cravings. Old-time views regarding temperate use of food. The sayings
  of Thomas Cogan. The teachings of Cornaro. Experimental results
  obtained by various physiologists. Work of the writer on true proteid
  requirements. Studies with professional men. Nitrogen equilibrium
  with small amounts of food. Sample dietaries. Simplicity in diet.
  Nitrogen requirement per kilogram of body-weight. Fuel value of the
  daily food. Experiments with university athletes. Nitrogen balance
  and food consumption. Sample dietaries. Adequacy of a simple diet.


Having acquired information regarding the principles of metabolism
and the general laws governing the nutrition of the body, we may
next consider briefly the dietetic habits of mankind, with a view
to learning how far such habits coincide with actual nutritive
requirements. Eventually, we shall need to ask the questions: What are
the _true_ nutritive requirements of the body? How much food and what
kinds of food does the ordinary individual doing an average amount
of work need each day in order to preserve body equilibrium, and to
maintain health, strength, and vigor under the varying conditions
of life? What amount of nitrogen or proteid, and what the total
calorific value required to supply the physiological needs of the
body? How closely do the so-called “normal diets” and “standard diets,”
which have met with such general acceptance, conform to a rational
conception of true physiological needs? These are vital questions of
great physiological and economic importance, and they are not easily
answered; but theoretical considerations based on scientific data, and
experimental evidence combined with practical experience, should point
the way to some very definite conclusions.

Observations made in many countries regarding the dietetic customs and
habits of the people have resulted in the establishment of certain
dietary standards, which have been more or less generally adopted
as representing the requirements of the body. As a prelude to the
discussion of this question, let us consider briefly some of the
results of these dietary studies. In Sweden, laborers doing hard work
were found by Hultgren and Landergren to consume daily, on an average,
189 grams of proteid, 714 grams of carbohydrate, and 110 grams of fat,
with a total fuel value for the day’s ration of 4726 large calories.
In Russia, workmen at moderately hard labor, having perfect freedom of
choice in their food, were found by Erisman to take daily 132 grams of
proteid, 584 grams of carbohydrate, and 79 grams of fat, this ration
having a fuel value of 3675 calories. In Germany, soldiers in active
service consumed daily, according to Voit, 145 grams of proteid, 500
grams of carbohydrate, and 100 grams of fat, with a fuel value of 3574
calories. In Italy, laborers doing a moderate amount of work were found
by Lichtenfelt to consume daily 115 grams of proteid, 696 grams of
carbohydrate, and 26 grams of fat, with a fuel value of 3655 calories.
In France, Gautier states that the ordinary laborer working eight
hours a day must have 135 grams of proteid, 700 grams of carbohydrate,
and 90 grams of fat daily, with a fuel value of 4260 calories. In
England, weavers were found to take daily 151 grams of proteid, with
carbohydrates and fats sufficient to make the total fuel value of the
day’s ration equal to 3475 calories. In Austria, farm laborers consumed
daily 159 grams of proteid, with carbohydrates and fats sufficient to
raise the fuel value of the food to 5096 calories.

  +----------------------------------+----------------+----------------+
  |             Subjects.            |Proteid consumed|Total Fuel Value|
  |                                  |     Daily.     | of Daily Food. |
  +----------------------------------+----------------+----------------+
  |                                  |     grams      |    calories    |
  |Swedish laborers, at hard work    |      189       |      4726      |
  |Russian workmen, moderate work    |      132       |      3675      |
  |German soldiers, active service   |      145       |      3574      |
  |Italian laborers, moderate work   |      115       |      3655      |
  |French laborers, eight hours’ work|      135       |      4260      |
  |English weavers                   |      151       |      3475      |
  |Austrian farm laborers            |      159       |      5096      |
  |                                  |                |                |
  |        American Subjects.        |                |                |
  |                                  |                |                |
  |Man with very hard muscular work  |      175       |      5500      |
  |Man with hard muscular work       |      150       |      4150      |
  |Man with moderately active        |                |                |
  |  muscular work                   |      125       |      3400      |
  |Man with light to moderate        |                |                |
  |  muscular work                   |      112       |      3050      |
  |Man at “sedentary” or woman with  |                |                |
  |  moderately active work          |      100       |      2700      |
  +----------------------------------+----------------+----------------+

Observations of this order might be multiplied indefinitely, but
the above will suffice to give a general idea of the average food
consumption of European peoples doing a moderate amount of work. These
data, however, must be supplemented by the observations made in our own
country, which have been very extensive, through the “investigations on
the nutrition of man in the United States,” carried on by the Office
of Experiment Stations in the Department of Agriculture, under the
efficient leadership of Atwater. As stated by Messrs. Langworthy and
Milner, in an official bulletin issued in 1904, dietary studies of the
actual food consumption of people of different classes in different
parts of the United States have been made during the years 1894 to
1904 on about 15,000 persons,--men, women, and children,--as a result
of which it is indicated that “the actual food requirements of persons
under different conditions of life and work” vary from 100 to 175 grams
of proteid per day, with a total fuel value ranging from 2700 to 5500
calories. For comparison, the various data may be tabulated as shown on
page 155.

These figures by no means represent maximum food consumption. Thus,
studies have been made on fifty Maine lumbermen,[53] where the intake
of proteid food averaged 185 grams per day, with a total fuel value of
6400 calories. Further, dietary studies of university boat crews[54]
have shown fairly high results. The Yale University crew, while at
Gales Ferry, averaged per man during seven days 171 grams of proteid,
171 grams of fat, and 434 grams of carbohydrate, with a total fuel
value of 4070 calories per day. The members of the Harvard University
crew showed an average daily consumption of 160 grams of proteid, 170
grams of fat, and 448 grams of carbohydrate, with a total fuel value
of 4074 calories. It is also reported that a football team of college
students in the University of California consumed daily, per man, 270
grams of proteid, 416 grams of fat, and 710 grams of carbohydrate, with
a total fuel value of 7885 calories. These figures may be contrasted,
however, with the data obtained in a study of the dietary habits of
fourteen professional men’s families, where the average amount of
proteid consumed daily was 104 grams, fat 125 grams, and carbohydrate
423 grams, with a total fuel value of 3325 calories.

  [53] Bulletin No. 149. Woods and Mansfield. Studies of the Food of
  Maine Lumbermen. U. S. Department of Agriculture, 1904.

  [54] Bulletin No. 75. Atwater and Bryant. Office of Experiment
  Stations, U. S. Department of Agriculture, 1900.

Leaving out of consideration the extremes given, it is undoubtedly true
that, within certain rather wide limits, there is an apparent tendency
for people of different nations, having a free choice of food and not
restricted by expense, to consume daily approximately the same amounts
of nutrients; to use what may be called liberal rather than small
amounts of food; and, lastly, to consume food somewhat in proportion
to the amount of work done. It is perhaps, therefore, not strange that
students of nutrition should have taken these results, obtained by the
statistical method, as indicating the actual needs of the body for
food, and that so-called “standard diets” and “normal diets” should
have been constructed, based upon these and corresponding data. Thus,
we have the widely adopted “Voit standard,” composed of proteid 118
grams, carbohydrate 500 grams, and fat 56 grams, with a total fuel
value of 3055 calories, as the amount of food required daily by a man
of 70 kilos body-weight doing a moderate amount of work. These figures
were obtained by Voit as an average of the food consumption of a large
number of laboring men in Germany, and they carried additional weight
because at that time Voit and others thought they had evidence that
nitrogenous equilibrium could not be maintained for any length of time
on smaller amounts of proteid.

The figures given in the preceding table under the head of American
subjects constitute the “Atwater standards,” and as already indicated,
are based upon the dietetic habits of over 15,000 persons under
different conditions of life and physical activity. In the words of
the official Bulletin, these standards covering the quantities of food
per day “are intended to show the actual food requirements of persons
under different conditions of life and work.” Here, however, lies an
assumption which seems to meet with wide acceptance, but for which it
is difficult to conceive any logical reason. The thousands of dietary
studies made on peoples all over the world, affording more or less
accurate information regarding the average amounts of proteid, fat,
and carbohydrate consumed under varying conditions, are indeed most
interesting and important, as affording information regarding dietetic
customs and habits; but, the writer fails to see any reason why such
data need be assumed to throw any light on the actual food requirements
of the body. In the words of another, “Food should be ingested in just
the proper amount to repair the waste of the body; to furnish it with
the energy it needs for work and warmth; to maintain it in vigor;
and, in the case of immature animals, to provide the proper excess
for normal growth, in order to be of the most advantage to the body”
(Benedict).

Any habitual excess of food, over and above what is really needed
to meet the actual wants of the body, is not only uneconomical, but
may be distinctly disadvantageous. Voit, among others, has clearly
emphasized the general principle that the smallest amount of proteid,
with non-nitrogenous food added, that will suffice to keep the body in
a state of continual vigor is the ideal diet. My own conception of the
true food requirements of the body has been expressed in the statement
that man needs of proteids, fats, and carbohydrates sufficient to
establish and maintain physiological and nitrogen equilibrium;
sufficient to keep up that strength of body and mind that is essential
to good health, to maintain the highest degree of physical and mental
activity with the smallest amount of friction and the least expenditure
of energy, and to preserve and heighten, if possible, the ordinary
resistance of the body to disease germs. The smallest amount of food
that will accomplish these ends is, I think, the ideal diet. There must
truly be enough to supply the real needs of the body, but any great
surplus over and above what is actually called for may in the long run
prove an undesirable addition. With these thoughts in mind, may we
not reasonably ask why it should be assumed that there is any tangible
connection between the dietetic habits of a people and their true
physiological needs?

Arguments predicated on custom, habit, and usage have no physiological
basis that appeals strongly to the impartial observer. Man is
a creature of habits; he is quick to acquire new ones when his
environment affords the opportunity, and he is prone to cling to old
ones when they minister to his sense of taste. The argument that
because the people of a country, constituting a given class, eat
a certain amount of proteid food daily, the quantity so consumed
must be an indication of the amount needed to meet the requirements
of the body, is as faulty as the argument that because people of a
given community are in the habit of consuming a certain amount of
wine each day at dinner their bodies must necessarily be in need of
the stimulant, and that consequently alcohol is a true physiological
requirement. A large proportion of mankind is addicted to the tobacco
habit, and to many persons the after-dinner cigar is as essential to
comfort as the dinner itself; but would any one think of arguing that
tobacco is one of the physiological needs of the body?

It is said that dietary studies made all over the civilized world
“show that a moderately liberal quantity of protein is demanded by
communities occupying leading positions in the world.... It certainly
seems more than a remarkable coincidence that peoples varying so widely
in regard to nationality, climatic and geographical conditions, and
dietetic habits, should show such agreement in respect to consumption
of protein and energy.” Again, we hear it said that “whatever may be
true of a few individuals, with communities a generally low condition
of mental and physical efficiency, thrift, and commercial success, is
coincident with a low proportion of protein in the diet.” The writer,
however, fails to find evidence in the results afforded by dietary
studies that there is any causal relationship between the amount of
proteid food consumed and the mental or physical supremacy of the
people of a given nation or community. Cause and effect are liable to
become reversed in arguments of this kind. It is certainly just as
plausible to assume that increase in the consumption of proteid follows
in the footsteps of commercial and other forms of prosperity, as to
argue that prosperity or mental and physical development are the result
of an increased intake of proteid food.

Proteid foods are usually costly, and the ability of a community to
indulge freely in this form of dietetic luxury depends in large measure
upon its commercial prosperity. The palate is an extremely sensitive
organ, and the average individual properly derives great satisfaction
from the pleasurable effects of tasty articles of food. Furthermore,
there are many curious and quite unphysiological notions abroad
regarding foods, which tend to incite persons to unnecessary excess
and extravagance whenever they acquire the means to do so. The latter
point is well illustrated by the more or less prevalent opinion that a
cut of tenderloin steak is more nutritious than a cut of round steak.
It is true that the former is apt to be more tender, to have a little
finer flavor; but the round steak, when properly prepared, is just as
nutritious, and equally capable of meeting the needs of the body, as
the more expensive tenderloin. With increasing prosperity, we turn at
once, as a rule, to the more tasty and appetizing viands, partly to
satisfy the craving of appetite and palate, and partly because there
is an inherent belief that these varied delicacies, accessible to the
prosperous community, count as an aid to health and strength. The poor
laborer, with his small wage, is restricted to a certain low level
of dietary variety, and must likewise be economical as to quantity,
but on the first opportunity afforded by a fuller purse he is apt
to pass from corned beef to a fresh roast with its more appetizing
flavor; to eschew brown bread in favor of the white loaf, and in many
other ways to evince his desire for a dietary which, though perhaps no
more nutritious, appeals because of its finer flavor, more appetizing
appearance, and greater variety. He is in the same position as the
smoker who, limited by his purse to a five-cent cigar after dinner,
quickly passes to a cigar of better flavor as soon as his finances
warrant the indulgence. At the same time, if prosperity continues, our
laborer will speedily pass to a higher level of proteid intake and
greater fuel value, through increased consumption of meat and butter,
together with other articles rich in proteid and fat.

In this connection, we may emphasize a fact of some significance in its
bearing on dietetic customs; viz., that ever since Liebig advanced his
theory that proteid material is the sole source of muscular energy,
there has been a deep-rooted belief that meat is the most efficient
kind of food for keeping up the strength of the body, and hence
especially demanded by all whose work is mainly physical. Although this
view, as we have seen, has been thoroughly disproved, the idea is still
more or less generally held that an abundance of meat is a necessary
requisite for a good day’s work, a view which undoubtedly accounts in
some measure for the tendency toward a high proteid intake, evinced by
many of the laboring class whose means will permit the necessary outlay.

Increased consumption of proteid food may be coincident with thrift and
commercial success, but there is no justification for the belief that
these are the result of changed dietary conditions. The dietary of our
New England forefathers was, according to all accounts, exceedingly
limited as compared with that of to-day, but it is doubtful if the
present generation is any better developed, physically or mentally,
than the stalwart and vigorous people who opened up this country to
civilization. To-day, as a nation, we have greater wealth, and our
commercial prosperity has become phenomenal; but would any one think
for a moment that these characteristics are attributable to the
large consumption of proteid food so common to this generation of
the American people? No, increased wealth simply paves the way for
greater freedom in the choice of food; increased commercial success
and business prosperity throw open avenues which formerly were closed;
greater variety of animal foods, and vegetable foods as well, rich in
proteid, are made easily accessible, and appeal to eye and palate on
all sides; appetite and craving for food are abnormally stimulated,
and dietetic habits and customs change accordingly. In the words of
another, “the one thing that primitive, barbarous, and civilized man
alike long for is an abundance of the ‘flesh-pots of Egypt.’ The
very first use the latter makes of his increased power and financial
resources is to buy new, rare, and expensive kinds of meat.” With these
facts before us, it is difficult to accept the assumption that dietetic
customs afford any indication of the food requirements of the body. To
the physiologist such a view does not appeal, since there is a lack of
any scientific evidence that carries conviction.

But it may be asked, is not appetite a safe guide to follow? Do not
the cravings of the stomach and the so-called pangs of hunger merit
consideration? Is it not the part of wisdom to follow inclination in
the choice and quantity of our food? Can we not safely rely upon these
factors as an index of the real needs of the body? If these questions
are to be answered in the affirmative, then it is plain that a study
of dietetic customs will tell us definitely how much food and what
kinds of food are required daily to supply the true wants of the body.
There are writers who claim that instinct is a perfectly safe guide to
follow; that it is far superior to reason; but it is to be noticed
that most of these writers, if they have any physiological knowledge
to draw upon, are sooner or later prone to admit that the body has
certain definite needs which it is the purpose of food to supply, with
the added implication that any surplus of food over and above what is
necessary to meet these demands is entirely uncalled for. Thus, one
such writer states that “the man in the street follows his God-given
instincts and plods peacefully along to his three square meals a day,
consisting of anything he can find in the market, and just as much of
it as he can afford, with special preference for rich meats, fats, and
sugars.” Yet this same writer, a little later, emphasizes the fact that
“every particle of the energy which sparkles in our eyes, which moves
our muscles, which warms our imaginations, is sunlight cunningly woven
into our food by the living cell, whether vegetable or animal. Every
movement, every word, every thought, every aspiration represents the
expenditure of precisely so much energy derived from food.” Why, then,
would it not be wise to ascertain how much energy is so expended, on
an average, during the day’s activity and govern the intake of food
accordingly? Why not apply an intelligent supervision in place of
following an instinct which, in the words of the author just quoted,
leads one on to consume “anything he can find in the market and just as
much of it as he can afford”? Truly, if dietetic customs and the habits
of mankind are the results of instinct working in this fashion, there
cannot be much value in the data obtained by observing the quantities
of food mankind is in the habit of eating. Dietary standards based on
such observations must be open to the suspicion of representing values
far above the actual needs of the body.

Habits and cravings are certainly very unreliable indices of true
physiological requirements. Man is constantly acquiring new habits, and
these in time become second nature, forcing him to practise that which
he has become accustomed to, regardless of whether it is beneficial
or otherwise. The celebrated philosopher, John Locke, in his essay on
education, says: “I do not think all people’s appetites are alike ...
but this I think, that many are made gourmands and gluttons by custom,
that were not so by nature; and I see in some countries, men as lusty
and strong, that eat but two meals a day, as others that have set their
stomachs by a constant usage, like Larums, to call on them for four or
five.” Again, the so-called cravings of appetite are largely artificial
and mainly the result of habit. A habit once acquired and persistently
followed soon has us in its grasp, and then any deviation therefrom is
very apt to disturb our physiological equilibrium. The system makes
complaint, and we experience a craving, it may be, for that to which
the body has become accustomed. There has thus come about a sentiment
that the cravings of the appetite for food are to be fully satisfied,
that this is merely obedience to nature’s laws. In reality, there is
no foundation for such a belief; any one with a little persistence can
change his or her habits of life, change the whole order of cravings,
thereby indicating that the latter are essentially artificial, and that
they have no necessary connection with the welfare or needs of the
body. The man who for some reason deems it advisable to adopt two meals
a day in place of three or four, at first experiences a certain amount
of discomfort, but eventually the new habit becomes a part of the daily
routine, and the man’s life moves forward as before, with perfect
comfort and without a suggestion of craving, or a pang of hunger.
Dietetic requirements, and standard dietaries, are not to be founded
upon the so-called cravings of appetite and the instinctive demands
for food, but upon reason and intelligence, reinforced by definite
knowledge of the real necessities of the bodily machinery.

The standards which have been adopted more or less generally throughout
the civilized world, based primarily on the assumption that man
instinctively and independently selects a diet that is best adapted
to his individual needs, are open to grave suspicion. The view that
the average food consumption of large numbers of individuals and
communities must represent the true nutritive requirements of the
people is equally untenable. Naturally, there is general recognition
of the principle that food requirements are necessarily modified by
a variety of circumstances, such as age, sex, body-weight, bodily
activity, etc. It is obvious that the man of 140 pounds body-weight
needs less proteid than the man of 170 pounds, and that the man who
does a large amount of physical work demands a larger calorific
value in his daily diet, _i. e._, more carbohydrate and fat, than
the sedentary individual. The growing child, in proportion to his
body-weight, plainly needs more proteid for the upbuilding of tissue,
and there are many conditions of disease where special dietetic
treatment is undoubtedly called for. Our contention, however, and one
which we believe to be perfectly justifiable, is that the true food
requirements of the body, under any conditions, cannot be ascertained
with any degree of accuracy by observations of what people are in the
habit of eating; that customs and habits are not a safe index of true
physiological needs. On the contrary, we are inclined to the belief
that direct physiological experimentation, covering a sufficient length
of time and with an adequate number of individuals, will prove far more
efficient in affording a true estimate of the quality and quantity of
food best adapted for the maintenance of good health, strength, and
vigor.

Before considering these latter points, it is interesting to note, in
passing, that during the last four centuries many thoughtful men have
called attention to the apparent excessive use of food. There seems
to have been in many quarters a more or less prevalent opinion that
custom and habit were leading people on to methods of living, which
were not in accord with the best interests of the community. It must be
remembered, however, that arguments of this kind, even fifty years ago,
could have been founded only on general observation and the application
of common sense, since there were then no sound physiological data
on which to predicate an opinion, or base a conclusion. Still, there
were men of authority who attempted to lay before the people a proper
conception of the temperate use of food. We have not the time here to
consider many of these pleas, but I venture to call attention to the
somewhat celebrated book published by the physician Thomas Cogan in
1596, under the title “The Haven of Health,” and dedicated “to the
right honorable and my verie good lord, Sir Edward Seymour, Knight and
Earl of Hertford.” Under the subject of diet, this old-time writer
says: “The second thing that is to be considered of meates is the
quantitie, which ought of all men greatly to be regarded, for therein
lyeth no small occasion of health or sickness, of life or death. For
as want of meate consumeth the very substance of our flesh, so doth
excesse and surfet extinguish and suffocate naturall heat wherein life
consisteth.” Again, “Use a measure in eating, that thou maist live
long: and if thou wilst be in health, then hold thine hands. But the
greatest occasion why men passe the measure in eating, is varitie of
meats at one meale. Which fault is most common among us in England
farre above all other nations. For such is our custome by reason of
plentie (as I think) that they which be of abilitie, are served with
sundry sortes of meate at one meale. Yea the more we would welcome our
friends the more dishes we prepare. And when we are well satisfied with
one dish or two, then come other more delicate and procureth us by that
meanes, to eate more than nature doth require. Thus varietie bringeth
us to excesse, and sometimes to surfet also. But Phisicke teacheth
us to faede moderately upon one kinde of meate only at one meale, or
at leastwise not upon many of contrarie natures.... This disease, (I
mean surfet) is verie common: for common is that saying and most true:
That more die by surfet than by the sword. And as Georgius Pictorius
saith, all surfet is ill, but of bread worst of all. And if nature be
so strong in many, and they be not sicke upon a full gorge, yet they
are drowsie and heavie, and more desirous to loyter than to labor,
according to that old maeter, when the belly is full, the bones would
be at rest. Yea the minde and wit is so oppressed and overwhelmed with
excesse that it lyeth as it were drowned for a time, and unable to use
his force.”

Cogan likewise makes some interesting statements regarding the effects
of custom on the consumption of proteid food, especially meats. Quoting
further from this author: “The fourth thing that is to be considered
in meats is custome. Which is of such force in man’s bodie both in
sicknesse and in health, that it countervaileth nature itselfe, and is
therefore called of Galen in sundry places, an other nature. Whereof he
giveth a notable example, where he sheweth that an olde woman of Athens
used a long time, to eate Hemlocke (which is a ranke poison) first a
little quantitie, and afterwarde more, till at length she could eate so
much without hurt as would presently poison another.... So that custome
in processe of time may alter nature.” Finally, we may quote one last
saying of Cogan’s, because of the good sense and wisdom displayed
in the sentiment, as true to-day as when it was written more than
three hundred years ago: “Neither is it good for any man that is in
perfect health, to observe any custome in dyet precisely, as Arnoldus
teacheth upon the same verses in these wordes: Every man should so
order himselfe, that he might be able to suffer heate and cold, and
all motions, and meats necessary, so as he might change the houres of
sleeping and waking, and his dwelling and lodging without harme: which
thing may be done if we be not too precise in keeping custome, but
otherwise use things unwonted. Which sentence of Arnoldus agraeth verie
well to that of Cornelius Celsus: He that is sound and in good health,
and at libertie, should bind himselfe to no rules of dyet. To need
neither Phisition or Chirurgion, he must use a diverse order of life,
and be sometimes in the countrie, sometime in the towne, sometimes
hunt, and sometime hawke. But some man may demand of me how this may
agree with that saying of the scholar of Salernus ‘if you would be free
from physicians, let these three be your physician, a cheerful mind,
rest, and a moderate diet.’ Whereunto I answer, that a moderate dyet
is alwaies good, but not a precise dyet: for a moderate diet is, as
Terence speaketh in Andria: To take nothing too much: which alwaies
is to be observed. But if a man accustome himselfe to such meats and
drinks as at length will breed some inconvenience in his bodie, or to
sleepe or to watch, or any other thing concerning the order of his
life, such custome must naedes be amended and changed, yet with good
discretion, and not upon the sudden: because sudden changes bring harme
and weaknesse, as Hippocrates teacheth. He therefore that will alter
any custome in dyet rightly, must do it with three conditions, which
are expressed by Hippocrates. Change is profitable, if it be rightly
used, that is, if it be done in the time of health, and at leisure, and
not upon the sudden.”

This noteworthy book written by Cogan was preceded by the writings
of Louis Cornaro, the Venetian, who forty years before had published
the first edition of his celebrated book, “The Temperate Life,” and
who was a most ardent advocate of the benefits to be derived by
living temperately, especially in matters of diet. The simple diet
which served for the nourishment of the oldest peoples of Syria,
Greece, Egypt, and of the Romans when they were at the height of
their prosperity and culture, was advocated by Cornaro as conducing
to longevity, better health, and greater comfort of mind and body.
Himself a striking example of the effects of a reasonable abstinence in
diet (the last edition of his book having been written at the age of
ninety-five), his teachings have continued to attract attention down to
the present day; and although we have no values in grams or calories
expressive of his average food consumption, it is quite evident that
Cornaro lived a very abstemious life, eating little of the heavier
articles of diet common to his time and country. It is perhaps not
strictly physiological to refer to these cases, yet they have value
as representing a sentiment, common to the centuries now passed, that
benefit was to be derived by mankind from greater care in the taking of
food; that prevalent customs and habits were leading the people into
intemperate modes of life, and that these were surely tending toward
the physical and mental deterioration of the nation. We may attach much
or little weight to these conclusions, but there is a certain degree of
significance in the views, current then as now, that dietetic customs
and habits have no real connection with bodily requirements.

Passing down to our own times, we find physiologists, by the aid
of scientific methods, studying the effects of smaller amounts of
food (smaller than custom prescribes) on the condition of the body,
thereby evincing a certain degree of skepticism concerning the dietary
standards based on habit and usage. This has been especially true
regarding the nitrogen requirement, or the need for proteid food.
As has been clearly pointed out in other connections, there are
two distinct needs which the body has for food; one for proteid or
nitrogen, the other for energy-yielding material. According to the
Voit standard, a man of average body-weight doing a moderate amount
of work requires daily 118 grams of proteid food, or about 16 grams of
metabolizable nitrogen, with fat and carbohydrate sufficient to yield
a total fuel value of over 3000 large calories. As we have seen, the
fuel value of the food must of necessity be a variable quantity because
of variations in bodily activity. The more muscular work performed,
the greater must be the intake of carbohydrate and fat, if the body
is to be kept in equilibrium. With proteid or nitrogen, however, the
case is quite different, since with adequate amounts of non-nitrogenous
food, proteid is not drawn upon for the energy of muscular work.
We can conceive of the nitrogen requirement, therefore, as being
a constant factor in the well-nourished individual and dependent
primarily upon body-weight, or more exactly, upon the weight of true
proteid-containing tissue. Obviously, whatever else happens, there must
be enough proteid food taken daily to maintain the body in nitrogen
equilibrium. If this can be accomplished only by the ingestion of 16
grams of metabolizable nitrogen, then it is plain that the daily ration
must contain at least 118 grams of proteid food; _i. e._, it must
conform approximately at least to ordinary usage.

This question has been studied by many investigators, with very
interesting and suggestive results. Thus, in 1887, Hirschfeld[55]
reported some experiments on himself, twenty-four years of age and
weighing 73 kilos. His ordinary diet contained daily 100 to 130 grams
of proteid, and the amount of nitrogen excreted varied from 16 to 20
grams per day, corresponding to a metabolism of proteid equal to the
amount ingested. In other words, the body was essentially in nitrogen
equilibrium. Then, for a period of fifteen days, during which he was
unusually active, he lived on a diet in which the content of proteid
corresponded to only 6 grams of nitrogen per day, and yet he remained
in nitrogen equilibrium. The diet made use of was composed essentially
of milk, eggs, rice, potatoes, bread, butter, sugar, and coffee, with
some wine and beer, and on two days a little meat. It is to be observed
that the nitrogen or proteid intake per day was only one-third of what
he was accustomed to consume. In a second experiment, covering ten
days, similar results were obtained. So that evidence was afforded that
a young and vigorous man can maintain his body in nitrogen equilibrium,
for fifteen consecutive days at least, on an amount of proteid food
equal to only one-third of the minimal requirement called for by common
usage. Plainly, the difference between a daily consumption of 118 grams
of proteid food and 40 grams represents a large percentage saving,
both of proteid and in the metabolism of proteid matter with all the
attendant transformations. In these experiments, however, the subject
consumed relatively large amounts of non-nitrogenous food, notably
butter, of which on some days he took as much as 100 grams. The average
fuel value of his food ranged from 3750 to 3916 calories per day; a
fact of some importance, since it is to be remembered that both fat and
carbohydrate tend to protect proteid metabolism.

  [55] Felix Hirschfeld: Untersuchungen über den Eiweissbedarf des
  Menschen. Pflüger’s Archiv f. d. gesammte Physiologie, Band 41, p.
  533.

In an experiment reported in 1889 by Carl Voit[56], on a vegetarian
weighing about 57 kilos, it was found that with a purely vegetable
diet the subject was able, for a few days at least, to maintain his
body in essentially a condition of nitrogen equilibrium on a daily
diet containing 8.4 grams of nitrogen, corresponding to 52.5 grams
of proteid. In addition, there was a large consumption of starchy
food with some fat. Klemperer,[57] experimenting with two young men,
having a body-weight of 64 and 65.5 kilos, respectively, was able to
keep them in a condition of nitrogenous equilibrium for a period of
eight days on 4.38 grams and 3.58 grams of nitrogen per day. The diet,
however, had a large fuel value, 5020 calories per day, and contained
in addition to the small amount of proteid, 264 grams of fat, 470
grams of carbohydrate, and 172 grams of alcohol. Breisacher,[58] in an
experiment on himself, using a mixed diet composed of 67.8 grams of
proteid, 494.2 grams of carbohydrate, and 60.5 grams of fat per day,
with a total fuel value of 2866 calories, observed a daily excretion
of nitrogen during thirty days of 8.23 grams. This corresponds to a
metabolism of 51.4 grams of proteid, thus showing that the 67 grams
of food-proteid taken was quite sufficient to maintain nitrogen
equilibrium for the above length of time.

  [56] Carl Voit: Ueber die Kost eines Vegetariers. Zeitschrift für
  Biologie, Band 25, p. 232.

  [57] Klemperer: Untersuchungen über Stoffwechsel und Ernährung in
  Krankheiten. Zeitschrift für klin. Medizin, Band 16, p. 550.

  [58] L. Breisacher: Ueber die Grösse des Eiweissbedarfs beim
  Menschen. Deutsche med. Wochenschrift. 1891. No. 48.

Caspari and Glässner[59] have reported observations made on two
vegetarians, a man and his wife, aged 49 and 48 years respectively, who
had lived for some years exclusively on a vegetable diet. The man had
a body-weight of 68.8 kilos, while the woman weighed 58 kilos. During
five days, the man consumed per day, on an average, 7.83 grams of
nitrogen and 4559 calories. This corresponds to 0.114 gram of nitrogen
per kilo of body-weight, and 66 calories per kilo. On this diet, the
man gained slightly in weight and showed a plus nitrogen balance of
5.2 grams for the five days. In other words, even this low nitrogen or
proteid intake was more than sufficient to meet the wants of his body.
The wife, during the same period of time, consumed per day 5.33 grams
of nitrogen and 2715 calories, corresponding to 0.092 gram of nitrogen
per kilo of body-weight and 47 calories per kilo. On this diet, the
woman gained 0.9 kilo in weight during the five days, and like the
man, she showed a plus nitrogen balance of 2.45 grams for the entire
period. The somewhat noted experiments of Sivén have been referred to
in another connection, and it will suffice to recall the fact that
he was able, with a body-weight of 60 kilos, to establish nitrogen
equilibrium on 6.26 grams of nitrogen, and for a day or two on 4.5
grams of nitrogen, with a total fuel value of only 2444 calories in the
day’s ration.

  [59] W. Caspari: Physiologische Studien über Vegetarianismus. Bonn.
  1905. p. 13.

These few illustrations will serve to indicate that, so far as the
maintenance of nitrogen equilibrium is concerned during short periods
of time, there is no necessity for the consumption of proteid food in
such amounts as common usage dictates. The high proteid intake called
for by the “standard dietaries,” and the ordinary practices of mankind,
is not needed to establish a condition of nitrogen equilibrium. It
would seem, however, as if results of this nature, presented from
time to time by various investigators, have been considered more in
the light of scientific curiosities than as data having an important
bearing on physiological processes. So strong has been the hold upon
the medical and physiological mind of the necessity of high proteid
that such figures as the above have merely excited comment, without
weakening in any measure the prevalent conviction that health,
strength, and the power to work necessitate a high rate of proteid
exchange.

To one willing to accept the data as having possible significance there
arises at once the question, How long can the body be maintained in
nitrogen equilibrium on such relatively small quantities of proteid
food? In other words, can experiments of this nature, extending over
comparatively short periods of time, be safely accepted as a reliable
means of measuring the proteid requirements of the body for indefinite
periods? Suppose, says the critic, we grant that the body can maintain
itself in nitrogen equilibrium for a week or two on a very small amount
of proteid food, what proof have we that in the long run the body will
be benefited thereby, or even able to exist in a condition of normal
strength and vigor? In other words, is a low proteid diet, one that
seems sufficient to maintain the body in nitrogen equilibrium, a wholly
safe one to follow? May there not be other elements to be considered,
aside from nitrogen equilibrium, which, if fully understood, would
satisfactorily account for the customs of mankind, in which perhaps
man’s instincts have been followed for the betterment of the race? It
was with a view to learning more concerning these questions that five
years ago the writer commenced systematic, experimental, work upon the
nutrition of man, with special reference to his nitrogen requirements.
The experiments and observations have been continued up to the present
time, with many suggestive results, some of which will now be referred
to.[60]

  [60] In presenting the general results of these experiments, the
  writer has drawn freely from his book, “Physiological Economy in
  Nutrition,” published by the Frederick A. Stokes Company, New York,
  1904.

One group of subjects was composed of professional men, professors and
instructors in the university, whose work was mainly mental rather
than physical, though by no means excluding the latter. Of this group,
two cases will be referred to with some regard for detail, since in
no other way can so striking a picture be presented of the effects
produced. The first subject weighed 65 kilos in the fall of 1902, and
at that time was nearly 47 years of age. His dietetic habits were in
accord with common practice, and his daily consumption of proteid food
averaged close to 118 grams. With a clear recognition of the principle
that the habits of a lifetime should not be too suddenly changed, a
very gradual reduction in the total amount of food, and especially of
proteid matter, was made. This finally resulted, with this particular
subject, in the complete abolition of breakfast, with the exception of
a small cup of coffee. A light lunch was taken at noontime, followed by
a more substantial dinner at night. There was no change to a vegetable
diet, but naturally any attempt to cut off largely the amount of
proteid food necessarily results in a marked diminution in the quantity
of animal food or meats. It is a somewhat singular though suggestive
fact, that a change of this order gradually results in a stronger
liking for simple foods, with their more delicate flavor, accompanied
by a diminished desire for the heavier animal foods.

As the day’s ration was gradually reduced in amount, the body-weight
began to fall off, until after some months it became stationary at 57
kilos, at which point it has remained practically constant for over
three years. The sixteen pounds of weight lost was composed, mainly at
least, of superfluous fat. For a period of nine months, from October,
1903, to the end of June, 1904, the amount of proteid material broken
down in the body was determined each day. The average daily metabolism
of nitrogen for the entire period of nearly nine months amounted to
5.69 grams. For the last two months, it averaged 5.4 grams per day.
Analyses made from time to time since these figures were obtained show
that the subject is still living at the same low level of nitrogen
metabolism. In fact, the data available afford satisfactory proof
that for a period covering over three years this particular person
has subsisted on an amount of proteid food equal to a metabolism of
not more than 5.8 grams of nitrogen per day. It may be asked why the
subject should have continued such a low proteid diet after the nine
months’ period was completed? In reply, it may be said that the new
habit has taken a firm hold, and entirely supplanted the dietetic
desires and cravings of the preceding years. Further, the improved
condition of health, freedom from minor ailments that formerly caused
inconvenience and discomfort, and the greater ability to work without
fatigue, have all combined to place the new habit on a firm basis, from
which there is no desire to change.

Consider for a moment what this lowered consumption of proteid food
really amounts to, as compared with ordinary usage and the so-called
dietary standards. The latter call for at least 118 grams of proteid
or albuminous food daily, of which 105 grams should be absorbable, in
order to maintain the body in a condition of nitrogen equilibrium,
and in a state of physical vigor and general tone. This would mean a
daily metabolism and excretion of at least 16 grams of nitrogen. Our
subject, however, excreted per day, during nine months, only 5.69 grams
of nitrogen, which means a metabolism of 35.6 grams of proteid; _i.
e._, about one-third the amount ordinarily deemed necessary to meet
man’s requirement for proteid food. But was our subject in nitrogen
equilibrium on this small amount of proteid food? We answer yes, as the
following balance period shows:

                                          Output.
             Nitrogen in   Nitrogen through   Weight of Excrement
                Food.          Kidneys.              (dry).
  March 20   6.989 grams.    5.91 grams.           3.6 grams.
        21   6.621           5.52                   ..
        22   6.082           5.94                 12.0
        23   6.793           5.61                 18.5
        24   5.057           4.31                 23.0
        25   6.966           5.39                 16.9
             -----           ----                 ----
                                                  74.0 grams contain
                                                       6.42% N.

            38.508          32.68       +         4.75 grams nitrogen.
            ------          --------------------------
            38.508 grams nitrogen.     37.43 grams nitrogen.

  Nitrogen balance for six days  =  +1.078 grams.
  Nitrogen balance per day       =  +0.179 gram.

In this particular period of six days, the body was really gaining a
little nitrogen, _i. e._, storing away a small amount of proteid for
future use, although it may be granted that the amount was too small to
have any special significance. During this period, the average daily
intake of nitrogen was 6.4 grams, equal to 40 grams of proteid food.
The average daily output of nitrogen through kidneys and excrement was
6.24 grams. The average daily output of metabolized nitrogen, through
the kidneys, was 5.44 grams, corresponding to the breaking down of 34
grams of proteid material. Further, it should be stated that the total
calorific value of the daily food during this period was less than 2000
calories. Let me add now a final balance period taken at the close of
the nine months’ trial:

                                         Output.
             Nitrogen in   Nitrogen through   Weight of Excrement
                Food.          Kidneys.             (dry).
  June 23    6.622 grams.    5.26 grams.          10.6 grams.
       24    6.331           5.30                 30.7
       25    4.941           4.43                 14.2
       26    5.922           4.66                 11.9
       27    5.486           4.98                 15.2
             -----           ----                 ----
                                                  82.6 grams contain
                                                       6.08% N.

            29.302          24.63       +       5.022 grams nitrogen.
            ------         --------------------------
            29.302 grams nitrogen.   29.652 grams nitrogen.


  Nitrogen balance for five days  = -0.350 gram.
  Nitrogen balance per day        = -0.070 gram.

In this period of five days, the average daily intake of nitrogen was
5.86 grams, corresponding to 36.6 grams of proteid food. The average
daily output of metabolized nitrogen was 4.92 grams, implying the
breaking down in the body of only 30.7 grams of proteid material
per day. The fuel value of the daily food, calculated as closely as
possible, was less than 2000 calories. The body was essentially in
nitrogen equilibrium, the minus balance being too small to have any
special significance.

It will be instructive to consider next the actual character and amount
of the diet made use of on several of these balance days:


_March 21._

  Breakfast.--Coffee 119 grams, cream 30 grams, sugar 9 grams.

  Lunch.--One shredded wheat biscuit 31 grams, cream 116 grams, wheat
  gem 33 grams, butter 7 grams, tea 185 grams, sugar 10 grams, cream
  cake 53 grams.

  Dinner.--Pea soup 114 grams, lamb chop 24 grams, boiled sweet potato
  47 grams, wheat gems 76 grams, butter 13 grams, cream cake 52 grams,
  coffee 61 grams, sugar 10 grams, cheese crackers 16 grams.

    Total nitrogen content of the day’s food = 6.621 grams.


_June 24._

  Breakfast.--Coffee 96 grams, cream 32 grams, sugar 8 grams.

  Lunch.--Creamed codfish 89 grams, baked potato 95 grams, butter 10
  grams, hominy gems 58 grams, strawberries 86 grams, sugar 26 grams,
  ginger snaps 47 grams, water.

  Dinner.--Cold tongue 14 grams, fried potato 48 grams, peas 60 grams,
  wheat gems 30 grams, butter 11 grams, lettuce-orange salad with
  mayonnaise dressing 155 grams, crackers 22 grams, cream cheese 14
  grams, ginger snaps 22 grams, coffee 58 grams, sugar 10 grams.

    Total nitrogen content of the day’s food = 6.331 grams.


_June 25._

  Breakfast.--Coffee 101 grams, cream 36 grams, sugar 13 grams.

  Lunch.--Omelette 50 grams, bacon 9 grams, French fried potato 23
  grams, biscuit 29 grams, butter 8 grams, ginger snaps 42 grams, cream
  cheese 17 grams, iced tea 150 grams, sugar 15 grams.

  Dinner.--Wheat popovers 57 grams, butter 10 grams, lettuce-orange
  salad with mayonnaise dressing 147 grams, crackers 22 grams, cream
  cheese 21 grams, cottage pudding 82 grams, coffee 48 grams, sugar 11
  grams.

    Total nitrogen content of the day’s food = 4.941 grams.


_June 27._

  Breakfast.--Coffee 112 grams, cream 22 grams, sugar 10 grams.

  Lunch.--Roast lamb 9 grams, baked potato 90 grams, wheat gems 47
  grams, butter 12 grams, iced tea 250 grams, sugar 25 grams, vanilla
  éclair 47 grams.

  Dinner.--Lamb chop 32 grams, creamed potato 107 grams, asparagus 49
  grams, bread 35 grams, butter 17 grams, lettuce-orange salad with
  mayonnaise dressing 150 grams, crackers 21 grams, cream cheese 12
  grams, coffee 63 grams, sugar 9 grams.

    Total nitrogen content of the day’s food = 5.486 grams.


It can be seen that there was nothing especially peculiar in these
dietaries, aside from their simplicity, except that the quantities
were small. Meat was not excluded; there was no approach to a cereal
diet; there were no fads involved, nothing but simple moderation
in the amounts of nitrogen-containing foods. Further, there was
perfect freedom of choice; full latitude to consider personal likes
and dislikes in the selection of foods; anything that appealed to
the appetite could be eaten, with the simple restriction that the
amount taken must be small. During the balance days, naturally, every
article of food had to be carefully weighed and analyzed, which fact
undoubtedly tended to limit in some degree the variety of foods chosen,
since increase in the number of articles meant increased labor in
analysis. Quite noticeable, however, was the extreme constancy in the
nitrogen-content of the daily diet, even on those days when the food
was not weighed. In other words, there had been gradually acquired
a new habit of food consumption, and the individual, unconsciously
perhaps, rarely overstepped the limits fixed by the new level of
proteid metabolism. This is a fact that has been conspicuous in nearly
all of our experiments, where freedom of choice in the taking of food
has been followed; and is in harmony with the view that after a lower
level of proteid metabolism has once been established, and the body has
become accustomed to the new conditions, there is little tendency for
any marked deviation from the new standards of food consumption.

With maintenance of body-weight, together with nitrogen equilibrium
through all these months; and with health, strength, and mental and
physical vigor unimpaired, there is certainly ground for the belief
that the real needs of the body were as fully met by the lowered
consumption of proteid food as by the quantities called for by the
customary standards. Finally, it should be noted that this particular
subject was small in weight, and hence did not need so much proteid
as a man of heavier body-weight would require. In recognizing this
principle, we may for future comparison calculate the nitrogen
requirement of the body, on the basis of the present results, per kilo
of body-weight. With the weight of the subject placed at 57 kilos, and
with an average daily excretion of nitrogen amounting to practically
5.7 grams, it is plain that this individual was quite able to maintain
a condition of equilibrium with a metabolism of 0.1 gram of nitrogen
per kilo of body-weight. Translated into terms of proteid matter, this
would mean a utilization by the body of 0.625 gram of proteid daily
per kilo of body-weight. Regarding the fuel value of the daily food,
we need not be more precise than to emphasize the fact that so far as
could be determined, on the basis of chemical composition, the heat
value of the food rarely exceeded 1900 calories per day. If we make a
liberal allowance, for the sake of precaution, it would seem quite safe
to say that this particular individual, under the conditions of life
and bodily activity prevailing, did not apparently need of fuel value
more than 2000 calories per day, which would correspond to 35 calories
per kilo of body-weight.

Let us turn now to the second subject in this group, a man of 76 kilos
body-weight, 32 years of age, and of strong physique. His active life
in the laboratory called for greater physical exertion than the former
subject, and consequently there was need for greater consumption of
non-nitrogenous food, with the accompanying increase in fuel value of
the day’s ration. As in the preceding case, there was no prescribing
of food, but a gradual and voluntary diminution of proteid material.
During the last seven months and a half of the experiment, the average
daily excretion of nitrogen through the kidneys amounted to 6.53 grams,
equivalent to a metabolism of 40.8 grams of proteid matter daily; a
little more than one-third the minimal quantity called for by common
usage. At first, the body-weight of the subject gradually fell until it
reached 70 kilos, at which point it remained fairly constant during the
last five months. That the quantity of food taken was quite sufficient
to maintain the body in a condition of nitrogen equilibrium is apparent
from the results of a comparison of income and outgo of nitrogen, as
shown in the following table:

                                        Output.
            Nitrogen in   Nitrogen through   Weight of Excrement
               Food.          Kidneys.             (dry).
  May 18    8.668 grams.     6.06 grams.         14 grams.
      19    6.474            7.17                39
      20    6.691            6.33                30
                                                 --
      21    8.345            6.78                83 contain 6.06% N.
                                                          = 5.03 grm. N.
      22    7.015            5.70                ..
      23    9.726            5.75                38
      24   10.424            6.39                57
           ------            ----                --
                                                 95 contain 5.76% N.
                                                          = 5.47 grm. N.
                                                            ----
                                                           10.50 grm. N.

           57.343           44.18 + 10.50 grams nitrogen.
           ------           -------------
           57.343 grams N.       54.68 grams nitrogen.

       Nitrogen balance for seven days = +2.663 grams.
       Nitrogen balance per day        = +0.380 gram.

The average daily intake of nitrogen was 8.192 grams, equivalent to
51.2 grams of proteid food. The average amount of nitrogen excreted
through the kidneys each day was 6.31 grams, corresponding to a
metabolism of 39.43 grams of proteid matter. The plus balance of 0.380
gram of nitrogen per day shows that not only was the amount of proteid
food consumed quite adequate to meet the demands of the body, but the
latter was able to store up 2.3 grams of proteid per day. Regarding
the character of the food taken by this subject, it should be stated
that there was gradually developed a tendency toward a pure vegetarian
diet. During the last seven months of the experiment, meats were
almost entirely excluded. The diet voluntarily selected thus differed
decidedly from that of the preceding subject in that it was much more
bulky, contained a larger proportion of undigestible vegetable matter,
and was richer in fats and carbohydrates, with a corresponding increase
in fuel value. The exact character of the daily dietary is indicated
by the following data of food consumption, on four of the days of the
above balance period:


_May 19._

  Breakfast.--Banana 102 grams, wheat rolls 50 grams, coffee 150 grams,
  cream 50 grams, sugar 21 grams.

  Lunch.--Omelette 20 grams, bread 57 grams, hominy 137 grams, syrup 68
  grams, potatoes 128 grams, coffee 100 grams, cream 50 grams, sugar 21
  grams.

  Dinner.--Tomato purée 200 grams, bread 24 grams, fried sweet potato
  100 grams, spinach 70 grams, Indian meal 100 grams, syrup 25 grams,
  coffee 100 grams, cream 40 grams, sugar 21 grams.

    Total nitrogen content of the day’s food = 6.474 grams.


_May 20._

  Breakfast.--Sliced orange 140 grams, coffee 100 grams, cream 30
  grams, sugar 21 grams.

  Lunch.--Lima beans 40 grams, mashed potato 250 grams, bread 28 grams,
  fried hominy 115 grams, syrup 48 grams, coffee 100 grams, cream 30
  grams, sugar 21 grams.

  Dinner.--Consommé 150 grams, string beans 140 grams, mashed potato
  250 grams, rice croquette 93 grams, syrup 25 grams, cranberry jam 95
  grams, bread 19 grams, coffee 100 grams, cream 30 grams, sugar 21
  grams.

    Total nitrogen content of the day’s food = 6.691 grams.


_May 21._

  Breakfast.--Banana 153 grams, coffee 150 grams, cream 30 grams, sugar
  21 grams.

  Lunch.--Potato croquette 229 grams, bread 25 grams, tomato 123 grams,
  Indian meal 109 grams, syrup 48 grams, coffee 100 grams, cream 20
  grams, sugar 14 grams.

  Dinner.--Bean soup 100 grams, bacon 5 grams, fried potato 200 grams,
  bread 31 grams, lettuce-orange salad 47 grams, prunes 137 grams,
  coffee 100 grams, cream 25 grams, sugar 21 grams, banana 255 grams.

    Total nitrogen content of the day’s food = 8.345 grams.


_May 23._

  Breakfast.--Banana 229 grams, coffee 125 grams, cream 25 grams, sugar
  21 grams.

  Lunch.--Consommé 75 grams, scrambled egg 15 grams, bread 58 grams,
  apple sauce 125 grams, fried potato 170 grams, rice croquette 197
  grams, syrup 68 grams, coffee 100 grams, cream 30 grams, sugar 21
  grams.

  Dinner.--Vegetable soup 100 grams, potato croquette 198 grams, bread
  73 grams, bacon 7 grams, string beans 120 grams, water ice 77 grams,
  banana 270 grams, coffee 100 grams, cream 30 grams, sugar 14 grams.

    Total nitrogen content of the day’s food = 9.726 grams.


While the critic might justly say that these dietaries lack variety
and would not appeal to a fastidious taste, there is force in the
illustration which they afford of a simple diet being quite adequate
to meet the wants of the body. Further, it should be emphasized that
there is no special virtue in any of these dietaries, aside from their
simplicity and low content of nitrogen. They represent individual
taste and selection. Any other form of diet would answer as well,
provided there was not too large an intake of proteid, and provided
further the fuel value of the day’s ration was sufficient to meet the
requirements for heat and work. Again, it might be said that with this
latter subject the daily consumption of proteid food was considerably
larger than with the first subject. This is indeed true, but it must be
remembered that the second subject had a body-weight of 70 kilos during
the last seven months, while the first subject weighed only 57 kilos.
Obviously, with this marked difference in the weight of living tissue
there must be a corresponding difference in the extent of proteid
katabolism, and consequently a difference in the demand for proteid
food.

As we have seen, the smaller subject for a period of many months
showed a proteid katabolism equal to 0.1 gram of nitrogen, per kilo
of body-weight, daily. The second and larger subject, on a totally
different diet, for seven months and a half, metabolized daily, on
an average, 6.53 grams of nitrogen. Taking the weight of the body at
70 kilos, it is readily seen that the nitrogen metabolized daily per
kilo of body-weight was 0.093 gram, almost identical with the rate of
nitrogen exchange found with the first subject. It is certainly very
suggestive that these two individuals with their marked difference
in body-weight, under different degrees of physical activity, and
living on different forms of diet, with only the one point in common
of voluntary restriction in the amount of proteid food, until a new
habit had been acquired and a new level of proteid metabolism attained,
should have quite independently reached exactly the same level of
nitrogen exchange per kilo of body-weight. And when it is remembered
that this was attained by the daily consumption of not more than
one-third to one-half the minimal amount of proteid food called for
by the dietetic customs of mankind, and with maintenance of all the
characteristics of good health through this comparatively long period
of time, there certainly seems to be justification for the opinion
that the consumption of proteid food, as practised by the people of
the present generation, is far in excess of the needs of the body.
Referring for a moment to the calorific value of the food used by the
second subject, in the last balance period, it is to be noted that the
heat value per day averaged 2448 calories, as estimated on the basis of
the chemical composition of the food. This would amount to 34 calories
per kilo. Whether this figure is strictly correct is immaterial; it is
certainly sufficiently so to warrant the statement that the needs of
the body were fully met by an intake of food below the standards set
by usage, and that maintenance of nitrogen equilibrium on a greatly
diminished consumption of proteid food is possible without increasing
the intake of non-nitrogenous matter.

Finally, as affording additional evidence, we may refer to a third
subject in this group, a man of 65 kilos body-weight, 26 years of age,
who for a period of six consecutive months maintained body-weight,
nitrogen equilibrium, and a general condition of good health, with a
proteid metabolism equal to 7.81 grams of nitrogen per day. During the
last two months of the experiment, the average excretion of nitrogen
per day amounted to 6.68 grams, corresponding to a metabolism of
0.102 gram of nitrogen per kilo of body-weight. This figure, it will
be noted, is practically identical with the values obtained with the
preceding subjects, calculated to the same unit of weight. Further,
this third subject did not reduce his nitrogen intake by an exclusion
of meat, but made use of his ordinary diet gradually reduced in amount.
His daily consumption of proteid food averaged 55 grams, or 8.83 grams
of nitrogen, and on this amount of proteid, without increasing the
intake of fats and carbohydrates, he was quite able to do his work with
preservation of physiological equilibrium.

Views so radically different from those commonly accepted can be made
to carry weight, only by the accumulation of supporting evidence
obtained under widely different conditions of life, and by methods
which will defy criticism. It might be argued, and with perhaps some
justification, that while professional men, with freedom from muscular
work, may be able to live without detriment on a relatively small
amount of proteid food, such a conclusion would not be warranted for
the great majority of mankind with their necessarily greater muscular
activity. We are confronted at once with the oft-heard statement that
the laboring man requires more proteid food; he has a more vigorous
appetite, and he must take an abundance of meat and other foods
rich in proteid, if he is to maintain his ability as a worker. Note
the statements already made in other connections regarding the food
consumption of Maine lumbermen, of men on the football team, of trained
athletes in general. These men consume large amounts of proteid daily,
because their work demands it. If the demand did not really exist, they
would not so agree in the use of high proteid standards, so runs the
argument. The custom certainly does exist and is almost universally
followed; men in training for athletic events deem it necessary to
consume large amounts of proteid food. Custom and long experience
sanction a high proteid diet, rich in nitrogen, for the development
and maintenance of that strength and vigor that help to make the
accomplished athlete. It is common knowledge to-day, however, that the
energy of muscle work does not have its origin in the breaking down of
proteid material, certainly not when there is an adequate amount of
fat and carbohydrate in the diet. A high proteid intake must therefore
be called for because of some subtle quality, not at present fully
understood. It must not be subjected to criticism, however, because it
is sanctioned by custom, habit, and common usage.

Still, I have ventured to experiment somewhat with a group of eight
university athletes, all trained men, and with some surprising results.
We have not space for details, but it may be mentioned that the men
were young, from 22 to 27 years of age, and were experts in some field
of athletic work. By a preliminary study of their ordinary dietetic
habits, it was found that they were all large consumers of proteid
food, with a corresponding high rate of proteid katabolism. One subject
of 92 kilos body-weight, during ten days, showed an average daily
excretion through the kidneys of 22.79 grams of nitrogen, implying a
metabolism of 142 grams of proteid matter per day. On one of these
days, the nitrogen excretion reached the high figure of 31.99 grams,
corresponding to a metabolism of about 200 grams of proteid matter.
Calculated per kilo of body-weight, this means a metabolism of 0.35
gram of nitrogen, or three and a half times the amount needed by the
three professional men for the maintenance of nitrogen equilibrium.
These subjects, with an intelligent comprehension of the point
at issue, and with full freedom in the choice of food, gradually
diminished their daily consumption of proteid material, at the same
time cutting down very markedly the total consumption of food. The
experiment extended through five months, and during the last two
months, the average daily excretion of metabolized nitrogen of the
eight men amounted to 8.81 grams per man. This corresponds to a
metabolism of 55 grams of proteid matter.

Further, the average daily output of nitrogen through the kidneys
during the preceding two months was in many cases nearly, if not quite,
as low as during the last two months of the experiment. If we contrast
this average daily exchange of 8.81 grams of nitrogen with the average
output prior to the change in diet, it is easy to see that the men were
living on about one-half the amount of proteid food they were formerly
accustomed to take. Moreover, if the metabolized nitrogen for each
individual, with one exception, is calculated per kilo of body-weight,
it is seen to vary from 0.108 gram to 0.134 gram; somewhat higher than
was observed with the older professional men, but not conspicuously so.
Again, it is to be emphasized that the lowered intake of proteid food
with these men was quite adequate to maintain their bodies in nitrogen
equilibrium. We may cite a single case by way of illustration:

                                       Output.
           Nitrogen of   Nitrogen through   Weight of Excrement
              Food.          Kidneys.             (dry).
  May 18   8.119 grams.     5.75 grams.         .. grams.
      19   9.482            6.64                15
      20  10.560            8.45                ..
      21   8.992            8.64                ..
      22   9.025            8.53                ..
      23   8.393            7.69                89
      24   7.284            7.34                24
           -----            ----               ---
                                               128 grams contain
                                                   6.40 % N.

          61.855           53.04   +   8.192 grams nitrogen.
          ------           -----------------
          61.855 grams nitrogen.  61.232 grams nitrogen.

      Nitrogen balance for seven days  =  +0.623 gram.
      Nitrogen balance per day         =  +0.089 gram.

The daily intake of nitrogen during this balance period averaged 8.83
grams, corresponding to 55.1 grams of proteid food. The metabolized
nitrogen eliminated through the kidneys averaged 7.58 grams per day,
thus showing a daily average metabolism of 47.37 grams of proteid
matter. With a body-weight of 63 kilos, this individual was maintaining
equilibrium on a metabolism of 0.120 gram of nitrogen per kilo of
body-weight. The fuel value of the day’s food as estimated did not
exceed 2800 calories, thus substantiating the general statement that
there is no need for increasing the fuel value of the food in any
attempt to maintain a lower nitrogen level. This particular individual,
in his choice of food, unconsciously drifted--as he expressed
it--toward a simple vegetable diet, without, however, excluding meat
entirely. The following four dietaries will serve to illustrate the
character and amount of his daily food:


_May 21._

  Breakfast.--Banana 106 grams, boiled Indian meal 150 grams, cream 50
  grams, sugar 21 grams, bread 59 grams, butter 16 grams.

  Lunch.--Lamb chop 37 grams, potato croquette 105 grams, tomato 216
  grams, bread 55 grams, butter 13 grams, sugar 14 grams, water ice 143
  grams.

  Dinner.--Bean soup 100 grams, bacon 10 grams, fried egg 22 grams,
  fried potato 100 grams, lettuce salad 63 grams, coffee 100 grams,
  cream 50 grams, sugar 21 grams, stewed prunes 247 grams.

    Total nitrogen content of the day’s food = 8.992 grams.


_May 22._

  Breakfast.--Orange 60 grams, oatmeal 207 grams, roll 46 grams, butter
  14 grams, coffee 150 grams, cream 150 grams, sugar 35 grams.

  Lunch.--Boiled potato 150 grams, boiled onions 145 grams, macaroni
  130 grams, fried rice 138 grams, syrup 48 grams, ice cream 160 grams,
  cake 26 grams.

  Dinner.--Celery soup 150 grams, spinach 100 grams, mashed potato 100
  grams, bread 19 grams, coffee 100 grams, cream 50 grams, sugar 7
  grams, strawberry short-cake 169 grams.

    Total nitrogen content of the day’s food = 9.025 grams.


_May 23._

  Breakfast.--Sliced banana 201 grams, cream 100 grams, sugar 28 grams,
  griddle cakes 103 grams, syrup 48 grams.

  Lunch.--Consommé 150 grams, rice croquette 140 grams, syrup 48 grams,
  fried potato 100 grams, bread 36 grams, butter 15 grams, apple sauce
  90 grams, coffee 75 grams, sugar 7 grams.

  Dinner.--Vegetable soup 100 grams, bacon 20 grams, potato croquette
  50 grams, string beans 120 grams, macaroni 104 grams, bread 26 grams,
  water ice 184 grams.

    Total nitrogen content of the day’s food = 8.393 grams.


_May 24._

  Breakfast.--Orange 80 grams, fried rice 186 grams, syrup 72 grams,
  coffee 100 grams, cream 50 grams, sugar 21 grams.

  Lunch.--Celery soup 125 grams, bread 34 grams, butter 19 grams,
  boiled onion 127 grams, boiled potato 150 grams, tomato sauce 50
  grams, stewed prunes 189 grams, cream 50 grams.

  Dinner.--Tomato soup 125 grams, bread 21 grams, fried potato 100
  grams, spinach 130 grams, cream pie 158 grams, coffee 100 grams,
  cream 50 grams, sugar 14 grams.

  Evening.--Ginger ale 250 grams.

    Total nitrogen content of the day’s food = 7.284 grams.


Here, again, we have dietaries not particularly attractive to every
one, but they represent the choice of an individual who was following
his own preferences, and like the preceding dietaries they are
characterized by simplicity. In any event, they were quite adequate
for the wants of the body, and their value to us lies in the proof
they afford that a relatively small intake of proteid food will not
only bring about and maintain nitrogen equilibrium for many months,
and probably indefinitely, but that such a form of diet is equally as
effective with vigorous athletes, accustomed to strenuous muscular
effort, as with professional men of more sedentary habits. Further,
these many months of observation with different individuals all lead to
the opinion that there are no harmful results of any kind produced by
a reduction in the amount of proteid food to a level commensurate with
the actual needs of the body. Body-weight, health, physical strength,
and muscular tone can all be maintained, in partial illustration of
which may be offered two photographs of one of the eight athletes
taken toward the end of the experiment; pictures which are certainly
the antithesis of enfeebled muscular structure, or diminished physical
vigor.

[Illustration: STAPLETON

_Photograph taken in the middle of the experiment, in April_]




CHAPTER VI

FURTHER EXPERIMENTS AND OBSERVATIONS BEARING ON TRUE FOOD REQUIREMENTS

  TOPICS: Dietary experiments with a detail of soldiers from the United
  States army. General character of the army ration. Samples of the
  daily dietary adopted. Rate of nitrogen metabolism attained. Effect
  on body-weight. Nitrogen balance with lowered proteid consumption.
  Influence of low proteid on muscular strength of soldiers and
  athletes. Effect on fatigue. Effect on physical endurance. Fisher’s
  experiments on endurance. Dangers of underfeeding. Dietary
  observations on fruitarians. Observations on Japanese. Recent dietary
  changes in Japanese army and navy. Observations of Dr. Hunt on
  resistance of low proteid animals to poisons. Conclusions.


General acceptance of a new theory, or a new point of view, can be
expected only when there is an adequate amount of scientific evidence
on which the theory can safely rest. Facts cannot be ignored, and the
larger the amount of supporting evidence the more certain becomes
the general truth of the theory to which it points. Corroborative
evidence, therefore, is always desirable, and he who would open up a
new point of view must be zealous in accumulating facts to uphold his
position. Critics there are without number who are ever ready to pick
flaws in an argument or overturn a theory, especially if the one or
the other stands opposed to their own point of view. This, however,
is highly advantageous for the advance of sound knowledge, since it
necessarily prompts the advocate to search in all directions for added
data, by which he can build a bulwark of fact sufficient to defy just
criticism. Further, the true scientific spirit demands persistent
and painstaking effort in the search after truth, that error and
misconception may be avoided.

In harmony with these ideas, our attempt to ascertain the real needs of
the body for proteid food led us to enlarge our evidence by a series
of experiments with still another body of men, _i. e._, a detail of
soldiers from the United States army.[61] This was a somewhat more
difficult and ambitious undertaking, since the number of subjects
involved was larger, and because with this group of men we could not
expect quite that high degree of intelligent co-operation afforded by
the preceding subjects. Still, this very fact was in a sense an added
inducement, since it offered the opportunity of experimenting with a
body of men who naturally would not take kindly to anything that looked
like deprivation, and whose continued co-operation could be expected
only by satisfying their natural demands for food. If this could be
accomplished by an intelligent prescription in their daily diet, and
the experiment brought to a successful conclusion, with maintenance of
body-weight, nitrogen equilibrium, health, strength, and general vigor;
with an intake of proteid food essentially equal to that adopted by the
preceding subjects, corroborative evidence of the highest value would
be obtained.

  [61] In presenting the general results of these experiments, the
  writer has drawn freely from his book, “Physiological Economy in
  Nutrition,” published by the Frederick A. Stokes Company, New York,
  1904.

The detail was composed of a detachment of twenty men from the
Hospital Corps of the army, under the command of a first lieutenant
and assistant surgeon. They were located in a convenient house near
to the laboratory, where they lived during their six months’ stay in
New Haven, under military discipline, and subject to the constant
surveillance of the commanding officer and the non-commissioned
officers. Having well-trained cooks and assistants, with all necessary
facilities for preparing and serving their food, with members of
the laboratory staff to superintend the weighing of the food as it
was placed before the men, and with intelligent clerks to attend
to the many details connected with such an undertaking, a somewhat
unique physiological experiment was started. Thirteen members of the
detachment really took part in the experiment as subjects, and they
represented a great variety of types: of different ages, nationalities,
temperaments, and degrees of intelligence. They were men accustomed
to living an active life under varying conditions, and they naturally
had great liking for the pleasures of eating. Further, it should be
remembered that, although the men had volunteered for the experiment,
they had no personal interest whatever in the principles involved,
and it could not be expected that they would willingly incommode
themselves, or suffer any great amount of personal inconvenience.
Again, there were necessary restrictions placed upon their movements,
when relieved from duty, which constituted something of a hardship in
the minds of many of the men and added to the irksomeness and monotony
of their daily life. Regularity of life was insisted upon, and this
was a condition which brought to some of the men a new experience.
These facts are mentioned because their recital will help to make clear
that, from the standpoint of the men, there were certain depressing
influences connected with the experiment which would add to any
personal discomfort caused by restriction of diet.

The ordinary army ration to which these men were accustomed was rich in
proteid, especially in meat, and during the first few days they were
allowed to follow their usual dietary habits, in order that data might
be obtained bearing on their average food consumption. The details of
one day’s food intake will suffice to show the average character and
amount of the food eaten per man:

  Breakfast.--Beefsteak 222 grams, gravy 68 grams, fried potatoes 234
  grams, onions 34 grams, bread 144 grams, coffee 679 grams, sugar 18
  grams.

  Dinner.--Beef 171 grams, boiled potatoes 350 grams, onions 55 grams,
  bread 234 grams, coffee 916 grams, sugar 27 grams.

  Supper.--Corned beef 195 grams, potatoes 170 grams, onions 21 grams,
  bread 158 grams, fruit jelly 107 grams, coffee 450 grams, sugar 21
  grams.

It is not necessary to comment upon the large proportion of proteid
matter in the day’s ration; the three large portions of meat testify
clearly enough to that fact, while the three equally large volumes of
coffee indicate a natural disposition toward generous consumption of
anything available. Habit, reinforced by inclination, had evidently
placed these men on a high plane of food consumption.

For a period of six months, a daily dietary was prescribed for the
subjects; the food for each meal and for every man being of known
composition, each article being carefully weighed, while the content of
nitrogen in the day’s ration was so graded as to bring about a gradual
reduction in the amount of proteid ingested. The rate of proteid
katabolism was likewise determined each day by careful estimation of
the excreted nitrogen, balance experiments being made from time to
time in order to ascertain if the men were in a condition of nitrogen
equilibrium. Finally, it should be mentioned that the subjects lived
a fairly active life, having each day a certain amount of prescribed
exercise in the university gymnasium, in addition to the regular drill
and other duties associated with their usual work.

[Illustration: _Photograph of the soldiers taken at the close of the
experiment_]

[Illustration: _Photograph of the soldiers taken at the close of the
experiment_]

As just stated, the amount of proteid food was gradually reduced,
three weeks being taken to bring the amount down to a level somewhat
commensurate with the estimated needs of the body. This naturally
resulted in diminishing largely the intake of meat, though by no means
entirely excluding it. Effort was constantly made to introduce as
much variety as was possible with simple foods, though the main problem
with this group of men was to keep the volume of the food up to such
a point as would dispel any notion that they were not having enough
to eat. A second problem, which at first threatened trouble, was the
fear of the men, as they saw the proportion of meat gradually drop
off, that they were destined to lose their strength; but fortunately,
they very soon began to realize that their fears in this direction
were groundless, and a little later their personal experience opened
their eyes to possible advantages which quickly drove away all further
thought of danger, and made them quite content to continue the
experiment. We may introduce here a few samples of the daily food given
to the men after they had reached their lower level of proteid intake:


_January 15._

  Breakfast.--Wheat griddle cakes 200 grams, syrup 50 grams, one cup
  coffee[62] 350 grams.

  Dinner.--Codfish balls (4 parts potato, 1 part fish, fried in pork
  fat) 150 grams, stewed tomato 200 grams, bread 75 grams, one cup
  coffee 350 grams, apple pie 95 grams.

  Supper.--Apple fritters 200 grams, stewed prunes 125 grams, bread 50
  grams, butter 15 grams, one cup tea 350 grams.

    Total nitrogen content of the day’s food = 8.560 grams.


  [62] The coffee was prepared with milk and sugar.


_January 16._

  Breakfast.--Soft oatmeal 150 grams, milk 100 grams, sugar 30 grams,
  bread 30 grams, butter 10 grams, one cup coffee 350 grams.

  Dinner.--Baked macaroni with a little cheese 200 grams, stewed tomato
  200 grams, bread 50 grams, tapioca-peach pudding 150 grams, one cup
  coffee 350 grams.

  Supper.--Fried bacon 20 grams, French fried potato 100 grams, bread
  75 grams, jam 75 grams, one cup tea 350 grams.

    Total nitrogen content of the day’s food = 7.282 grams.


_March 1._

  Breakfast.--Fried rice 150 grams, syrup 50 grams, baked potato 150
  grams, butter 10 grams, one cup coffee 350 grams.

  Dinner.--Thick pea soup 250 grams, boiled onions 150 grams, boiled
  sweet potato 150 grams, bread 75 grams, butter 20 grams, one cup
  coffee 350 grams.

  Supper.--Celery-lettuce-apple salad 120 grams, crackers 32 grams,
  American cheese 20 grams, potato chips 79 grams, one cup tea 350
  grams, rice custard 100 grams.

    Total nitrogen content of the day’s food = 7.825 grams.


_March 3._

  Breakfast.--Boiled hominy 175 grams, milk 125 grams, sugar 25 grams,
  baked potato 150 grams, butter 10 grams, one cup coffee 350 grams.

  Dinner.--Hamburg steak with much bread, fat, and onions 150 grams,
  boiled potato 250 grams, bread 75 grams, butter 10 grams, one cup
  coffee 350 grams.

  Supper.--Tapioca-peach pudding 250 grams, bread 75 grams, butter 20
  grams, jam 75 grams, one cup tea 350 grams.

    Total nitrogen content of the day’s food = 8.750 grams.


_March 6._

  Breakfast.--Sliced banana 100 grams, fried Indian meal 150 grams,
  syrup 50 grams, baked potato 150 grams, butter 10 grams, one cup
  coffee 350 grams.

  Dinner.--Corned beef 50 grams, boiled cabbage 200 grams, mashed
  potato 250 grams, bread 75 grams, fried rice 100 grams, jam 75 grams,
  one cup coffee 350 grams.

  Supper.--Crackers 32 grams, butter 10 grams, sardine 14 grams, sponge
  cake 150 grams, apple sauce 150 grams, one cup tea 350 grams.

    Total nitrogen content of the day’s food = 10.265 grams.


_March 30._

  Breakfast.--Sliced banana 250 grams, fried hominy 150 grams, butter
  10 grams, syrup 75 grams, one cup coffee 350 grams.

  Dinner.--Codfish balls 125 grams, mashed potato 250 grams, stewed
  tomato 200 grams, bread 35 grams, apple sauce 200 grams, one cup
  coffee 350 grams.

  Supper.--Chopped fresh cabbage with salt, pepper, and vinegar 75
  grams, bread 50 grams, butter 20 grams, fried sweet potato 250 grams,
  cranberry sauce 200 grams, sponge cake 50 grams, one cup tea 350
  grams.

    Total nitrogen content of the day’s food = 9.356 grams.


_March 31._

  Breakfast.--Fried Indian meal 100 grams, syrup 75 grams, baked potato
  250 grams, butter 20 grams, one cup coffee 350 grams.

  Dinner.--Tomato soup, thick, with potatoes and onions boiled in, 300
  grams, scrambled egg 50 grams, mashed potato 200 grams, bread 50
  grams, butter 10 grams, one cup coffee 350 grams.

  Supper.--Fried bacon 20 grams, boiled potato 200 grams, butter 10
  grams, bread pudding 150 grams, sliced banana 200 grams, one cup tea
  350 grams.

  Total nitrogen content of the day’s food = 8.420 grams.


_April 1._

  Breakfast.--Fried hominy 150 grams, syrup 75 grams, baked potato 200
  grams, butter 20 grams, one cup coffee 350 grams.

  Dinner.--Baked spaghetti 200 grams, mashed potato 250 grams, boiled
  turnip 150 grams, bread 35 grams, butter 10 grams, apple sauce 200
  grams, one cup coffee 350 grams.

  Supper.--Fried bacon 25 grams, fried sweet potato 200 grams, bread
  35 grams, butter 20 grams, jam 100 grams, apple-tapioca pudding 300
  grams, one cup tea 350 grams.

    Total nitrogen content of the day’s food = 7.342 grams.


These dietaries are fair samples of the daily food given the men
during the last five months of the experiment. If we place the intake
of nitrogen at 8.5 grams per day, or even 9 grams daily, it would
mean at the most an average daily consumption of 56 grams of proteid;
viz., about one-third the amount they were accustomed to take under
their ordinary modes of life. Of greater interest, however, is
the rate of proteid katabolism shown by these men under the above
conditions of diet, during the five months’ period. The average
daily output of metabolized nitrogen for each man ranged from 7.03
grams--the lowest--to 8.91 grams--the highest. An excretion of 7.03
grams of nitrogen per day means a katabolism, or breaking down, of
43.9 grams of proteid matter; while the excretion of 8.91 grams of
nitrogen corresponds to a katabolism of 55.6 grams of proteid. The
grand average, _i. e._, the average daily output of nitrogen of all
the men for the five months’ period amounted to 7.8 grams per man,
corresponding to an average daily katabolism of 48.75 grams of proteid.
The heaviest man of the group had a body-weight of 74 kilograms, while
his average daily output of metabolized nitrogen amounted to 7.84
grams. This corresponds to 0.106 gram of metabolized nitrogen per kilo
of body-weight; a figure which agrees quite closely with the lowest
figures obtained with the preceding subjects when calculated to the
same unit of weight. Many of the men, however, metabolized considerably
more nitrogen or proteid in proportion to their body-weight, due in a
measure at least to the fact that they were being fed more liberally
with proteid food than was really necessary for the needs of the body.
In this group, we have a body of men doing a reasonable amount of
physical work, who lived without discomfort for five consecutive months
on a daily consumption of proteid food not much, if any, greater than
one-third the amount called for by common usage, and the average fuel
value of which certainly did not exceed 3000 calories per day. Indeed,
so far as could be determined on the basis of chemical composition, the
heat value of the food was quite a little less than this figure would
imply.

If the relatively small amount of proteid food made use of in this
trial was inadequate for the real necessities of the body, some
indication of it would be expected to reveal itself, with at least some
of the men, by the end of the period. One criticism frequently made is
that the subject draws in some measure upon his store of body material.
Should this be the case, it is evident that body-weight--in such a
long experiment as this--will gradually but surely diminish. Further,
the subject will show a minus nitrogen balance, _i. e._, there will
be a constant tendency for the body to give off more nitrogen than it
takes in. As bearing on the first point, the following table showing
the body-weights of the men at the commencement of the experiment
in October, and at the close of the experiment in April will be of
interest:


TABLE OF BODY-WEIGHTS

  +-------------+----------------+----------------+
  |             |  October, 1903 |  April, 1904   |
  +-------------+----------------+----------------+
  |             |      kilos     |    kilos       |
  | Steltz      |      52.3      |    53.0        |
  | Zooman      |      54.0      |    55.0        |
  | Coffman     |      59.1      |    58.0        |
  | Morris      |      59.2      |    59.0        |
  | Broyles     |      59.4      |    61.0        |
  | Loewenthal  |      60.1      |    59.0        |
  | Sliney      |      61.3      |    60.6        |
  | Cohn        |      65.0      |    62.6        |
  | Oakman      |      66.7      |    62.1        |
  | Henderson   |      71.3      |    71.0        |
  | Fritz       |      76.0      |    72.6        |
  | Bates       |      72.7      |    64.3 (Feb.) |
  | Davis       |      59.3      |    57.2 (Jan.) |
  +-------------+----------------+----------------+

As is readily seen, five of the men practically retained their weight
or made a slight gain. Of the others, Coffman, Loewenthal, Sliney, and
Cohn lost somewhat, but the amount was very small. Further, the loss
occurred during the first few weeks of the experiment, after which
their weight remained practically stationary. Fritz and Oakman lost
weight somewhat more noticeably, but this loss likewise occurred during
the earlier part of the trial. The accompanying photographs of Fritz,
taken at the close of the experiment, show plainly that such loss
of weight as he suffered did not detract from the appearance of his
well-developed musculature. Certainly, the photographs do not show any
signs of nitrogen starvation, or suggest the lack of any kind of food.

Of all the men, Bates was the only one who underwent any great loss of
weight. He, however, was quite stout, and the work in the gymnasium,
reinforced by the change in diet, brought about what was for him a very
desirable loss of body-weight. It is evident, therefore, that there was
no marked or prolonged loss of body-weight as a result of the continued
use of the low proteid diet. Regarding the second point, viz., nitrogen
equilibrium, the following illustrations will suffice to indicate the
relationship existing between the income and outgo of nitrogen. A
balance experiment with each of the men, lasting seven days, February
29 to March 6, is here shown, the figures given being the daily
averages for the period:

  +-------------+----------+-----------+-------------+----------+
  |             | Nitrogen | Nitrogen  | Nitrogen of | Nitrogen |
  |             | of Food. | of Urine. | Excrement.  | Balance. |
  +-------------+----------+-----------+-------------+----------+
  |             |  grams   |   grams   |    grams    |  grams   |
  | Oakman      |   9.52   |    7.24   |     1.76    |  +0.52   |
  | Henderson   |   9.40   |    7.90   |     1.00    |  +0.50   |
  | Morris      |   9.49   |    6.05   |     2.30    |  +1.14   |
  | Coffman     |   9.53   |    7.92   |     1.47    |  +0.14   |
  | Steltz      |   9.62   |    7.16   |     1.95    |  +0.51   |
  | Loewenthal  |   9.64   |    7.00   |     1.71    |  +0.95   |
  | Cohn        |   9.27   |    7.63   |     1.41    |  +0.23   |
  | Zooman      |   9.49   |    7.13   |     1.76    |  +0.60   |
  | Sliney      |   9.52   |    8.08   |     1.92    |  -0.48   |
  | Broyles     |   9.43   |    7.01   |     1.19    |  +1.23   |
  | Fritz       |   9.37   |    6.36   |     1.81    |  +1.20   |
  +-------------+----------+-----------+-------------+----------+

[Illustration: FRITZ

_At the close of the experiment_]

With one exception, all of the men were plainly having more proteid
food than was necessary to maintain the body in nitrogen
equilibrium, the plus nitrogen balance in most cases being fairly
large. It is only necessary to remember that a gain to the body of 1
gram of nitrogen means a laying by of 6.25 grams of proteid, and with
such a gain per day it is apparent that the men were really being
supplied with an excess of proteid food. This view is supported by the
fact that a later balance experiment, when considerably less proteid
food was being given, still showed many of the men in a condition
of plus balance, or with a minus balance so small as to indicate
essentially nitrogen equilibrium. The following figures, being daily
averages of a balance period about the first of April, may be offered
in evidence:

  +-------------+----------+-----------+-------------+----------+
  |             | Nitrogen | Nitrogen  | Nitrogen of | Nitrogen |
  |             | of Food. | of Urine. | Excrement.  | Balance. |
  +-------------+----------+-----------+-------------+----------+
  |             |   grams  |   grams   |    grams    |  grams   |
  | Broyles     |   8.66   |    6.63   |     1.87    |  +0.16   |
  | Fritz       |   8.13   |    5.77   |     1.63    |  +0.73   |
  | Loewenthal  |   8.51   |    6.51   |     2.02    |  -0.02   |
  | Steltz      |   8.32   |    6.50   |     1.88    |  -0.06   |
  | Cohn        |   8.29   |    6.25   |     1.55    |  +0.49   |
  | Morris      |   8.45   |    6.49   |     2.27    |  -0.31   |
  | Oakman      |   8.62   |    7.04   |     1.87    |  -0.29   |
  +-------------+----------+-----------+-------------+----------+

A daily intake of 8.5 grams of nitrogen means the consumption of
53 grams of proteid. Under these conditions of diet, the average
daily amount of nitrogen metabolized was 6.45 grams, corresponding
to 40.3 grams of proteid. The men were practically in a condition
of nitrogen equilibrium, so that we are apparently justified in the
general statement that the simple dietary followed with these men
during the six months’ experiment, and which was accompanied by an
average daily metabolism, after the first three weeks, of 7.8 grams of
nitrogen, was certainly sufficient to maintain both body-weight and
nitrogen equilibrium. Lastly, emphasis may be laid upon the fact that
these values for nitrogen do not necessarily represent the minimal
proteid requirement of the human body, since it is a well-established
physiological principle that by increase of non-nitrogenous food
the rate of proteid katabolism can always be further diminished; a
principle which is plainly in harmony with the view that a high rate of
proteid exchange is not a necessary requisite for the welfare of the
body.

The experimental results presented afford very convincing proof that
so far as body-weight and nitrogen equilibrium are concerned, the
needs of the body are fully met by a consumption of proteid food
far below the fixed dietary standards, and still further below the
amounts called for by the recorded habits of mankind. General health
is equally well maintained, and with suggestions of improvement that
are frequently so marked as to challenge attention. Most conspicuous,
however, though something that was entirely unlooked for, was the
effect observed on the muscular strength of the various subjects.
When the experiments were planned, it was deemed important to arrange
for careful quantitative tests of the more conspicuous muscles of the
body, with a view to measuring any loss of strength that might occur
from the proposed reduction in proteid food. The thought that prompted
this action was a result of the latent feeling that somehow muscular
strength must be dependent more or less upon the proteid constituents
of the muscles, and that consequently the cutting down of proteid food
would inevitably be felt in some degree. The most that could be hoped
for was that muscle tone and muscular strength might be maintained
unimpaired. Hence, we were at first quite astonished at what was
actually observed.

With the soldier detail, fifteen distinct strength tests were made
with each man during the six months’ period, by means of appropriate
dynamometer tests applied to the muscles of the back, legs, chest,
upper arms, and forearms, reinforced by quarter-mile run, vault, and
ladder tests, etc. The so-called “total strength” of the man was
computed by multiplying the weight of the body by the number of times
the subject was able to push up (strength of triceps muscles) and pull
up (strength of biceps muscles) his body while upon the parallel bars,
to this product being added the strength (dynamometer tests) of hands,
legs, back, and chest. It should be added that all of these tests were
made quite independently in the university gymnasium by the medical
assistants and others in charge of the work there. It will suffice for
our purpose to give here the strength tests of the various members of
the soldier detail at the beginning and close of the experiment.


TOTAL STRENGTH

  +------------+----------+---------+
  |            | October. |  April. |
  +------------+----------+---------+
  | Broyles    |   2560   |   5530  |
  | Coffman    |   2835   |   6269  |
  | Cohn       |   2210   |   4002  |
  | Fritz      |   2504   |   5178  |
  | Henderson  |   2970   |   4598  |
  | Loewenthal |   2463   |   5277  |
  | Morris     |   2543   |   4869  |
  | Oakman     |   3445   |   5055  |
  | Sliney     |   3245   |   5307  |
  | Steltz     |   2838   |   4581  |
  | Zooman     |   3070   |   5457  |
  +------------+----------+---------+

Without exception, we note with all of the men a phenomenal gain in
strength, which demands explanation. Was it all due to the change in
diet? Probably not, for these men at the beginning of the experiment
were untrained, and it is not to be assumed that months of practical
work in the gymnasium would not result in a certain amount of physical
development, with corresponding gain in muscular skill and power.
Putting this question aside for the moment, however, it is surely
proper to emphasize this fact; viz., that although the men for a
period of five months were restricted to a daily diet containing
only one-third to one-half the amount of proteid food they had been
accustomed to, there was no loss of physical strength; no indication
of any physical deterioration that could be detected. In other words,
the men were certainly not being weakened by the lowered intake of
proteid food. This is in harmony with the principle, already discussed,
that the energy of muscle work comes primarily from the breaking down
of non-nitrogenous material, and consequently a diminished intake of
proteid food can have no inhibitory effect, provided, of course, there
is an adequate amount of proteid ingested to satisfy the endogenous
requirements of the tissues.

On the other hand, recalling the large number of nitrogenous cleavage
products which result from the breaking down of proteid material, we
can conceive of an exaggerated exogenous proteid katabolism which
may flood the tissues and the surrounding lymph with a variety of
nitrogenous waste products, having an inhibitory effect upon the
muscle fibres themselves, or upon the peripheral endings of the motor
nerves, by which the muscles are prevented, directly or indirectly,
from working at their highest degree of efficiency. This being true,
a reduction of the exogenous katabolism to a level more nearly
commensurate with the real needs of the body might result in a marked
increase in the functional power of the tissue. However this may
be, the fact remains that all of the subjects showed this great
gain in strength; and furthermore, there was a noticeable gain in
self-reliance and courage in their athletic work, both of which are
likewise indicative of an improved condition of the body. How far these
improvements are attributable to training and to the more regular life
the men were leading, and how far to the change in diet, cannot be
definitely determined. We may venture the opinion, however, for reasons
to be made clear shortly, that the change in diet was in a measure
at least responsible for the increased efficiency. As the writer has
already expressed it, there must be enough food to make good the
daily waste of tissue, enough food to furnish the energy of muscular
contraction, but any surplus over and above what is necessary to supply
these needs is not only a waste, but may prove an incubus, retarding
the smooth working of the machinery and detracting from the power of
the organism to do its best work.

Let us now turn our attention for a moment to the group of university
athletes, remembering that these men had been in training for many
months, and some of them for several years, prior to the commencement
of the trial with a reduced proteid intake. In the words of the
director of the gymnasium, “These eight men were in constant practice
and in the pink of condition; they were in ‘training form’ when they
began the changed diet.” Some of them had gained marked distinction for
their athletic work; one during the early months of the test won the
Collegiate and All-around Inter-collegiate Championship of America.
Compare now the strength tests of these men as taken at the beginning
and end of the five months’ experiment, during which they reduced their
daily intake of proteid food more than fifty per cent:


TOTAL STRENGTH

  +----------------+----------+----------+
  |                | January. |   June.  |
  +----------------+----------+----------+
  | G. W. Anderson |   4913   |   5722   |
  | W. L. Anderson |   6016   |   9472   |
  | Bellis         |   5993   |   8165   |
  | Callahan       |   2154   |   3983   |
  | Donahue        |   4584   |   5917   |
  | Jacobus        |   4548   |   5667   |
  | Schenker       |   5728   |   7135   |
  | Stapleton      |   5351   |   6833   |
  +----------------+----------+----------+

It is to be observed that the majority of these trained men showed at
the first trial in January a total strength test approximately equal to
that of the soldier detail at the close of their experiment. This by no
means implies that the latter men owed their gain in strength wholly
to the systematic training they had undergone, but it is certainly
plausible to assume that in a measure this was the case. In any event,
it is plain that the long-continued low proteid diet of the soldiers
had not interfered with a progressive muscular development, and the
attainment of a high degree of muscular strength.

The noticeable feature in the figures obtained with the athletes,
however, is the striking difference between the January and June
results. Every man, without exception, showed a decided gain in his
muscular power as measured by the strength tests. This improvement, to
be sure, was not so marked as with the soldiers; a fact to be expected,
since with these men the element of training and the acquisition
of proficiency in athletic work could have played no part in the
observed gain. Further, most of the tests indicated that the gain was
progressive, each month showing an improvement, in harmony with the
growing effect of the diminished proteid intake. With these subjects,
the only tangible change in their mode of life which could in any sense
be considered as responsible for their gain in strength was the change
in diet. Consequently, it seems perfectly justifiable to conclude that
the observations presented afford reasonable proof of the beneficial
effects of a lowered proteid intake upon the muscular strength of man.

The significance of such a conclusion is manifestly obvious. It
confirms and gives added force to the observations that man can
profitably maintain nitrogen equilibrium, and body-weight, upon a
much smaller amount of proteid food than he is accustomed to consume.
It harmonizes with the view that the normal requirements of the body
for food, under which health, strength, and maximum efficiency are
best maintained, are on a far lower level than the ordinary practices
of mankind would lead one to believe. The widespread opinion that
a rich proteid diet, with the correspondingly high rate of proteid
metabolism, is a necessity for the preservation of bodily strength and
vigor, is seen to be without foundation; for even the most conservative
estimate of the real value of these strength tests must carry with
it the conviction that lowering the consumption of proteid food does
not at least result in any weakening of the body. This is a fact of
vital importance, for it needs no argument to convince even the most
optimistic that while it might be possible to maintain body-weight
and nitrogen equilibrium on a small amount of proteid food, such a
form of physiological economy would not only be of no advantage to
the individual, but would be positively injurious if there was a
gradual weakening of the muscles of the body with decrease of physical
strength, vigor, and endurance.

Another fact to be emphasized in this connection was the conviction,
gradually acquired by many of the subjects, that they suffered
less from fatigue after vigorous muscular effort than formerly.
This was especially conspicuous in the case of Donahue, whose work
on the Varsity basket-ball team called for vigorous exercise. It is
interesting to note that this athlete, of 63 kilos body-weight, for the
last four months of the experiment showed an average daily katabolism
of 7.45 grams of nitrogen, corresponding to a breaking down of 46.5
grams of proteid material daily. Yet, with this low rate of proteid
exchange, he maintained his position on the team with satisfaction to
all, and with the consciousness of improved physical condition and
greater freedom from fatigue. Other subjects, as the laboratory workers
of the professional group, observed that the customary late afternoon
fatigue, coincident with the continued walking and standing about the
laboratory, gradually became far less conspicuous than usual; so that
there seemed to be a consensus of opinion that in some way the change
in diet was conducive to greater freedom from muscular weariness.

It is well understood by physiologists that the ability of a muscle
to do work is inhibited by any condition that tends to depress the
general nutritive state of the body, or that interferes with the local
nutrition of the muscle or muscles involved. On the other hand, there
are certain well-recognized conditions that tend to augment the power
of the muscle, notably an increased circulation of blood through the
tissue, the taking of food, and especially the introduction of sugar.
Further, experiments have shown that when a given set of muscles has
been made to work excessively, other muscles of the body quite remote
will share in the fatigue, thus implying that muscular weariness and
the diminished power to do work are connected with what may be termed
fatigue products, which are distributed by means of the circulation. In
this way, muscles and nerve endings alike are exposed to the inhibitory
influence of waste products of unknown composition, formed in the
muscle, and as previously stated, we may conceive of an exaggerated
exogenous katabolism, with excessive proteid intake, by which muscular
fatigue and weariness may be augmented; hence, the beneficial effect in
this direction of a more rational food consumption, by which proteid
katabolism shall be reduced to a true physiological level.

With these marked effects on strength and fatigue, it is reasonable
to assume that some corresponding action may be exerted on physical
endurance. As is well known, strength and endurance, though related,
are quite distinct and can be separately measured. Strength tests,
however, as usually carried out in gymnasium work, do involve in
considerable degree the question of endurance, since it is customary to
use as one of the factors in estimating total strength the number of
times the man can pull up, or push up, his body on the parallel bars.
Strictly speaking, however, the strength of a muscle is measured by the
maximum force it can exert in a single contraction, while its endurance
is estimated from the number of times it can contract well within the
limit of its strength.

It is well known that endurance, both physical and mental, is one of
the most variable of the human faculties, and it is usually considered
that exercise or training is the chief cause of the differences so
frequently seen. The Maine guide will row a boat or paddle a canoe
for the entire day without undue fatigue, while the novice, though
he may have the necessary strength, lacks the endurance to continue
the task longer than a few hours. As expressed by Professor Fisher,
“Some persons are tired by climbing a flight of stairs, whereas the
Swiss guides, throughout the summer season, day after day spend the
entire time in climbing the Matterhorn and other peaks; some persons
are ‘winded’ by running a block for a street car, whereas a Chinese
coolie will run for hours on end; in mental work, some persons are
unable to apply themselves more than an hour at a time, whereas
others, like Humboldt, can work almost continuously through eighteen
hours of the day.” Again, Fisher states that “among some 75 tests
of different persons holding their arms horizontal, many were found
whose arms actually dropped against their will inside of 10 minutes,
whereas several were able to hold them up over 1 hour, and one man held
them 3 hours and 20 minutes, or a round 200 minutes, and then dropped
them voluntarily. Similarly with deep knee-bending, some persons were
found physically unable to rise again from the stooping posture after
accomplishing less than 500 bendings, whereas several succeeded in
stooping 1000 times, and in one case, 2400.” Here, we have inherent
differences in endurance not associated with training or exercise, and
the question may well be asked, What is the cause of these radical
variations in the ability to repeat a simple muscular exertion?

Hitherto, little attention has been paid to the possible influence of
diet upon this faculty. It has always been assumed that endurance,
like physical strength, is augmented by a rich proteid diet, but it
has never been considered that diet by itself was a factor of any
great moment as compared with training or persistent exercise. It is
true that claims have been advanced from time to time concerning the
beneficial effects on endurance of a vegetable diet, and vegetarians
have frequently presented glowing reports of the great increase in
endurance they have experienced, but little attention has been given to
such statements, and the matter has remained more or less in obscurity.

Recently, Professor Irving Fisher,[63] of Yale, has conducted an
interesting experiment on the influence of a change in diet on
endurance, having the co-operation of nine healthy students as
subjects. The experiment extended through five months, with endurance
tests at the beginning, middle, and end of the period. At the outset,
the men consumed daily an average of 2830 calories, of which 210
were in the form of flesh foods, such as meats, poultry, fish and
shell-fish; 2.6 calories of proteid being ingested for each pound of
body-weight. At the close of the experiment, the per capita calories
had fallen to 2220, of which only 30 were in flesh foods, and the
proteid had fallen to 1.4 calories per pound of body-weight. In other
words, the total calories of the daily ration had dropped off about 25
per cent, the proteid about 40 per cent, and the flesh foods over 80
per cent, or to about one-sixth of their original amount.

  [63] Through the kindness of Professor Fisher, the writer has had the
  opportunity of reading the report of this work, which at this writing
  is not published, and he has drawn upon it freely for the following
  statements of fact.

To determine the endurance of the subjects, six simple gymnastic
tests were employed, and one of mental endurance. The physical tests
consisted of (1) in rising on the toes as often as possible; (2) deep
knee-bending, or stooping as far as possible and rising to the standing
posture, repeating as often as possible; (3) while lying on the back,
raising the legs from the floor to a vertical position and lowering
them again, repeating to the point of physical exhaustion; (4) raising
a 5-lb. dumb-bell (with the triceps) in each hand from the shoulder up
to the highest point above the head, repeating to the point of physical
exhaustion; (5) holding the arms from the sides horizontally for as
long a time as possible; (6) raising a dumb-bell (with the biceps) in
one hand from a position in which the arm hangs free, to the shoulder
and back, repeating to the point of physical exhaustion. This test was
taken with four successive dumb-bells of decreasing weight, viz., 50,
25, 10, and 5 pounds respectively. The mental test consisted in adding
specified columns of figures as rapidly as possible, the object being
to find out whether the rapidity of performing such work tended to
improve during the experiment.

The following table shows the results of the three sets of physical
tests made in January, March, and June:


TESTS OF PHYSICAL ENDURANCE WITH THE NINE SUBJECTS

  +-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
  |             |Time.|  B. |  E. | Lq. | Lw. |  M. |  P. |  R. |  T. |  W. |
  +-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
  |1. Rising on |Jan. | 300 |1007 | 333 |  69 | 127 |1482 | 702 | 900 |1263 |
  |   toes      |Mar. | 400 |1265 |2620 |  65 | 400 |     | 831 |1500 |     |
  |             |June | 500 |1061 |3000 |  85 |1500 |1800 |1263 |1800 |3350 |
  |             |     |     |     |     |     |     |     |     |     |     |
  |2. Deep knee-|Jan. |  82 | 142 |  70 |  48 | 132 | 208 | 374 | 129 | 404 |
  |   bending   |Mar. |     |     | 191 |  47 |     |     |     |     |     |
  |             |June | 200 |  81 | 202 |  58 | 155 | 230 | 453 | 250 | 508 |
  |             |     |     |     |     |     |     |     |     |     |     |
  |3. Leg       |Jan. |  25 |  52 |   9 |  22 |  30 |  27 |  50 |  23 |  30 |
  |   raising   |Mar. |     |     |     |  33 |     |  34 |     |     |  40 |
  |             |June |  33 |  38 |  20 |  35 |  31 |  37 | 103 |  19 |  53 |
  |             |     |     |     |     |     |     |     |     |     |     |
  |4. 5lb.      |Jan. |  75 | 138 |  78 |  38 |  51 |  44 | 100 |  83 | 185 |
  |   Dumb-bell |Mar. |     |     | 106 |     |     |     |     |     |     |
  |   (triceps) |June | 127 |  59 |  80 |  51 |  75 |  56 | 104 | 101 | 501 |
  |             |     |m. s.|m. s.|m. s.|m. s.|m. s.|m. s.|m. s.|m. s.|m. s.|
  |5. Holding   |Jan. | 5–0 | 1–33| 4–7 | 3–37| 3–30| 5–39| 2–5 | 3–22|11–0 |
  |   arms      |Mar. |     |     |     |     | 5–49|     |     |     |15–35|
  |   horizontal|June | 9–36| 2–56| 3–50| 3–0 | 6–5 |10–1 | 3–16| 3–24|23–45|
  |             |     |     |     |     |     |     |     |     |     |     |
  |6. 25lb.     |Jan. |  50 |  18 |  16 |   6 |  20 |  11 |  10 |  25 |  54 |
  |   Dumb-bell |June | 105 |  10 |  26 |  33 |  30 |  29 |  27 |  75 | 108 |
  |   (biceps)  |     |     |     |     |     |     |     |     |     |     |
  +-------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+

The data presented show a marked improvement in March and June over the
record made at the beginning of the experiment in January, except in
the case of one subject, E. As Fisher states, the increased endurance
observed can be ascribed only to dietetic causes, since no other
factors of known significance could have aided in the result. The
dietetic changes, as we have seen, consisted in a slight reduction of
the total amount of food consumed daily, but with a large reduction of
the proteid element, especially from flesh foods. It is significant,
says Fisher, that the only man whose strength and endurance showed any
decrease was E, “whose case was exceptional in almost all respects.
His reduction in quantity of food, except for a spurt at the end, was
less than of most of the men; his reduction in proteid, with the same
exception, was the least of all; his reduction in quantity of flesh
foods was the least of all.” He stands out conspicuously as the one man
whose endurance failed to improve. The mental test carried out with the
subjects pointed to “a slight increase in mental quickness,” but the
adding test was too short to be of great value.

We see in these results another confirmation of the view that the
welfare of the body is not impaired by a marked reduction in the amount
of proteid food; on the contrary, benefit results in the increased
efficiency which manifests itself in various directions. Physical
endurance is an asset not to be ignored, and like the strength of an
individual, it may well be fostered by the recognition and practice of
a principle which seemingly has a firm physiological basis. Whether
the fatigue poisons come from the excessive exogenous katabolism of
proteids in general, or whether they are derived directly in a measure
from flesh foods, need not be considered here; the main point is that
by lowering the rate of proteid katabolism, which necessarily compels a
reduction in the amount of flesh foods, there is a diminished quantity
of nitrogenous waste floating about in the body. Further, we need
not criticise too closely the method by which the reduction of food
is accomplished; whether it be by encouraging mastication, with a
view to better tasting and fuller enjoyment of the food, to the point
of involuntary swallowing; or whether we follow natural taste and
appetite, reinforced by the use of reason, with a full appreciation
of the principle that the welfare of the body is best subserved by a
quantity of food commensurate with true physiological needs.

In making this presentation of the true food requirements of the
body as based on the results of physiological experimentation and
observation, I am by no means unmindful of the dangers of underfeeding;
but this is a condition comparatively rare. When occurring, as stated
by Dr. Curtis, “it is either because of dyspepsia, in which case it
really is involuntary, or comes from some silly notion born of a
combination of innate mental crookedness and that ‘little knowledge’
that is a dangerous thing.” Overfeeding is the predominant dietetic
sin, and with the prevailing dietary standards, as fixed by common
usage, there is good ground for believing that it will continue for
many years to come. Reason tells us, however, in the practice of our
personal nutrition, to steer a middle course between physiological
excess on the one side, and the minimal food requirement on the other.
To quote again from Dr. Curtis,[64] who has expressed the matter very
forcibly, “The physiological chemist can easily draw a line on the
Scylla (starvation) side of the channel. A dietary whereby the system
gets less than it pays out is, obviously, a dangerous veer toward
starvation rock. But on the Charybdis (stuffing) side, just as the
whirlpool itself has no well-defined border, the channel boundary is
not so easily marked. The case is exactly analogous to the stoking of
a furnace. The proportion of ash to live coals is a telltale as to
_under_feeding, but not as to _over_feeding. With undersupply of fuel
the ashes overbalance the live coals, and the fire is thus foretold to
be going out. But with an oversupply the fire simply burns the faster:
all the fuel continues to be consumed; the more coal simply makes the
more ash, so that equilibrium is not disturbed, although maintained
at a higher level. To argue, therefore, that a given dietary is none
too large, because the balance between the material receipts and
expenditures of the economy is not upset, would be like saying that a
given furnace-fire is certainly none too hot, since the ashes raked
out of the fire-box just correspond to the amount of coal shovelled
in. The same would be equally true of a slower fire consuming much
less fuel. The philosophy of the matter is, then, to find the minimum
of steam that will run the engine, and then maintain a fire somewhat
hotter than the exact requirement, in order to run no risk of failure;
or, to return to the metaphor already employed, the would-be careful
liver must simply note how close to Scylla other voyagers have sailed
with safety, and then steer his own bark accordingly.”

  [64] Edward Curtis, M. D.: Nature and Health. New York, Henry Holt &
  Co. 1906. p. 71.

As one looks through the many careful dietary studies that have been
made in recent years, it is easy to find striking illustrations of
people, and communities of people, who have lived for long periods
of time on dietaries so strikingly simple and meagre that it seems
difficult at first glance to believe their daily needs could have been
entirely satisfied. Yet, such observations are quite in accord with
the facts we have been presenting, and they afford additional evidence
that the artificial dietary standards that have been set up are widely
at variance with the real requirements of the body for food. It may
be quite true that many of the people referred to have been and are
faddists, with peculiar notions regarding food, based on religious or
other scruples, but that has no bearing on the main contention that
they have lived for many years on amounts of food ridiculously small
as compared with the ordinary customs of mankind. Thus, in Professor
Jaffa’s report[65] of investigations made among fruitarians and Chinese
of California is an interesting account of a dietary study of a family
of fruitarians, consisting of two women and three children. They had
all been fruitarians from five to seven years, their diet being limited
to nuts and fruit, except for the addition of celery, honey, olive
oil, and occasionally a small amount of prepared cereal food. This
family was in the habit of taking only two meals a day; at 10.30 in
the morning and at 5 o’clock in the afternoon. The first meal always
consisted of nuts and fruit, the nuts being eaten first. At the second
meal, nuts were usually replaced by olive oil and honey. The nuts made
use of were almonds, Brazil nuts, pine nuts, pignolias (a variety of
pine nuts), and walnuts. Fruits, both fresh and dried, were used,
the former including apples, apricots, bananas, figs, grapes, olives
(pickled), oranges, peaches, pears, plums, and tomatoes. The dried
fruits were dates and raisins.

  [65] Bulletin No. 107, Office of Experiment Stations, U. S.
  Department of Agriculture, 1901, from which the descriptions given
  have been taken.

On this limited dietary of raw, uncooked food, with a complete absence
of the high-proteid animal foods, and the ordinary vegetables, legumes,
etc., and without eggs or milk, this family, with three growing
children, had lived all these years. Note now what Jaffa observed
regarding their food consumption. The first subject, a woman 33 years
of age and weighing 90 pounds, was studied for twenty consecutive days,
all the food eaten being carefully weighed and its chemical composition
determined. As a result, it was found that the average amount of food
consumed per day was: proteid, 33 grams; fat, 59 grams; carbohydrate,
150 grams; with a total fuel value of 1300 calories. The other members
of the family were studied in a similar manner, one of the children
being the subject on two separate occasions. The table (on page 217),
showing the average daily food consumption, gives a summary of the
results obtained.

  +--------------------------------+--------+-----+------+--------+--------+
  |                                |        |     |      |        |Proteid |
  |                                |        |     |Carbo-|  Fuel  |per Kilo|
  |                                |Proteid.|Fat. | hyd- | Value. | Body-  |
  |                                |        |     | rate.|        |weight. |
  +--------------------------------+--------+-----+------+--------+--------+
  |                                | grams  |grams|grams |calories| grams  |
  |Woman, 33 years old,            |        |     |      |        |        |
  | Weight 90 lbs. (40.9 kilos)    |   33   |  59 | 150  |  1300  |  0.80  |
  |Woman, 30 years old,            |        |     |      |        |        |
  | Weight 104 lbs. (47.3 kilos)   |   25   |  57 |  90  |  1040  |  0.52  |
  |Girl, 13 years old,             |        |     |      |        |        |
  | Weight 75-1/2 lbs. (34.3 kilos)|   26   |  52 | 157  |  1235  |  0.75  |
  |Boy, 9 years old,               |        |     |      |        |        |
  | Weight 43 lbs. (19.5 kilos)    |   27   |  56 | 152  |  1255  |  1.38  |
  |Girl, 6 years old,              |        |     |      |        |        |
  | Weight 30-1/2 lbs. (13.9 kilos)|   24   |  58 | 134  |  1190  |  1.72  |
  |Girl, 7 years old,              |        |     |      |        |        |
  | Weight 34 lbs. (15.4 kilos)    |   40   |  72 | 134  |  1385  |  2.59  |
  +--------------------------------+--------+-----+------+--------+--------+

As Professor Jaffa states, the tentative dietary standard for a
woman at light work calls for 90 grams of proteid daily, with a fuel
value of 2500 calories. Both of these women were light in weight,
and furthermore had no occasion to do much physical work; but even
so, a daily consumption of only 0.8 gram and 0.52 gram of proteid,
respectively, per kilo of body-weight, with the small calorific values
indicated, represents a phenomenally small amount of food. And yet
Jaffa, in referring to the woman with the lowest intake of food, states
that even this small quantity of food, judging from the appearance
and manner of the subject, “seemed sufficient for her needs, enabling
her to do her customary housework and take care of her two nieces
and nephew.” Regarding the children, it is stated that the commonly
accepted American dietary standard for a child 13 years old and of an
average activity calls for about 90 grams of proteid and 2450 calories.
As is seen from the table, however, the 13-year-old girl consumed of
proteid less than one-third, and of fuel value only about 60 per cent
of the amount called for; yet, says Jaffa, “notwithstanding the facts
brought out by this comparison, the subject had all the appearances of
a well-fed child in excellent health and spirits.”

We need not consume time in discussing the details of this experimental
study, though the facts are interesting and suggestive, for it is only
the general question of proteid requirement and calorific value that
has interest for us at present. The fact is perfectly clear that this
family of fruitarians, young and old, were quite able to live and
thrive on a diet, the value of which in proteid and calories was at as
low a level as was attained in our experimental studies. The rock of
starvation, however, was not touched or even sighted by the voyagers
down this stream of nutrition. We may all agree that it would be
preferable, as a rule, to acquire the proteids, fats, and carbohydrates
of our diet from a greater variety of sources than did the fruitarians;
we might well complain at a dietary so limited in quality; but the
point to be emphasized is that the low intake of proteid and the low
fuel value were quite adequate for meeting the needs of the body. “It
is a difficult matter,” says Professor Jaffa, “to draw any general
conclusions from the foregoing dietaries without being unjust to
the subjects. It would appear, upon examining the recorded data and
comparing the results with commonly accepted standards, that all the
subjects were decidedly undernourished, even making allowances for
their light weight. But when we consider that the two adults have lived
upon this diet for seven years, and think they are in better health
and capable of more work than they ever were before, we hesitate to
pronounce judgment. The three children, though below the average in
height and weight, had the appearance of health and strength. They ran
and jumped and played all day like ordinary healthy children, and were
said to be unusually free from colds and other complaints common to
childhood.”

Turning now to a larger community,--the island nation of Japan,--whose
exploits in war have recently attracted the attention of the civilized
world, we find a people the great majority of whom have remained
untouched by the prodigality of western civilization, and whose customs
and habits still bear the imprint of simplicity and frugality. After
the restoration of Japan and the reorganization of the government in
1867, much attention was directed to the methods of living and to the
dietary habits of the people, with the result that during the last
twenty-five years there have been slowly accumulating many important
data bearing on the food consumption of the people. These have recently
been brought together in an interesting volume by Kintaro Oshima, and
published[66] in the English language.

  [66] A Digest of Japanese Investigations on the Nutrition of Man.
  Bulletin No. 159, Office of Experiment Stations, U. S. Department of
  Agriculture, 1905.

  +-----------------------+--------+--------------------------------+
  |                       |        |Digestible Nutrients and Energy |
  |                       |        |        per Man per Day.        |
  |       Subjects.       | Body-  +--------+-----+--------+--------+
  |                       |weight. |Proteid.| Fat.| Carbo- |  Fuel  |
  |                       |        |        |     |hydrate.| Value. |
  +-----------------------+--------+--------+-----+--------+--------+
  |                       | kilos  | grams  |grams|  grams |calories|
  |School business agent  |57.5    |  65.3  |11.3 |  493.8 |  2467  |
  |Physician              |....    |  61.9  | 8.0 |  468.5 |  2315  |
  |Merchant               |47.6    |  81.5  |19.6 |  366.2 |  2082  |
  |Medical student        |49.0    |  74.8  |11.2 |  326.9 |  1811  |
  |Medical student        |48.5    |  64.7  | 5.1 |  469.6 |  2305  |
  |Military cadets        |....    |  72.3  |11.7 |  618.1 |  3021  |
  |Prisoners without work |47.6[67]|  36.3  | 5.6 |  360.4 |  1726  |
  |Prisoners at light work|48.0[67]|  43.1  | 6.2 |  443.9 |  2112  |
  |Prisoners at hard work |....    |  56.7  | 7.5 |  610.8 |  2884  |
  |Physician              |40.2    |  48.3  |15.5 |  438.2 |  2201  |
  |Hygienic assistant     |40.5    |  46.5  |19.7 |  485.3 |  2430  |
  |Medical student        |51.0    |  42.8  |14.0 |  438.2 |  2163  |
  |Police prisoners       |....    |  42.7  | 8.7 |  387.3 |  1896  |
  |Army surgeon           |54.0    |  79.3  |11.7 |  502.0 |  2567  |
  |Soldier                |66.7    |  75.8  |13.5 |  563.8 |  2828  |
  |Soldier                |61.0    |  58.8  |11.3 |  467.8 |  2330  |
  |Soldier                |56.7    |  55.2  |10.9 |  459.6 |  2276  |
  +-----------------------+--------+--------+-----+--------+--------+

  [67] Average weight of twenty subjects.

As is well known, the great majority of the people of Japan live
mainly on a vegetable diet. It is also known to physiologists at least
that Japanese dietaries are characterized by a relatively small amount
of proteid, though since the passage of the Food Supply Act of the navy
in 1884, the proteid-content of the navy ration has been decidedly
increased. It will be interesting to note a few of the results collated
by Oshima, and some of the conclusions that he draws from the data
presented. The foregoing table shows a few of the more striking results
of the dietary studies obtained with various classes of people, where
the food used was largely vegetable, but generally with some admixture
of fish or meat.

The figures presented, which represent the actual amounts of food
consumed, with proper correction for the indigestible portion, show
a much smaller intake of proteid than is common with European and
American people; indeed, both proteid and fuel value are very much
less than common practices call for among western peoples, even when
due allowance is made for differences in body-weight. To quote from
Oshima, “Probably the most interesting of the dietary studies are
those with poorer classes, which comprise by far the larger part of
the population. The dietaries of the miscellaneous class, including
employees, prisoners, etc., consisted largely of vegetable foods
and supplied on an average 59 grams of proteid and 2190 calories of
energy per man per day.” Especially suggestive were the results of
a study made with a military colonist, a type of man very common in
Japan; in reality farmers who live at home, but have military drill at
certain fixed times. The subject was carefully selected under advice
of officers in charge of the district, and weighed 59.9 kilograms. His
diet consisted solely of cereals and vegetables, being identical with
that of the people in the rural districts of Japan. His daily food was
found to be composed of 46.3 grams of digestible proteid, with a fuel
value of 2703 calories.

Even more striking were the results obtained in a study of the dietary
habits of three healthy natives of Formosa, employed as day laborers
at the military hospital. They weighed respectively 60.9, 55, and 54.8
kilograms. The main portion of their diet was rice, supplemented,
however, by a little salt fish, salted melon, spinach, ginger, and
greens. The daily amount of proteid ingested was 48.0 grams (37.4 grams
of digestible proteid), with a total fuel value of 1948 calories. A
composite sample of urine covering seven days showed an average daily
output of metabolized nitrogen of 6.93 grams, corresponding to a
breaking down of 43.3 grams of proteid.

Especially interesting also is a series of experiments with
professional men, reported by Oshima, in which attention was paid to
nitrogen balance. The following table shows the essential results:

  +--------+-------+----------+-----------------------------------------+
  |        |       |          |     Digestible Nutrients and Energy     |
  |        |       |          |             per Man per Day.            |
  |Subject.| Body- |Character +--------+-----+--------+--------+--------+
  |        |weight.| of Food. |Proteid.| Fat.| Carbo- |  Fuel  |Nitrogen|
  |        |       |          |        |     |hydrate.| Value. |Balance.|
  +--------+-------+----------+--------+-----+--------+--------+--------+
  |        | kilos |          |  grams |grams|  grams |calories|        |
  |  N. K. |  43.1 |mixed diet|  72.7  |18.3 |  380.7 |  2091  |   +    |
  |  S. A. |  49.5 |mixed diet|  69.8  |20.2 |  410.7 |  2222  |   +    |
  |  N. K. |  42.9 |mixed diet|  64.4  | 8.5 |  396.3 |  2028  |   +    |
  |  N. K. |  43.2 |mixed diet|  62.8  | 8.7 |  433.2 |  2178  |   +    |
  |  N. K. |  43.0 |vegetable |  68.5  |19.7 |  433.0 |  2303  |   +    |
  |  N. K. |  43.9 |vegetable |  36.8  | 6.6 |  381.0 |  1824  |   -    |
  |  N. K. |  42.4 |vegetable |  40.5  | 8.7 |  462.6 |  2200  |   +    |
  |  S. A. |  49.6 |vegetable |  34.4  | 7.5 |  451.9 |  2119  |   -    |
  |  S. A. |  49.9 |vegetable |  43.5  | 9.1 |  500.0 |  2376  |   +    |
  +--------+-------+----------+--------+-----+--------+--------+--------+

It is to be observed that in all of the above experiments, excepting
two, the subjects gained nitrogen even with the low proteid intake and
the small fuel value of the day’s food. Particularly noteworthy, in
harmony with previous statements, are the results of the sixth and
seventh experiments. In the sixth experiment, the subject was not able
to maintain nitrogen equilibrium on a diet containing 36.8 grams of
digestible proteid and having a fuel value of 1825 calories, but by
raising the intake of carbohydrate food (seventh experiment) to 462
grams daily, thereby increasing the fuel value of the daily ration
to 2200 calories (with a slight increase in the proteid incidental
thereto), the body was able to change its previous loss of nitrogen
into a gain; in other words, the added carbohydrate served as a
protector of proteid.

The series of experiments as a whole, however, is to be considered
in the light of additional data bearing on the dietary customs of a
people who for generations have apparently lived and thrived on a daily
ration noticeably low in its content of proteid, as well as low in its
calorific value. As Oshima states, “It is probably fair to infer that
the amount of proteid in the dietaries of the classes living largely on
vegetable foods (and they constitute the larger part of the population)
may not be very far from 60 grams per day,” or 45 grams of digestible
proteid. It is reasonable to assume that the people live in this way
from force of habit or of necessity, and we may agree with Baelz, a
professor connected with the medical faculty of Tokyo University, “that
their diet is sufficient from a physiological standpoint.” Doubtless a
mixed diet, with a larger proportion of animal food, did their means
readily permit, would offer some advantages from the standpoint of
palatability and variety, but it is questionable if any material gain
in health or strength would result. “It is sometimes remarked,” says
Oshima, “that the peasants in the rural districts of Japan, living
largely on vegetable food, are really healthier and stronger than
people of the better classes, who live on a mixed diet, and the better
physical condition of the former is commonly believed to be due to
their diet.” This, however, is a difficult matter to decide, since
there are so many other factors that are liable to play a part, such as
the general conditions of life which are so widely different in the two
classes.

It is plainly evident that the daily diet of the great bulk of the
Japanese people has been characterized by a very low proteid standard,
as contrasted with the standards and usages of the majority of
European and American people. The fact is brought forward merely as
confirmatory evidence, on a large scale, of the perfect safety of
lowering the consumption of proteid food to somewhere near the level of
the physiological requirements of the body. Generations of low proteid
feeding, with the temperance and simplicity in dietary matters thereby
implied, have certainly not stood in the way of phenomenal development
and advancement when the gateway was opened for the ingress of modern
ideas from western civilization. Many changes are sure to follow in
the footsteps of the nation’s progress, and among these it is safe to
prophesy that as public and private wealth, and resources in general,
increase, the dietary of the people will gradually assume a more varied
character with corresponding increase in volume. Whether such a change
will prove of real benefit to the race, time alone can determine.

Having said so much concerning the Japanese, it is proper that a few
additional statements should be made. The stature and general physique
of the people could be advantageously improved. Is this a question of
dietary, or is it connected with some condition of life on which the
daily food has no bearing; or is it, perchance, a racial characteristic
so deeply ingrained that conditions of environment are without
noticeable influence? These questions cannot be definitely answered
at present. Finally, we may call attention to the dietary changes
inaugurated in recent years in connection with the new organization of
the imperial army and navy. With a view to increasing the efficiency of
the men, following the customs of other countries, an act was passed
increasing the amount of proteid food in the navy dietary. Oshima’s
report of the various steps taken to accomplish this end, with the
results that followed, is interesting in several ways.

“A large part of the rice was to be replaced by bread, and meats were
to be used liberally. The experience, during the first year that
this ration was tried, indicated that bread and meat could not be
advantageously substituted immediately for the rice, because most of
the marines were unaccustomed to these food materials; consequently,
a modification of the ration was introduced in 1885, whereby a
rice-barley mixture was adopted in place of the bread. Barley was
considered at that time as a better article of food than rice, on
account of its higher proteid content, but later investigations showed
that the digestibility of the nutrients of barley was small. In 1886,
an effort was again made to substitute bread for the rice-barley
mixture. In 1890, the ration allowance was reduced by one-fifth and
an amount of money equivalent to the cost of the reduction in diet
was given to each marine with which to buy accessory food according
to his own choice. In 1898, the reduction was made one-tenth, instead
of one-fifth as in previous years. In 1900, the cash allowance was
abolished and a new ration adopted.” This ration contains about 150
grams of proteid (animal and vegetable food) and has a fuel value of
over 3000 calories. In all of these changes, the proportion of rice was
greatly reduced.

Probably, one of the chief reasons why persistent efforts were made
to improve the dietary of the navy was the prevalence among the men
of the disease known as beriberi. “While no satisfactory explanation
as to the cause of the disease was offered, it was generally believed
that there was some very close relation between the disease and the
rice diet” (Oshima). During the years 1878–1883 inclusive, nearly 33
per cent of the marines suffered from beriberi. With the adoption of
the new ration in 1884, in which a large part of the rice was replaced
by bread and other articles, and with better hygienic conditions,
this disease immediately began to disappear, and during the six years
after the adoption of the new diet only 16 per cent of the marines
were affected by the disease. Later on, hardly more than two or three
cases a year were recorded. Advocates of a high proteid diet bring
forward this illustration as an evidence of the danger connected with
a lowered proteid intake; _i. e._, that the nutrition of the body
will be impaired and diseases of various sorts liable to follow. Yet,
Oshima is very careful to state, “It should be especially noted that
here no attempt has been made to indicate the cause of beriberi or the
relation between the disease and the diet.” That rice in itself can be
a cause of the disease is not to be considered for a moment. Further,
so far as any facts are concerned, the writer can see no ground for
considering that a low rate of proteid metabolism has in itself any
direct connection with the disease. From a dietary standpoint, it seems
far more plausible to assume that the great restriction in variety of
foods, so strikingly manifest in the dietary of the poorer people of
Japan, results in a lack of some one or more elements which conduces to
the disease, just as in scurvy the lack of _fresh_ vegetables on long
voyages was liable to be followed by an epidemic of this disease.

Consider the natural character of the dietary of the great bulk
of the Japanese people, determined as it was by adverse financial
circumstances. As Oshima states, “The rural population of the
interior depends very largely or entirely upon a vegetable diet.
Fish is eaten perhaps once or twice a month, and meat once or twice
a year, if at all. The poorer working classes in the cities also
use very little animal food. But the poorer classes in the city and
the peasantry of the rural districts comprise nearly 75 per cent of
the total population, and it is therefore safe to assume that this
proportion lives chiefly, or wholly, upon vegetable diet. And this,
it may be observed, means vegetarianism literally. The so-called
lacto-vegetarianism is unknown in Japan. Cows are scarce, and milk
and other dairy products are expensive, and such as are available are
consumed almost entirely by the wealthier people in the cities.” It is
also to be noted that the amount of fat in Japanese dietaries is very
small. The reported data indicate that the usual vegetable dietaries
contain only about 10 grams of fat per day, while even in the average
mixed dietaries the amount rarely rises above 20 grams per day. In
other words, the ordinary food of the Japanese was characterized by
great lack of variety, and with such a preponderance of carbohydrate
materials of a limited kind that it is easy to conceive of a possible
dearth of some essential or accessory element, necessary for the
preservation of that nutritive balance which aids in protection against
disease.

If the resistance of the body to disease germs and toxic influences in
general is really diminished by reducing the consumption of proteid
food below the set dietary standards, then obviously here lies a
tangible reason for the maintenance of a high proteid intake. I know
of only one series of scientific observations that bears directly on
this question. Dr. Reid Hunt of Washington has studied recently the
power of resistance to the poison acetonitrile of animals kept for some
time upon a reduced proteid diet. “My experiments,” says Dr. Hunt,
“showed in all cases that the resistance was much increased.” In other
words, the animals that had been fed a low proteid ration were able to
endure a much larger dose of the poison than corresponding animals on
their customary diet; “they resisted 2–3 times the ordinary fatal dose
of acetonitrile.” This general subject, however, is obviously a very
important one, and merits further experimental study under a diversity
of conditions.

In conclusion, the facts here presented bearing on food requirements,
especially those that relate to the need for proteid food, are
seemingly harmonious in indicating that the physiological necessities
of the body are fully met by a much more temperate use of food than is
commonly practised. Dietary standards based on the habits and usages
of prosperous communities are not in accord with the data furnished
by exact physiological experimentation. Nitrogen equilibrium can be
maintained on quantities of proteid food fully fifty per cent less
than the every-day habits of mankind imply to be necessary, and this
without increasing unduly the consumption of non-nitrogenous food. A
daily metabolism of proteid matter equal to an exchange of 0.10–0.12
gram of nitrogen per kilogram of body-weight is quite adequate for
physiological needs, provided a sufficient amount of non-nitrogenous
foods--fats and carbohydrates--is taken to meet the energy requirements
of the body.

The long-continued experiments on many individuals, representing
different types and degrees of activity, all agree in indicating that
equilibrium can be maintained indefinitely on these smaller quantities
of food, and that health and strength can be equally well preserved,
to say nothing of possible improvement. The lifelong experience of
individuals and of communities affords sufficient corroborative
evidence that there is perfect safety in a closer adherence to
physiological needs in the nutrition of the body, and that these needs,
so far as proteid food is concerned, are in harmony with the theory
of an endogenous metabolism, or true tissue metabolism, in which the
necessary proteid exchange is exceedingly limited in quantity. There
are many suggestions of improvement in bodily health, of greater
efficiency in working power, and of greater freedom from disease, in
a system of dietetics which aims to meet the physiological needs of
the body without undue waste of energy and unnecessary drain upon the
functions of digestion, absorption, excretion, and metabolism in
general; a system which recognizes that the smooth running of man’s
bodily machinery calls for the exercise of reason and intelligence, and
is not to be intrusted solely to the dictates of blind instinct or to
the leadings of a capricious appetite.




CHAPTER VII

THE EFFECT OF LOW PROTEID DIET ON HIGH PROTEID ANIMALS

  TOPICS: A wide variety of foods quite consistent with temperance in
  diet. Safety of low proteid standards considered. Arguments based
  on the alleged effects of low proteid diet on high proteid animals.
  Experiments of Immanuel Munk with dogs. Experiments of Rosenheim.
  Experiments of Jägerroos. Comments on the above experiments. The
  experiments of Watson and Hunter on rats. The writer’s experiments
  with dogs. Details of the results obtained with six dogs. Comparison
  of the results with those of previous investigators. Effect of a
  purely vegetable diet on dogs. Different nutritive value of specific
  proteids considered. Possible influence of difference in chemical
  constitution of individual proteids. Effect of low proteid diet on
  the absorption and utilization of food materials in the intestine
  of dogs. General conclusions from the results of experiments with
  animals.


Man is by choice an omnivorous creature; he reaches out ordinarily in
all directions for as wide a variety of foods as his circumstances and
surroundings will allow. He rightly cultivates a taste for foods that
have individuality of flavor, and derives pleasure and satisfaction
from the eating of delicacies that appeal to palate and to reason.
All this he can do without becoming an epicure or a glutton, and
without violation of physiological laws or disregard of the teachings
of temperance. As a being endowed with reason and intelligence he is,
however, necessarily mindful of the possible deleterious effect of
undue quantities of food, as he is likewise mindful of the desirability
of avoiding certain varieties of food which personal experience has
taught him are fraught with possible danger. Care and prudence in diet
are legitimate outcomes of a reasonable interest in the welfare of the
body, upon which so largely depend the happiness and working power of
the individual.

The adoption of dietary habits that aim to accord with the
physiological requirements of the body does not compel a crucifying of
the flesh or a disregard of personal likes and dislikes. A reasonable
intelligence combined with a disposition to exercise the same degree
of judgment and care in the nutrition of the body as is expended on
other matters, of no greater importance, pertaining to the individual,
to the household, or to business interests, are all that is needed to
bring about harmony between every-day dietary habits and the nutritive
requirements of the body. There is no occasion, unless one finds
pleasure and satisfaction in so doing, to resort to a limited dietary
of nuts and fruits, to become an ardent disciple of vegetarianism,
to adopt a cereal diet, to abjure meats entirely, or to follow in an
intensive fashion any particular dietary hobby, except so far as may
be necessary to insure an adequate amount of non-nitrogenous foods to
meet the energy requirements of the body without unduly increasing
the intake of proteid or nitrogenous food. Naturally, a man leading
a life of great physical activity with the consequent demand for a
large energy-yielding intake will be compelled to resort largely to
vegetable foods, rich in starch and poor in proteid, or to eat largely
of fatty foods. Reliance on meats and animal foods in general, under
such conditions, would plainly involve a high proteid intake with a
consequent high nitrogen metabolism, with the chance that even then the
energy requirement would not be fully met.

In view of all that has been said, reinforced by the various facts
brought forward as evidence, we must recognize the value of the
non-nitrogenous foods as a source of energy, and this means plainly
food from the plant kingdom. In any rational diet, vegetable foods of
low nitrogen-content must predominate, while animal foods with their
higher nitrogen values must be greatly subordinate in amount, if the
nitrogen or proteid metabolism of the body is to be maintained at a
level commensurate with true physiological requirements. But there
comes the ever-recurring question, Are the lower proteid standards
quite safe to follow? Are we warranted in turning aside from the
teachings based on the habits and customs of mankind? Many reasons have
already been presented which seemingly justify an affirmative answer,
while the experimental results and the observations on various groups
of people, covering years of time, speak with no uncertainty regarding
the element of safety, and indicate clearly that the absolute proteid
requirement of the body is quite small; much smaller indeed than
the amount of proteid food consumed by the average individual would
seemingly imply.

Probably the most striking evidence, certainly of an experimental
nature, so far presented against the safety of a relatively low
proteid diet for man is that based on the results of several studies
made to ascertain the effect of a reduced proteid intake on so-called
high proteid animals. Animal kind may be divided into three groups
according to the nature of their food, viz., high proteid feeders, such
as carnivorous animals in general, of which the dog is a good type;
omnivorous or moderate proteid consumers, to which class man belongs;
and low proteid consumers, such as herbivorous animals. Three series of
experiments have been reported by independent workers on the effects of
reducing the amount of proteid food in the diet of dogs. The results
of these experiments were of such a character that it has come to be
understood that animals of this type cannot exist for any great length
of time on a low proteid diet. It is affirmed that in a relatively
short period the animals reach such a state that they either die, or
are in such poor condition that they must be fed a more liberal amount
of proteid to maintain them alive. The explanation offered is that the
low proteid diet results “in a loss of the power of absorption from the
intestinal tract, caused apparently by a change in the condition of the
epithelial cells, as well as by a diminished secretion of the digestive
juices.”

The argument based on this evidence is that while a high proteid animal
feels at once, or almost immediately, the deleterious effect of a
reduction in the amount of proteid food, an omnivorous animal may be
more tardy in manifesting the injurious action, which, however, is sure
to follow sooner or later from any material reduction of proteid below
the customary standards. In other words, man as a moderate proteid
consumer can endure for a time even large reductions in the amount
of proteid food, but eventually there will be manifested some of the
disastrous results obtained with dogs. Here, we have a somewhat serious
indictment, one that merits careful consideration. To be sure, it may
be objected that between dog and man there is a wide gulf, and that
there is no justification for assuming that these two types of animal
life have anything in common. Still, the experience of many years
has taught the physiologist that much light can be thrown upon the
processes of higher types of life by a study of what occurs in lower
forms, and on the subject of nutrition any one of experience would
hesitate to cast out of court the evidence gathered from observation of
what occurs among the higher animals. It will be the part of wisdom,
therefore, to scrutinize somewhat carefully the character of this
evidence obtained from a study of the behavior of dogs toward a low
proteid diet.

The first series of experiments was made in 1891 by the late Immanuel
Munk of Berlin, privat docent of physiology at the University, followed
by further experiments in 1893.[68] Four dogs in all were studied. The
diet made use of was “fleischmehl” (dried meat ground to a powder),
fat (suet), and rice boiled together with water. We may refer briefly
to the details of one experiment. The dog weighed 10.4 kilograms, and
at first was given a daily diet composed of 85 grams of rice, 29 grams
of fat, and 30 grams of the flesh meal. This ration contained 30.3
grams of proteid, 31 grams of fat, and 66 grams of carbohydrate, with
a total fuel value of 663 calories, or 63 calories per kilogram of
body-weight. On this diet, there was at the outset a slight loss of
body-weight, after which both body equilibrium and nitrogen equilibrium
were practically maintained. After this preliminary period of three
weeks, the day’s diet was altered by replacing 15 grams of the proteid
by 15 grams of rice, so that the daily ration consisted of 15.3 grams
of proteid (with 2.42 grams of nitrogen), 31 grams of fat, and 81 grams
of carbohydrate, with essentially the same fuel value per kilo of
body-weight as before. Later, the fuel value of the food was further
increased by raising the amount of rice to 125 grams per day, the day’s
ration then consisting of 15.5 grams of proteid, 37 grams of fat, and
96 grams of carbohydrate, with a total fuel value of 780 physiological
heat units, or 78 calories per kilo. On this diet, nitrogen equilibrium
was maintained and the animal gained somewhat in body-weight. By the
seventh week, however, Munk reports that the animal began to show
signs of change; there was loss of appetite, absorption of the daily
food was impaired, both proteid and fat failing in large degree to be
utilized, while nitrogen equilibrium could no longer be maintained.
This condition continued during the next week, aggravated by vomiting
and accompanied by loss of strength and vigor. At the beginning of
the tenth week of this low proteid ration, the animal was in a very
poor condition, with complete loss of appetite, little inclination to
take food, etc. On feeding a liberal diet of fresh meat, as much as
250 grams per day, with some fat (50 grams a day), the animal speedily
recovered its appetite, and in a short time was in normal condition,
absorption of food and utilization of the same being as complete as at
the beginning of the experiment.

  [68] Ueber die Folgen einer ausreichenden, aber eiweissarmen Nahrung.
  Ein Beitrag zur Lehre vom Eiweissbedarf. Virchow’s Archiv für
  pathologische Anatomie und Physiologie, Band 132, p. 91.

It is not necessary to give further details bearing on the three
additional experiments. It will suffice to quote the general
conclusions which Munk drew from the various results obtained, viz.,
that a low proteid intake in the case of dogs causes a loss of
appetite, weakness, vomiting, etc., while body-weight and nitrogen
equilibrium are difficult or impossible to maintain. More specifically,
Munk’s observations led him to state that for dogs of ten kilograms
body-weight a daily intake of 0.255 gram of nitrogen per kilo of
body-weight is not sufficient to maintain the normal condition of the
body, even when the fuel value of the day’s food amounts to more than
100 calories per kilo. In order to have the animal continue in nitrogen
and body equilibrium, the daily food must contain at least 0.31 gram of
nitrogen per kilogram of body-weight, with sufficient non-nitrogenous
food to yield over 100 calories per kilo.

Let us now pass to the experiments made by Rosenheim,[69] which were
carried on at about the same date as Munk’s. In the first experiment,
the dog weighed 11.3 kilograms, and was fed daily a low proteid ration
having a fuel value of 1447 calories and containing 2.825 grams of
nitrogen. This ration was reduced in a short time to a still lower
plane, viz., to 1066 calories and 2.525 grams of nitrogen daily. The
food as then given was composed of 170 grams of rice, 50 grams of
fat, and 25 grams of chopped meat, on which the dog gained weight and
preserved nitrogen equilibrium. For six weeks, or thereabouts, the
animal maintained its normal condition, after which it began to show
symptoms of a general disturbance, with lack of appetite and weakness
accompanied by a condition of icterus. Addition of meat extract to
the diet to improve the flavor was without any appreciable effect.
During the next two weeks, the condition of the animal steadily grew
worse, although the body-weight remained practically stationary and
nitrogen equilibrium was maintained. A week later, the animal died in
a condition of exhaustion, without having manifested any symptoms of
disturbed metabolism. There was found a marked catarrhal condition
of the mucous membrane of the gastro-intestinal tract, with a fatty
degeneration or metamorphosis of the glandular apparatus, but nothing
sufficiently specific to account for the peculiar manner of death.

  [69] Theodor Rosenheim: Ueber den Gesundheitsschädigenden Einfluss
  eiweissarmer Nahrung. DuBois-Reymond’s Archiv für Physiologie,
  1891, p. 341. Also, Weiterer Untersuchungen über die Schädlichkeit
  eiweissarmer Nahrung. Pflüger’s Archiv f. d. gesammte Physiologie,
  Band 54, p. 61, 1893.

A second experiment with a dog of 5.8 kilograms, fed on meat, fat, and
rice, led to essentially the same results as the preceding experiment.
At the end of the first month, there appeared indications that the
animal was not well, loss of appetite being marked, with disturbance
of the stomach accompanied by occasional vomiting. These symptoms
disappeared quickly when the animal was given for a few days large
quantities of meat. On returning to the original low proteid diet,
with its large content of rice, the symptoms gradually reappeared.
At the end of two months the animal had again lost its appetite, and
before the end of the fifth month the subject was dead. Post-mortem
examination showed especially a strong fatty degeneration of the
epithelial cells of the mucous membrane of the stomach and intestine.
Rosenheim concludes that a diet poor in proteid is unhealthful for
dogs, and that a daily ration containing even 0.32 gram of nitrogen
per kilogram of body-weight, and with a fuel value of 110 calories per
kilo, is not sufficient to maintain the animal in a condition of health.

The next series of experiments was made by Jägerroos[70] of Finland.
This investigator was evidently impressed by the unfavorable and
monotonous character of the diet made use of by the preceding
investigators, and sought to introduce a little variety, recognizing
also that with a carnivorous animal it is difficult to reduce the
proteid to a low level and maintain the necessary fuel value, without
introducing foodstuffs to which the animal is wholly unaccustomed. In
the first experiment, the dog had a body-weight of 5.77 kilograms, and
at the beginning was fed daily 40 grams of meat and 100 grams of sugar,
equal to 0.31 gram of nitrogen and 80 calories per kilo of body-weight.
The experiment continued for eight months, sugar being replaced in part
by butter, and occasionally bread, fat, and wheat meal being used in
proper amount to yield the given nitrogen and fuel values. During the
last five months, the intake of nitrogen per day averaged 0.29 gram
per kilo, with a fuel value amounting to 89 calories daily per kilo of
body-weight. During this period, the animal maintained a plus nitrogen
balance for a large part of the time. The experiment was then continued
for two months longer, with a gradual diminution in the nitrogen of the
food and in the fuel value, the animal dying at the end of the tenth
month.

  [70] B. H. Jägerroos: Ueber die Folgen einer ausreichenden, aber
  eiweissarmen Nahrung. Skandinavisches Archiv für Physiologie, Band
  13, p. 375, 1902.

In a second experiment, the dog made use of weighed at the beginning
11.97 kilograms. During the first five months, the average intake of
nitrogen amounted daily to 0.29 gram per kilo, while the average fuel
value of the food (meat, fat, and sugar) was 76 calories per kilo
daily. In the middle of the seventh month the animal was quite ill,
with poor appetite, vomiting, etc. Body-weight began to fall off, and
the animal soon died. With both of these animals, the experiment ended
suddenly by a sharp and short illness.

Jägerroos, however, believed that both animals died from a severe
case of infection, and not as the result of the diminished intake of
proteid. This view was fully substantiated, in his opinion, by the
evidence furnished on bacteriological and morphological examination.
There was no pathological alteration and no fatty degeneration in the
intestinal epithelium; nothing to indicate any connection between the
lowered proteid intake and the death of the animal. To be sure, the
long-continued diet poor in nitrogen might have diminished the power
of resistance of the body, but no proof of this is offered. There
was indicated merely a simple infection, as shown by the presence
of Streptococcus and Bacterium coli communis in the blood. But, as
Jägerroos states, one might well conceive of a lowered power of
resistance on the part of the body, due not to any change in diet,
but to the long-continued confinement in a cage with the enforced
inactivity and lack of freedom. It is to be noted, furthermore, that
here there was no sign of a gradual and progressive weakening of
the body, no indication of any disturbance of the digestive tract
with diminished power of absorption of either fat or proteid. On the
contrary, there was a sudden and sharp attack of some infectious
disease by which the animals quickly succumbed. Jägerroos was of the
opinion that in the absence of this infection the animals would have
continued to live for a long period of time.

If a low proteid diet works so inimically on high proteid animals as
Munk and Rosenheim thought, it would naturally be expected that the
small proteid ration followed so long by Jägerroos would have resulted
in the appearance of marked symptoms, at least a gradual and persistent
falling off in body-weight, inability to maintain nitrogen equilibrium,
etc.; but none of these things occurred. In Munk’s first experiment,
the animal was given no fresh meat whatever during four weeks. Is it
not quite possible that in the abrupt cutting off of this wonted form
of food a disturbance may have been set up in the gastro-intestinal
tract, which paved the way for the more serious results that followed?
Jägerroos used only fresh, uncooked meat in his experiments, and laid
great stress upon the importance of not departing any more than was
necessary from the accustomed form of diet. The writer is strongly of
the opinion that sufficient stress has not been laid upon this phase of
the subject. A satisfactory diet for dog as for man must meet ordinary
hygienic requirements; it must not only be sufficient in amount, but
it must be easily digestible, of accustomed flavor, appealing to eye,
nostrils, and palate, with reasonable variation occasionally and of
moderate volume. With due regard to these conditions, I believe with
Jägerroos that not much attention need be paid to the proportion
of nitrogen therein, for however small the amount it will be found
sufficient to meet the needs of the body.

These are the results, collectively, so frequently used to point
a moral for man: Beware of the possible danger of reducing the
consumption of proteid food below the commonly accepted dietary
standards! We must admit, however, that there is a woeful lack of
agreement in these results, and it is difficult to prevent a shadow of
doubt from creeping over us as we try to depict for ourselves the way
in which a low proteid ration exerts its deleterious effect on dogs.
I do not believe that radical changes in diet, whether they involve
increase or decrease in total quantities, or in specific elements of
the diet, can be made suddenly without danger of some disturbance of
the gastro-intestinal tract or other parts of the economy, either
in dog or man. It is reasonable to believe also that a high proteid
feeder, like a dog, with his more limited dietary, will be far more
sensitive to great changes than omnivorous man with his wider range of
foodstuffs. Moreover, there is just as good ground for believing that
in any animal, excess of proteid is as dangerous as a low proteid diet.
Too great a disturbance in the nutritive balance, whether it involves
excess or reduction in the amount of a given foodstuff, is liable to be
attended with serious disturbance in any sensitive organism.

In illustration of these statements, we have some recent results
obtained by Watson and Hunter[71] upon the influence of diet on
growth and nutrition. These investigators find that young rats--two
and a half months old--when fed upon a diet composed exclusively of
horse-flesh, which is chiefly proteid matter with some fat, succumb
very quickly, for some reason. Of fourteen young rats fed on this meat
diet, six died on the third day. On the morning of this day, as the
authors state, “the rats appeared to be in their usual health, but
an hour after feeding one of them was lying on its side apparently
unconscious. In a few minutes others were affected. They appeared to be
paralyzed, they felt cold to the touch, exhibited symptoms of tetany,
and speedily became unconscious. Six succumbed within half-an-hour. Of
the remainder, some showed similar symptoms, although in less degree,
and they recovered when the diet was changed to bread and skim milk.”
After two days of the so-called normal diet, composed of bread and skim
milk, the remaining eight rats were again placed on an exclusive meat
diet. They appeared now to have acquired a certain degree of immunity,
for although they exhibited symptoms of deranged nutrition, these were
gradually recovered from and they gained in weight. At the end of the
eighth month, five of the animals were still alive and in apparent good
health, but their growth was permanently stunted. With an exclusive
diet of ox-flesh, young rats were much more liable to thrive, although
their growth was distinctly retarded.

  [71] Chalmers Watson, M.D., and Andrew Hunter, M.B.: Observations
  on Diet. The Influence of Diet on Growth and Nutrition. Journal of
  Physiology, Vol. XXXIV, p. 112, 1906.

This difference in the behavior of the animals towards the two
forms of proteid food is to be attributed to the fact that ox-flesh
contains more fat than horse-flesh, and consequently the diet with
this form of meat was less exclusively proteid in character. Further,
there were some indications that horse-flesh is less digestible than
ox-flesh. Another fact, showing the far-reaching effect of a distinctly
unphysiological diet, is the marked influence of pure meat food on the
progeny. Thus, of 93 rats born of meat-fed parents only 19 were alive
at the end of two months, while of 97 young born of bread and milk-fed
rats, 82 were alive and in apparent health at the end of the same
period.

As illustrating how foods that have, superficially at least,
approximately the same chemical composition may react differently
in the animal body we have the observations of Watson on rats fed
with porridge, made by boiling oatmeal with water and skim milk,
as contrasted with a diet of bread and skim milk, the two diets
having essentially the same composition. Of fourteen young rats fed
exclusively on porridge, all, with the exception of two that were
withdrawn, succumbed within five months, while the bread and milk-fed
animals thrived as usual. Adult rats, however, can live for prolonged
periods and maintain their weight on a porridge diet. It is believed
that the difference in the behavior of young rats to these two closely
allied forms of diet, is due to a difference in the digestibility of
the food, the porridge being presumably less readily digested by the
young animals than bread. With the more fully developed digestive
powers of the adult animals, however, this difference in availability
practically disappears as a potent factor in their nutrition. Finally,
mention may be made of the fact that a pure rice diet, notably
deficient in proteid, arrests the growth of young rats and leads to a
fatal issue within three months, while adult rats placed on such a
diet lose weight rapidly and die in about the same time. All of these
facts bearing on the nutrition of animals quite remote from man have
significance as showing how any wide departure from a physiological
diet, for that particular species or type, may lead to very undesirable
results, and they warn us not to be too hasty in drawing far-reaching
conclusions and sweeping deductions from a few experiments with a given
species of animal.

Recurring now to the experiments made with dogs, there is certainly
suggested an element of danger in a low proteid diet, which, if the
experiments are taken at their face value and the conclusions derived
therefrom applied to man, needs careful consideration. Jägerroos
plainly was not inclined toward the belief that a low nitrogen intake
was the cause of the unfortunate results that attended his experiments.
Still, his animals did die from some cause, and thereby his position
was weakened. Munk and Rosenheim, on the other hand, from their
experiments were apparently convinced that a low proteid intake was
inimical to dogs, and it will be remembered Rosenheim concluded that
“a daily ration containing even 0.32 gram of nitrogen per kilogram of
body-weight, and with a fuel value of 110 calories per kilo, is not
sufficient to maintain the animal in a condition of health.” If this
is really true, there is some ground for the arguments advanced by
critical writers regarding the general subject of nitrogen requirements
of man. The evidence and the arguments, however, have always seemed to
the present writer frail and faulty; but recognizing the hold they have
taken on physiologists and the way they are usually applied to man, I
have attempted to test the matter experimentally under conditions which
would yield trustworthy and conclusive results.

The question how far results obtained with dogs can be applied safely
to man may be open to discussion, but we must first be sure of our
facts before arguments or conclusions of any kind are warranted. It is
to be remembered that dogs are as sensitive in many ways as man, and no
physiological experiment covering a long period of time can be carried
out with any hope of success unless there is due regard for proper
hygienic conditions, some degree of variety in diet, and reasonable
opportunities for fresh air and occasional exercise. I fancy that
even the most vigorous and hardy man, if confined for six consecutive
months in a room just large enough to furnish requisite air-space and
to permit of extending his body at full length, would find himself
at the end of such a period in a condition far from healthful, even
though there were perfect freedom of choice in diet. If, however, there
were added to the above conditions monotony in diet extending through
many months, there would be no occasion for surprise if the individual
lost appetite and strength, and showed signs of disturbance of the
gastro-intestinal tract.

It is doubtful if there is full appreciation of the possible effect
of monotony, in the ordinary dietary experiments on dogs. Man quickly
feels the effect; the sportsman camping in the woods by brook or lake
enjoys his first meal of speckled trout and has no thought of ever
becoming tired of such a delicacy; but as trout cooked in various ways
continue to be placed before him three times a day, and with perhaps
very little else, he soon passes into a frame of mind where salt pork
would be a luxury, and where he would prefer to go hungry rather than
eat the delicacy, if indeed he has appetite to eat anything. Is it
strange that dogs confined in cages barely large enough to permit of
their turning around, and fed day after day and month after month with
exactly the same amount of desiccated meat, fat, and rice, should show
signs and symptoms, if nothing worse, of disturbed nutrition? It is
necessary in experiments of this kind that the animals be confined
for given periods, at least, since otherwise it would be impossible
to determine the extent of nitrogen excretion and the rate of proteid
katabolism, etc. It is possible, however, to limit the time of close
confinement to, say, ten consecutive days, this to be followed by a
like period of comparative freedom, thus insuring opportunities for an
abundance of fresh air and exercise.

The experiments of which I wish to speak, and which had for their
object a study of the effect of low proteid diet on dogs, as types
of high proteid animals, were carried out at our laboratory in the
Sheffield Scientific School and were made possible by liberal grants
from the Carnegie Institution of Washington, thus providing means for
securing the requisite number of chemical assistants. The experiments
were conducted on a somewhat large scale, over twenty dogs being made
use of, while many of the experiments extended through a full year.
The results in their entirety are not yet ready for publication, but
I am able to present in a general way observations on six dogs, which
will serve as an ample illustration of what may be expected with high
proteid animals when living on a low proteid diet under healthful
conditions. All of the six dogs whose cases are here presented were fed
on a mixed diet, with some fresh meat each day; bread, cracker dust,
milk, lard, and rice being the other foods drawn upon to complete the
dietary. The animals were fed twice a day, each meal being accurately
weighed and of definite chemical composition. A large, light, and
airy room, kept scrupulously clean, and in the winter time properly
heated by steam, served as their main abiding place. In this room
were a suitable number of smaller compartments, the walls of which
were composed of open lattice work (of iron), so as not to interfere
with light or air, and yet adequate to keep the dogs apart. These
compartments were not cages in the ordinary sense, but were truly large
and roomy. The entire floor under the dogs was composed of metal, the
joints all soldered, the floor being sloped to a metal gutter in front
so that all the compartments could be flushed out each morning and kept
sweet and clean. In pleasant weather, immediately after their first
meal, the dogs were taken out of doors to a large enclosure near by,
where they were allowed perfect freedom until about four o’clock, when
they were taken in for their second meal (between four and five o’clock
in the afternoon). The outdoor enclosure was inaccessible to every one
except the holder of the key, and the dogs while there were wholly free
from annoyance. Once every month, during a period of ten consecutive
days, each dog was confined in the metabolism cage so as to admit of
the collection of all excreta, in order to make a determination of
the nitrogen balance. Practically, therefore, each dog was in close
confinement only one-third of the month, the remaining two-thirds
being spent in much more congenial surroundings. I have entered thus
fully into a description of the conditions prevailing, because I deem
them exceedingly important, and because therein undoubtedly lies the
explanation of the striking contrast between our results and those of
the earlier investigators of this subject.

In considering the outcome of our experiments, it may be wise to enter
into some detail concerning the first case to be presented. The animal
employed in this experiment was designated as No. 5, and weighed on
July 27, 1905, 17.2 kilograms; it was apparently full grown, but was
thin and had the appearance of being underfed. At first, it was given
daily 172 grams of meat, 124 grams of cracker dust, and 72 grams of
lard, the day’s ration containing 8.66 grams of nitrogen and having a
fuel value of 1389 calories.[72] These figures are equivalent to 80
calories, and 0.50 gram of nitrogen, per kilogram of body-weight. The
animal took kindly to the diet, but on August 3 it refused to eat and
seemed to have a little fever. The next day it was better, but for
the three following days its appetite was poor, and only a portion
of the daily food was eaten. Body-weight began to fall off, and was
soon at 15.5 kilograms. On the 7th of August, a dose of vermifuge
was given, after which the appetite returned and the animal appeared
in good spirits. From this time forward it seemed in perfect health,
with good appetite, and showed the usual vivacity and playfulness of
dog-kind. The diet as specified was continued unchanged until August
25, a balance experiment covering a period of ten days, from the 15th
to the 24th of August inclusive, being carried out, in which the
nitrogen of the intake was compared with the output for each day. From
the accompanying table, where are given the average values of all the
balance periods of the experiment, it is to be seen that during this
first period the animal was laying on or gaining an average of 2 grams
of nitrogen per day.

  [72] The fuel value of the food was calculated from the data given
  in Bulletin No. 28, U. S. Department of Agriculture. All figures for
  nitrogen were obtained by exact chemical analysis.


SUBJECT No. 5. DAILY AVERAGES

  +----------------+-------+-----------------------+-------------------------+------+
  |                |       |         Food.         |          Output.        |      |
  |                |       +------+-------+--------+---------+-------+-------+      |
  |                | Body- |      |Nitro- |  Fuel  |  Nitro- | Nitro-| Nitro-|Nitro-|
  |      Date.     |weight.|Total |gen per| Value  |   gen   |  gen  |  gen  | gen  |
  |                |       |Nitro-| Kilo  |per Kilo| through |through|through| Bal- |
  |                |       | gen. | Body- | Body-  |  Kid-   |Excre- | Hair. | ance |
  |                |       |      |weight.| weight.|neys.[73]| ment. |       |+ or -|
  +----------------+-------+------+-------+--------+---------+-------+-------+------+
  |       1905     | kilos | grams|  gram |calories|  grams  |  gram |  gram |grams |
  |Aug. 15-Aug. 24 |  15.8 | 8.66 |  0.54 |  87.3  |   5.44  |  0.70 |  0.52 |+2.00 |
  |Sept. 6-Sept. 15|  17.1 | 4.76 |  0.27 |  72.4  |   3.41  |  0.32 |  0.48 |+0.55 |
  |Oct.  8-Oct. 17 |  17.6 | 4.76 |  0.27 |  71.8  |   3.54  |  0.54 |  0.49 |+0.19 |
  |Nov.  22-Dec. 1 |  16.9 | 4.77 |  0.28 |  72.0  |   3.76  |  0.39 |  0.32 |+0.30 |
  |      1906      |       |      |       |        |         |       |       |      |
  |Jan.  2-Jan. 11 |  17.2 | 4.07 |  0.23 |  72.0  |   3.19  |  0.54 |  0.35 |-0.01 |
  |Jan. 30-Feb. 8  |  18.0 | 4.07 |  0.23 |  69.0  |   2.87  |  0.54 |  0.62 |+0.04 |
  |Feb. 27-Mar. 8  |  18.2 | 5.18 |  0.28 |  73.0  |   3.69  |  0.66 |  0.74 |+0.09 |
  |Mar. 27-Apr. 5  |  18.3 | 5.23 |  0.28 |  73.0  |   3.66  |  0.84 |  0.48 |+0.25 |
  |Apr. 24-May 3   |  19.1 | 5.22 |  0.27 |  68.0  |   3.76  |  0.38 |  0.48 |+0.60 |
  |May  22-May 31  |  19.4 | 5.22 |  0.26 |  65.0  |   3.44  |  0.31 |  0.48 |+0.99 |
  |June 17-June 26 |  20.0 | 5.24 |  0.26 |  67.0  |   3.50  |  0.71 |  0.48 |+0.55 |
  +----------------+-------+------+-------+--------+---------+-------+-------+------+

  [73] All through the balance periods the dogs were catheterized each
  morning to insure complete collection of the twenty-four hours’ urine.

On August 25, a radical change was made in the diet, by reducing the
amount of meat to 70 grams daily, thereby lowering the intake of
nitrogen to 4.76 grams, or 0.27 gram per kilo of body-weight; the
cracker dust and lard being kept at essentially the same levels as
before. This diet was continued through the next balance period, the
dog in the meantime gaining in body-weight, and showing for the second
balance period an average gain by the body of half a gram of nitrogen
per day. The food was then altered by substituting bread for the
cracker dust, but so adjusted that the nitrogen and fuel values of the
day’s food remained practically unchanged. There was still, however, a
gain in body-weight and a slight gain in body nitrogen. At the close
of the third balance period, the diet was again altered, one-half of
the meat being replaced by milk, while cracker dust was substituted
for the bread. The morning meal consisted of 170 grams of milk, 86
grams of cracker dust, and 18 grams of lard, while the afternoon meal
was composed of 35 grams of meat, 63 grams of cracker, and 35 grams of
lard. The day’s ration, however, still contained 4.76 grams of nitrogen
and had a fuel value of 1249 calories. This diet was maintained until
November 20, when the animal was again placed on a daily ration of meat
(69 grams), bread (166 grams), and lard (80 grams), with a total fuel
value of 1228 calories and 4.77 grams of nitrogen. This was continued
until December 2, the dog still showing a plus nitrogen balance, but
with a little loss in body-weight. On December 2, the diet was again
changed by substituting milk for a portion of the meat, but the
nitrogen and fuel values were maintained at the same level as before.
After a week, December 9, the food was modified as follows: the morning
meal contained 170 grams of milk, 110 grams of rice, and 11 grams of
lard, while the afternoon meal was composed of 35 grams of meat, 81
grams of rice, and 30 grams of lard. The total nitrogen content of the
day’s ration was 4.07 grams, while the fuel value was 1255 calories. At
this time, the animal weighed 17.1 kilograms, consequently the intake
of nitrogen had been reduced to 0.23 gram per kilo of body-weight,
while the fuel value stood at 73 calories per kilogram. This diet was
continued until February 9, the balance period, between January 2 and
11, showing that the animal was in nitrogen equilibrium, in spite of
the material reduction in the intake of proteid, and that body-weight
was increasing. The next balance period, January 30 to February 8,
showed still further gain in weight with continuance of nitrogen
equilibrium. On February 9, the diet was changed by returning to 70
grams of meat, 158 grams of cracker dust, and 60 grams of lard, with a
daily intake of 0.28 gram of nitrogen per kilo of body-weight.

In this manner, the experiment was continued with frequent changes in
the character of the diet, but always maintaining essentially the same
values in nitrogen and calories as shown in the table, until June 27;
having extended through just eleven months, with the animal at the
close of the experiment still gaining in body-weight, with a steady
plus balance of nitrogen, and with every indication of good health and
strength. For ten months the animal lived with perfect comfort and in
good condition on an average daily intake of 0.26 gram of nitrogen
per kilogram of body-weight, and with an average fuel value of 70.3
calories per kilo. Further, it is to be observed that at no time
during the ten months did the daily intake of nitrogen rise above 0.28
gram per kilo, while during one month it fell to 0.23 gram per kilo.
Similarly, the fuel value of the daily food never exceeded 73 calories
per kilo, while at times it dropped as low as 67 and 65 calories per
kilo. That this diet was more than sufficient, both in nitrogen and
fuel value, is indicated by the steady increase in body-weight and by
the plus nitrogen balances observed in most of the periods throughout
the experiment. Indeed, with the comparatively low degree of muscular
activity which this animal was accustomed to, it would have been unwise
to have kept the subject much longer on a diet so rich as the above,
since there would have been danger of detriment to its health and good
condition. When these results are contrasted with the statements of
Munk and Rosenheim, the latter of whom found that even 0.32 gram of
nitrogen and 110 calories per kilo were insufficient to maintain dogs
in a condition of health, it is plain that for some reason our results
are quite at variance with their findings.

The accompanying photographs, taken on August 19, 1905, February 27,
April 24, and at the close of the experiment on June 27, 1906, show the
appearance of the animal at the respective dates, and indicate more
clearly than words can express the actual condition of the animal.

[Illustration: _Subject No. 5._      _August 19, 1905_]

[Illustration: _Subject No. 5._      _November 18, 1905_]

[Illustration: _Subject No. 5._      _April 24, 1906_]

[Illustration: _Subject No. 5._      _June 27, 1906_]

Turning now to a second subject, designated as dog No. 3, the
experiment with which lasted for nearly an entire year, the following
general statements may be made. The animal was a small black and white
fox terrier, weighing on July 6, 1905, 6.5 kilograms. It was a nervous,
affectionate little creature, far less phlegmatic than the animal just
described, always on the alert for a petting, and unceasingly active.
For these reasons, it seemingly required per kilogram of body-weight a
little more food than the preceding animal; a fact also in harmony with
the general law that small animals, per unit of body-weight, need more
food than larger ones. The diet made use of was of the same general
character as employed with the preceding animal, and was changed
from time to time to give requisite variety and to insure freedom from
too great monotony. The accompanying table, showing daily averages
during the twelve balance periods, gives all necessary information
regarding the outcome of the experiment.


SUBJECT No. 3. DAILY AVERAGES

  +----------------+-------+-----------------------+-----------------------+------+
  |                |       |         Food.         |        Output.        |      |
  |                |       +------+-------+--------+-------+-------+-------+      |
  |                | Body- |      |Nitro- |  Fuel  | Nitro-| Nitro-| Nitro-|Nitro-|
  |      Date.     |weight.|Total |gen per| Value  |  gen  |  gen  |  gen  | gen  |
  |                |       |Nitro-| Kilo  |per Kilo|through|through|through| Bal- |
  |                |       | gen. | Body- | Body-  | Kid-  |Excre- | Hair. | ance |
  |                |       |      |weight.| weight.| neys. | ment. |       |+ or -|
  +----------------+-------+------+-------+--------+-------+-------+-------+------+
  |      1905      | kilos | grams|  gram |calories| grams |  gram |  gram | gram |
  |July 18-July 28 |  6.8  | 5.88 |  0.84 |  79.0  |  5.58 |  0.43 |  0.05 |-0.18 |
  |Aug. 15-Aug. 24 |  7.1  | 3.44 |  0.49 |  77.4  |  3.35 |  0.17 |  0.13 |-0.21 |
  |Sept. 6-Sept. 15|  6.9  | 2.11 |  0.30 |  80.0  |  1.93 |  0.21 |  0.07 |-0.10 |
  |Oct.  8-Oct. 17 |  6.9  | 2.10 |  0.30 |  80.0  |  1.83 |  0.20 |  0.07 |  0   |
  |Nov. 22-Dec. 1  |  6.0  | 1.83 |  0.31 |  80.0  |  1.48 |  0.21 |  0.11 |+0.03 |
  |      1906      |       |      |       |        |       |       |       |      |
  |Jan. 2-Jan. 11  |  5.6  | 1.63 |  0.29 |  81.0  |  1.54 |  0.17 |  0.08 |-0.16 |
  |Jan. 30-Feb. 8  |  5.5  | 1.63 |  0.30 |  82.0  |  1.60 |  0.15 |  0.05 |-0.17 |
  |Feb. 27-Mar. 8  |  5.5  | 1.78 |  0.32 |  84.0  |  1.66 |  0.17 |  0.05 |-0.10 |
  |Mar. 27-Apr. 5  |  5.7  | 1.98 |  0.34 |  81.0  |  1.75 |  0.21 |  0.06 |-0.04 |
  |Apr. 24-May 3   |  5.7  | 1.98 |  0.34 |  83.0  |  1.68 |  0.13 |  0.13 |+0.04 |
  |May  22-May 31  |  5.8  | 1.98 |  0.34 |  80.0  |  1.77 |  0.13 |  0.11 |-0.03 |
  |June 17-June 26 |  6.0  | 1.98 |  0.33 |  77.0  |  1.53 |  0.21 |  0.07 |+0.17 |
  +----------------+-------+------+-------+--------+-------+-------+-------+------+

It will be observed that during the first three months the animal
showed a tendency to gain in weight slightly, recalling that its
initial weight on July 6 was 6.5 kilograms. Later, the weight fell
off a little, but in March it showed an upward movement, though very
gradual. With the amount of proteid food given, it is evident that
the animal needed about 80 calories per kilo to maintain a condition
of body-equilibrium. Nitrogen equilibrium was practically maintained
throughout the larger portion of the twelve months, but evidently the
animal required 0.31–0.33 gram of nitrogen per kilogram of body-weight.
Attention may be directed, in view of the results reported by Munk
regarding loss of the power of absorption and utilization of proteid
food, to the figures showing the average daily output of nitrogen
through the excrement. It is plain from the data presented, that this
animal was not suffering from any trouble of this order; indeed, the
utilization of proteid food throughout the entire experiment was
exceedingly complete, as shown by the relatively small loss of nitrogen
through the excrement, thus implying vigorous and unimpaired digestion,
together with thorough absorption of the products formed.

The accompanying photographs show the appearance of the animal on
August 19, 1905, November 18, 1905, April 3 and June 27, 1906, the
close of the experiment.

[Illustration: _Subject No. 3._      _August 19, 1905_]

[Illustration: _Subject No. 3._      _November 18, 1905_]

[Illustration: _Subject No. 3._      _April 24, 1906_]

[Illustration: _Subject No. 3._      _June 27, 1906_]

Passing now to the third subject, we have an experiment of somewhat
shorter duration, viz., of nine months, but sufficiently long to
afford ample opportunity for any deleterious effect to manifest
itself. The initial weight of the dog, No. 13, was 14.5 kilograms on
September 14. The lowest intake of nitrogen was 0.26 gram per kilo
of body-weight per day, while the fuel value of the daily food was
during one period reduced to 55 calories per kilo. A daily proteid
consumption equalling 0.30 gram of nitrogen per kilo, with a total
fuel value in the day’s food of 66–70 calories per kilo, was clearly
quite sufficient to maintain nitrogen equilibrium and body-weight;
indeed, toward the end of the experiment, the animal commenced to gain
in weight quite noticeably on the above diet, and was laying by fairly
large amounts of nitrogen daily. The accompanying table gives the
average daily nitrogen exchange, etc., of the nine balance periods,
while the photographs, taken on the dates indicated under each, show
the appearance of the animal at various times.

SUBJECT No. 13. DAILY AVERAGES

  +---------------+-------+-----------------------+-----------------------+------+
  |               |       |         Food.         |        Output.        |      |
  |               |       +------+-------+--------+-------+-------+-------+      |
  |               | Body- |      |Nitro- |  Fuel  | Nitro-| Nitro-| Nitro-|Nitro-|
  |      Date.    |weight.|Total |gen per| Value  |  gen  |  gen  |  gen  | gen  |
  |               |       |Nitro-| Kilo  |per Kilo|through|through|through| Bal- |
  |               |       | gen. | Body- | Body-  | Kid-  |Excre- | Hair. | ance |
  |               |       |      |weight.| weight.| neys. | ment. |       |+ or -|
  +---------------+-------+------+-------+--------+-------+-------+-------+------+
  |     1905      | kilos | grams|  gram |calories| grams |  gram |  gram | gram |
  |Sept. 24-Oct. 3| 14.0  | 7.22 |  0.52 |  86.0  | 6.40  |  0.71 |  0.19 |-0.08 |
  |Nov. 5-Nov. 14 | 13.0  | 4.78 |  0.35 |  80.0  | 4.29  |  0.37 |  0.25 |-0.13 |
  |Dec. 19-Dec. 28| 13.4  | 3.70 |  0.27 |  72.0  | 2.86  |  0.49 |  0.13 |+0.22 |
  |     1906      |       |      |       |        |       |       |       |      |
  |Jan. 16-Jan. 25| 14.1  | 3.72 |  0.26 |  70.0  | 3.16  |  0.61 |  0.16 |-0.21 |
  |Feb. 13-Feb. 22| 14.3  | 4.26 |  0.30 |  78.0  | 3.54  |  0.67 |  0.37 |-0.32 |
  |Mar. 13-Mar. 22| 14.1  | 3.62 |  0.26 |  55.0  | 3.29  |  0.46 |  0.14 |-0.27 |
  |Apr. 10-Apr. 19| 14.2  | 4.59 |  0.32 |  73.0  | 2.84  |  0.51 |  0.10 |+1.14 |
  |May 8-May 17   | 14.2  | 4.59 |  0.32 |  71.0  | 3.56  |  0.48 |  0.18 |+0.37 |
  |June 5-June 14 | 15.3  | 4.58 |  0.30 |  66.0  | 2.98  |  0.55 |  0.28 |+0.77 |
  +---------------+-------+------+-------+--------+-------+-------+-------+------+

[Illustration: _Subject No. 13._      _January 2, 1906_]

[Illustration: _Subject No. 13._      _February 27, 1906_]

[Illustration: _Subject No. 13._      _April 24, 1906_]

[Illustration: _Subject No. 13._      _June 19, 1906_]

Results of the same general tenor with dogs No. 15 and No. 20 are seen
in the appended tables, while the accompanying photographs testify
clearly to the general good condition of the animals up to the end of
the experiments. In No. 20 particularly, the great gain in body-weight
is to be noted, even though the fuel value of the food was reduced as
low as 64 calories per kilo, with the nitrogen intake at 0.28 gram per
kilo daily. Plainly, the day’s food could have been diminished still
more, with perfect safety to both body and nitrogen equilibrium.


SUBJECT No. 15. DAILY AVERAGES

  +---------------+-------+-----------------------+-----------------------+------+
  |               |       |         Food.         |        Output.        |      |
  |               |       +------+-------+--------+-------+-------+-------+      |
  |               | Body- |      |Nitro- |  Fuel  | Nitro-| Nitro-| Nitro-|Nitro-|
  |      Date.    |weight.|Total |gen per| Value  |  gen  |  gen  |  gen  | gen  |
  |               |       |Nitro-| Kilo  |per Kilo|through|through|through| Bal- |
  |               |       | gen. | Body- | Body-  | Kid-  |Excre- | Hair. | ance |
  |               |       |      |weight.| weight.| neys. | ment. |       |+ or -|
  +---------------+-------+------+-------+--------+-------+-------+-------+------+
  |      1905     | kilos | grams|  gram |calories| grams |  gram |  gram | gram |
  |Nov.  5-Nov. 14|  9.2  | 3.35 |  0.36 |  82.0  | 2.95  |  0.11 |  0.14 |+0.15 |
  |Dec. 19-Dec. 28|  8.9  | 2.61 |  0.30 |  75.0  | 2.47  |  0.12 |  0.12 |-0.10 |
  |      1906     |       |      |       |        |       |       |       |      |
  |Jan. 16-Jan. 25|  8.7  | 2.60 |  0.30 |  79.9  | 2.15  |  0.21 |  0.16 |+0.08 |
  |Feb. 13-Feb. 16|  8.5  | 2.61 |  0.30 |  82.0  | 2.37  |  0.20 |  0.15 |-0.11 |
  |Mar. 13-Mar. 22|  8.7  | 2.82 |  0.32 |  80.0  | 2.68  |  0.17 |  0.19 |-0.22 |
  |Apr. 10-Apr. 19|  9.0  | 2.80 |  0.31 |  82.0  | 2.14  |  0.26 |  0.09 |+0.31 |
  |May   8-May  17|  9.5  | 2.83 |  0.30 |  75.0  | 2.26  |  0.30 |  0.12 |+0.15 |
  |June  5-June 14| 10.2  | 2.81 |  0.27 |  70.0  | 2.26  |  0.28 |  0.24 |+0.03 |
  +---------------+-------+------+-------+--------+-------+-------+-------+------+

[Illustration: _Subject No. 15._      _January 2, 1906_]

[Illustration: _Subject No. 15._      _February 27, 1906_]

[Illustration: _Subject No. 15._      _April 24, 1906_]

[Illustration: _Subject No. 15._      _June 19, 1906_]


SUBJECT No. 20. DAILY AVERAGES

  +---------------+-------+-----------------------+-----------------------+------+
  |               |       |         Food.         |        Output.        |      |
  |               |       +------+-------+--------+-------+-------+-------+      |
  |               | Body- |      |Nitro- |  Fuel  | Nitro-| Nitro-| Nitro-|Nitro-|
  |      Date.    |weight.|Total |gen per| Value  |  gen  |  gen  |  gen  | gen  |
  |               |       |Nitro-| Kilo  |per Kilo|through|through|through| Bal- |
  |               |       | gen. | Body- | Body-  | Kid-  |Excre- | Hair. | ance |
  |               |       |      |weight.| weight.| neys. | ment. |       |+ or -|
  +---------------+-------+------+-------+--------+-------+-------+-------+------+
  |      1905     | kilos | grams|  gram |calories| grams |  gram | gram  | gram |
  |Dec. 6-Dec. 15 | 15.9  | 8.35 |  0.52 |  82.0  | 6.03  |  0.74 | 0.38  |+1.20 |
  |      1906     |       |      |       |        |       |       |       |      |
  |Jan. 16-Jan. 25| 16.4  | 4.47 |  0.27 |  73.0  | 3.61  |  0.55 | 0.15  |+0.16 |
  |Feb. 13-Feb. 22| 17.2  | 4.45 |  0.25 |  72.0  | 3.92  |  0.36 | 0.13  |+0.04 |
  |Mar. 13-Mar. 22| 17.4  | 5.00 |  0.28 |  72.0  | 5.49  |  0.33 | 0.10  |-0.92 |
  |Apr. 10-Apr. 19| 18.4  | 5.60 |  0.30 |  69.0  | 4.88  |  0.52 | 0.18  |+0.02 |
  |May 8-May  17  | 19.6  | 5.58 |  0.28 |  69.0  | 3.85  |  0.75 | 0.38  |+0.60 |
  |June 5-June 14 | 19.7  | 5.59 |  0.28 |  64.0  | 4.69  |  0.45 | 0.40  |+0.05 |
  +---------------+-------+------+-------+--------+-------+-------+-------+------+

[Illustration: _Subject No. 20._      _January 2, 1906_]

[Illustration: _Subject No. 20._      _February 27, 1906_]

[Illustration: _Subject No. 20._      _April 24, 1906_]

[Illustration: _Subject No. 20._      _June 19, 1906_]

The illustrations so far presented, with the general agreement in the
character of the results, might perhaps be interpreted as indicating
that there is no difficulty whatever in bringing a high proteid
consumer, like a dog, down to a low level of proteid consumption.
This, however, would be a false impression. Much depends upon the
character of the proteid food, at least where any attempt at rapid
change is made, for a certain modicum of meat or other animal food
seems a necessary part of the daily diet if health and strength are
to be maintained. A dog transferred suddenly from a daily ration in
which meat and milk are conspicuous elements to a diet in which these
are wholly wanting is very liable to show disturbing symptoms almost
immediately. One case may be cited in illustration of these statements.
On September 29, 1905, dog No. 17, weighing 18.2 kilos, was placed on
a daily diet composed of 70 grams of fresh meat, 442 grams of milk,
300 grams of bread, and 28 grams of lard. This ration contained 9.06
grams of nitrogen and had a fuel value of 1465 calories, or 0.5 gram
of nitrogen and 80 calories per kilogram of body-weight. On October
11, the animal weighed 18.6 kilograms and was in perfect condition. On
the 13th, the meat was reduced to 34 grams per day, but the milk was
increased in amount so as to maintain the same nitrogen intake and fuel
value as before. This diet was continued until November 3, a balance
experiment covering ten days from October 22 to the 31 inclusive,
showing that the animal was laying by a little nitrogen. On November
3, the diet was changed to milk, bread, and lard, the fuel value being
maintained at 80 calories per kilo daily, while the nitrogen intake
was reduced to 0.30 gram per kilo. On this diet, the animal seemed to
thrive perfectly, and at the end of two weeks showed a body-weight of
18.2 kilograms. November 19, the milk was withdrawn, the bread being
increased so as to keep the daily nitrogen intake and the fuel value
unchanged. The day’s food was now composed of bread and lard solely,
but, as just stated, the nitrogen and fuel values were unaltered. In
four days’ time, however, a change began to creep over the animal; the
appetite diminished, and there was apparent a condition of lassitude
and general weakness which deterred the animal from moving about as
usual.

During the next week the animal grew steadily worse, and would eat
only when coaxed with a little milk or with bread softened with milk,
the diet of bread and lard being invariably refused. There was marked
disturbance of the gastro-intestinal tract; bloody discharges were
frequent; the mucous membrane of the mouth was greatly inflamed and
very sore; body-weight fell off, and the animal was in a very enfeebled
condition. This continued until December 4, with every indication that
the animal would not long survive, but by feeding carefully with a
little milk and occasionally some meat, improvement finally manifested
itself, and by December 18 there was good appetite, provided bread was
not conspicuous in the food. Body-weight, which had fallen to 15.5
kilos, was being slowly regained, and on December 30 the animal was
again placed on a weighed diet, consisting of 70 grams of meat, 442
grams of milk, 210 grams of cracker dust, and 10 grams of lard. This
diet contained 8.26 grams of nitrogen and had a fuel value of 1330
calories, equivalent to 0.5 gram nitrogen and 80 calories per kilogram
of body-weight. On January 12, 1906, the weight of the animal was 16.7
kilos, while in general condition there was nothing to be desired. The
food was then modified by diminishing the amounts of meat and milk fed
daily by one-half, thus reducing the nitrogen intake to 0.35 gram per
kilo of body-weight, but maintaining the fuel value of the food at 80
calories per kilo. Under this régime, body-weight still increased,
and on January 27 was 17.5 kilograms. A balance period, shown in the
accompanying table, extending from January 30 to February 8, affords
ample evidence that the body was laying by nitrogen.


SUBJECT No. 17. DAILY AVERAGES

  +---------------+-------+-----------------------+-----------------------+------+
  |               |       |         Food.         |        Output.        |      |
  |               |       +------+-------+--------+-------+-------+-------+      |
  |               | Body- |      |Nitro- |  Fuel  | Nitro-| Nitro-| Nitro-|Nitro-|
  |      Date.    |weight.|Total |gen per| Value  |  gen  |  gen  |  gen  | gen  |
  |               |       |Nitro-| Kilo  |per Kilo|through|through|through| Bal- |
  |               |       | gen. | Body- | Body-  | Kid-  |Excre- | Hair. | ance |
  |               |       |      |weight.| weight.| neys. | ment. |       |+ or -|
  +---------------+-------+------+-------+--------+-------+-------+-------+------+
  |      1905     | kilos | grams|  gram |calories| grams |  gram |  gram | gram |
  |Oct. 22-Oct. 31| 18.3  | 9.06 |  0.49 |  80.0  | 7.73  |  0.66 |  0.28 |+0.39 |
  |      1906     |       |      |       |        |       |       |       |      |
  |Jan. 30-Feb. 8 | 17.6  | 5.77 |  0.33 |  78.0  | 4.12  |  0.44 |  0.21 |+1.00 |
  |Feb. 27-Mar. 8 | 17.9  | 5.31 |  0.30 |  72.0  | 4.59  |  0.59 |  0.37 |-0.24 |
  |Mar. 27-Apr. 5 | 18.1  | 5.33 |  0.29 |  70.0  | 5.63  |  0.89 |  0.27 |-1.52 |
  |Apr. 24-May 3  | 18.4  | 5.90 |  0.32 |  68.0  | 5.06  |  0.49 |  0.30 |+0.05 |
  |May  22-May 31 | 18.6  | 5.90 |  0.31 |  67.0  | 5.25  |  0.53 |  0.43 |-0.31 |
  |June 17-June 26| 19.9  | 5.89 |  0.29 |  70.0  | 4.29  |  0.39 |  0.28 |+0.93 |
  +---------------+-------+------+-------+--------+-------+-------+-------+------+

In all of the subsequent months, a small amount of meat was a part of
the daily food, but as is seen from the table of balance periods, the
total nitrogen intake and the fuel value of the food were reduced to
even lower levels per kilogram of body-weight. Yet the animal gained
steadily, until at the latter part of June the weight was considerably
above that noted at the commencement of the experiment in the preceding
October. Further, the animal was in nitrogen equilibrium or even
gaining nitrogen, and in perfect condition of health and vigor, as
is indicated by the accompanying photographs taken at the different
periods stated. Especially to be emphasized is the fact that during the
last six months of the experiment, the daily intake of nitrogen and the
fuel value of the food were as low or even lower than in November, when
the daily diet was limited to bread and lard. The disastrous result
which showed itself at once on this latter diet, with all animal food
excluded, was not due to low proteid or to deficiency in fuel value,
but simply to the fact that the animal for some reason could not
adjust itself to a simple dietary of bread and fat, although there was
ample available nitrogen and fuel value for the body’s needs. Something
was lacking, which meat or milk could supply, and this something was
indispensable for the maintenance of the normal nutritional rhythm.

[Illustration: _Subject No. 17._      _January 2, 1906_]

[Illustration: _Subject No. 17._      _February 27, 1906_]

[Illustration: _Subject No. 17._      _April 24, 1906_]

[Illustration: _Subject No. 17._      _June 27, 1906_]

This is by no means an exceptional case, but we can cite many other
examples of like results where the animal when restricted to a purely
vegetable diet, such as bread, pea-soup, bean soup, etc., reinforced by
an animal fat, quickly passed from a condition of health into a state
of utter wretchedness, with serious gastro-intestinal disturbance.
The results are not to be attributed to the lower utilization of the
vegetable food, for the disastrous effect is too quickly manifest, and
further, often shows itself when the animal plainly has a large store
of available nutriment in its own tissues.

This experiment with dog No. 17 has been dwelt upon at some length,
because it illustrates a very important principle in the nutrition
of a high proteid and carnivorous animal. As before stated, it is
not a question of high or low proteid simply, but involves possibly
the more subtle question of the relative value of specific forms of
proteid food. It will be noted that this statement is made somewhat
guardedly, in harmony with the caution necessarily called for in view
of our lack of knowledge regarding the possible need of the animal’s
body for extraneous principles which only meat, milk, or other animal
products can supply. Inorganic salts, nitrogenous extractives, and
other substances without any appreciable fuel value, are quite likely
to be of primary importance in controlling and regulating the various
processes of the body, which combine to maintain the condition of
normal nutrition. With a diet restricted to one or two vegetable
products, it is quite conceivable that something may be lacking
which the system demands, though it cannot be measured in terms of
nitrogen or calories. It may be said that man thrives on a purely
vegetable diet, but while this is unquestionably true, it must be
remembered that man with his free choice of food has recourse, as a
rule, to a large variety of vegetable products from many sources,
and consequently there is great likelihood of his absorbing from
these varied products such supplementary matters as may be needed. On
this question, we are in a realm of doubt and uncertainty, but the
possibilities suggested must not be ignored, for they may contain a
germ of truth of the utmost importance. The fact remains, however, that
a dog when restricted to a purely vegetable dietary does not thrive; a
little animal food seems necessary to keep up health and strength, and
this suffices even though the daily nitrogen intake and fuel value of
the food are restricted to a level below that of the vegetable dietary.

With these facts before us, it is difficult to avoid the conclusion
that some significance may attach to the specific nature of the
proteid. Of course, we must not overlook the radical difference in
dietary habits of man and dog. Man as an omnivorous creature has for
generations been accustomed to partake largely of vegetable foods,
and as a result his digestive tract and his system as a whole has
become acclimated, as it were, to the nutritive effects of vegetable
matter. Dogs, on the other hand, are typical carnivores, and their
habits for generations have led in an opposite direction, so that their
gastro-intestinal tracts and their systems have become accustomed
to the effects of a diet in which animal food largely predominates.
Whether these deeply ingrained characteristics are responsible in
any large measure for the difference in behavior of man, on a purely
vegetable diet, and dogs is open to question. It would certainly not be
strange if such were the case, but as we look at the facts collected in
our study of this subject, it is somewhat impressive to note how well
dogs thrive on a relatively large amount of vegetable food, provided
there is a modicum of animal food added thereto. In other words,
these high proteid consumers are apparently quite able to utilize the
vegetable foods, but there is something lacking in such a dietary
which the body has great need of. Is it not quite possible, as already
suggested, that the specific nature of the proteid counts for something
in nutrition? The question cannot be answered definitely at present,
but there are certain facts slowly accumulating which make the question
a pertinent one in this connection.

Thus, it is becoming evident, as was pointed out in an earlier chapter,
that the many proteid substances occurring in the animal and vegetable
kingdoms are more or less unlike each other in their chemical make-up.
They yield different decomposition products, or the same products
in widely different proportion, when broken down by the action of
hydrolyzing agents; and when we recall that the digestive enzymes of
the body convert the proteids of the food into these same end-products,
it is plain that in the assimilation and utilization of the proteid
foodstuffs the body has to deal with these various chemical units.
Hence, an animal suddenly restricted to a dietary in which all of the
proteid is furnished by bread might be seriously incommoded, either by
the excess of certain amino-acids resulting therefrom, or by a lack
of certain other end-products to which its body is accustomed. As an
example, we may take the three typical proteids of the wheat kernel,
gliadin, glutenin, and leucosin, and note the very striking difference
in the proportion of certain of the decomposition products of each, as
reported by Osborne and Clapp.[74]

  [74] See Osborne and Clapp: The Chemistry of the Protein Bodies of
  the Wheat Kernel. American Journal of Physiology, vol. 17, p. 231.

  +-----------------+-------------+-------------+-------------+
  |                 |             |             |             |
  |                 |  Gliadin.   |  Glutenin.  |  Leucosin.  |
  |                 |             |             |             |
  +-----------------+-------------+-------------+-------------+
  |                 |   per cent  |   per cent  |   per cent  |
  | Leucin          |     5.61    |     5.95    |    11.34    |
  | Lysin           |      0      |     1.92    |     2.75    |
  | Arginin         |     3.16    |     4.72    |     5.94    |
  | Glutaminic acid |    37.33    |    23.42    |     6.73    |
  | Ammonia         |     5.11    |     4.01    |     1.41    |
  | Aspartic acid   |     0.58    |     0.91    |     3.35    |
  | Tyrosin         |     1.20    |     4.25    |     3.34    |
  +-----------------+-------------+-------------+-------------+

It is obvious from these figures that the three proteids of the wheat
kernel are radically different from each other. Contrast, for example,
the content of glutaminic acid in gliadin with the amount in leucosin.
With such striking differences in chemical make-up, it is reasonable
to assume that corresponding differences in physiological action or
food values may exist. Further, “in respect to the amount of these
amino-acids, leucosin more nearly resembles the animal proteins than
the seed proteins thus far examined, and in this connection it is
interesting to note that leucosin occurs chiefly if not wholly in the
embryo of this seed and is probably one of its ‘tissue’ proteins, in
contrast to the ‘reserve’ proteins of the endosperm of which gliadin
and glutenin form the chief part” (Osborne and Clapp). In other words,
animal proteids, such as those of meat, are characterized like leucosin
by a small content of glutaminic acid and ammonia; while leucin, lysin,
aspartic acid, and arginin are relatively more abundant. Until we know
more on this subject, however, any broad generalization would be out
of place, but certainly there is justification for the supposition
that in these differences in chemical constitution are to be found
explanation of some of the peculiarities common to certain varieties of
proteid food. Wheat flour, aside from its starch, is composed mainly
of glutenin and gliadin with their large content of glutaminic acid.
Meat proteids, on the other hand, like leucosin, contain only a small
fraction of this acid, and, with the other differences indicated,
meat proteid and wheat proteid as food for dogs or other high proteid
consumers may reasonably be expected to have at the least very unequal
values. And if we go a step beyond this and suppose that in the
formation of true tissue proteid or the living protoplasm of the cell,
certain of these end-products of proteid decomposition are absolutely
indispensable, we can easily picture for ourselves a dearth of such
building stones in the long-continued use of a diet which lacks that
particular proteid from which the necessary building stones can be
split off in adequate number.

It has been said, notably by Munk, that in dogs fed for some time on
a low proteid diet there is a diminished power of absorption from the
intestinal tract, associated with weakened digestion. If it is true
that a lowered proteid intake results in a diminished utilization of
the ingested food, that efficiency in the digestion and absorption of
foodstuffs is impaired, it can only be interpreted as meaning that
some injurious influence has been exerted on the epithelial cells of
the intestine or the adjacent gland cells. We have, however, failed
to find any evidence of deleterious action in the dogs that we have
experimented with, where due regard was paid to maintaining a diet
suitable for the physiological needs of the body. In the experiments
that we have cited, both nitrogen intake and the fuel value of the food
per day were lower than in Munk’s experiments, but the utilization of
fat and proteid was not sensibly affected. The following tables give
the results with ten dogs (including the six dogs already described)
for lengths of time ranging from seven to twelve months, the periods
indicated being each of ten days’ duration and occurring once each
month. In the first table, the utilization of fat is shown, the
figures given being based on determinations of the amount of fat
contained in the excrement. Knowing the amount of fat in the daily
food and the amount which passed through the intestine, it is easy to
calculate the percentage of fat utilized.


UTILIZATION OF FAT IN PERCENTAGES.

  +----------+-------------------------------------------------+
  |          |                     Dogs.                       |
  | Periods. +----+----+----+----+----+----+----+----+----+----+
  |          |  1 |  2 |  3 |  4 |  5 | 12 | 13 | 15 | 17 | 20 |
  +----------+----+----+----+----+----+----+----+----+----+----+
  |    1     | 97 | 96 | 93 | 97 | 97 | 96 | 96 | 98 | 98 | 95 |
  |    2     | 96 | 96 | 98 | 98 | 98 | 94 | 95 | 97 | 98 | 95 |
  |    3     | 98 | 97 | 97 | 99 | 96 | 97 | 97 | 98 | 94 | 98 |
  |    4     | 98 | 96 | 97 | 97 | 96 | 94 | 95 | 98 | 97 | 97 |
  |    5     | 96 | .. | 94 | 98 | 97 | 95 | 95 | 98 | 97 | 96 |
  |    6     | 97 | 98 | 94 | 98 | 97 | 96 | 94 | 97 | 96 | 97 |
  |    7     | 97 | 98 | 98 | 97 | 96 | 93 | 95 | 97 | 98 | 96 |
  |    8     | .. | .. | 98 | 96 | 96 | 96 | 93 | 97 | .. | .. |
  |    9     | .. | .. | 98 | 97 | 98 | .. | 97 | 98 | .. | .. |
  |   10     | .. | .. | 98 | 97 | 98 | .. | .. | .. | .. | .. |
  |   11     | .. | .. | 97 | 92 | 97 | .. | .. | .. | .. | .. |
  |   12     | .. | .. | 97 | 97 | .. | .. | .. | .. | .. | .. |
  +----------+----+----+----+----+----+----+----+----+----+----+

It is perfectly plain from these results that there was no falling off
in the utilization of fat; the percentage amount digested and absorbed,
as in dogs 3 and 4, was just as large at the end of the twelve months’
experiment as at the beginning. Clearly, a so-called low nitrogen
intake with dogs does not lead to any loss of power in the utilization
of the fat of the food. This being so, it is equally clear that the
arguments based on Munk’s results in this direction, and applied to
man, are without adequate foundation.


UTILIZATION OF NITROGEN IN PERCENTAGES.

  +----------+-------------------------------------------------+
  |          |                     Dogs.                       |
  | Periods. +----+----+----+----+----+----+----+----+----+----+
  |          |  1 |  2 |  3 |  4 |  5 | 12 | 13 | 15 | 17 | 20 |
  +----------+----+----+----+----+----+----+----+----+----+----+
  |    1     | 95 | 91 | 92 | 94 | 91 | 91 | 90 | 93 | 92 | 91 |
  |    2     | 92 | 94 | 94 | 95 | 93 | 90 | 92 | 96 | 92 | 87 |
  |    3     | 91 | 92 | 90 | 91 | 88 | 89 | 86 | 95 | 89 | 91 |
  |    4     | 90 | 85 | 90 | 92 | 91 | 82 | 83 | 91 | 83 | 93 |
  |    5     | 90 | 82 | 88 | 92 | 86 | 85 | 84 | 96 | 91 | 90 |
  |    6     | 86 | 87 | 89 | 83 | 86 | 89 | 87 | 94 | 91 | 86 |
  |    7     | 87 | 87 | 90 | 83 | 87 | 83 | 88 | 90 | 93 | 91 |
  |    8     | .. | .. | 90 | 83 | 84 | 81 | 89 | 89 | .. | .. |
  |    9     | .. | .. | 89 | 87 | 92 | .. | 87 | 89 | .. | .. |
  |   10     | .. | .. | 93 | 85 | 94 | .. | .. | .. | .. | .. |
  |   11     | .. | .. | 93 | 81 | 86 | .. | .. | .. | .. | .. |
  |   12     | .. | .. | 89 | 92 | .. | .. | .. | .. | .. | .. |
  +----------+----+----+----+----+----+----+----+----+----+----+

The figures in the above table were obtained by determining the
amount of nitrogen in the dried excrement from the animals, _i. e._
the amount that passed through the intestine unchanged;[75] and
knowing the content of nitrogen in the daily food, the percentage of
unabsorbed nitrogen was then easily calculated, after which by simple
subtraction the percentage of utilized nitrogen was found. At first
glance, it would appear that as the experiments proceeded utilization
of nitrogen was less complete. In a sense, this was true, but it was
not connected with any impairment of the digestive or absorptive
powers of the intestine. It must be remembered that in the earlier
periods a larger proportion of the ingested nitrogen was in the form of
readily digestible meat, but as the latter was reduced in amount larger
proportions of vegetable food were introduced in order to maintain the
desired fuel value, and consequently the percentage of non-absorbable
nitrogen was increased. The well-known difference in the availability
of animal and vegetable proteid has already been referred to in other
connections; a difference due not so much to any inherent quality in
the digestibility of the two forms of proteid as to the presence of
cellulose and other material in the vegetable food which retards in
some measure the action of the digestive juices. To this cause must
be ascribed the slight falling off in the utilization of nitrogen
noticeable in most of the experiments. If, however, the figures are
compared with those usually obtained on a diet largely vegetable in
nature, it will be seen that the utilization of nitrogen by these dogs
was in no sense abnormal.

  [75] There is an unavoidable error here, since the excrement contains
  not only undigested food, but also contains some nitrogenous matter
  derived from the secretions of the intestine, etc.

These experiments on the influence of a low proteid diet on dogs, as
a type of high proteid consumers, taken in their entirety, afford
convincing proof that such animals can live and thrive on amounts of
proteid and non-nitrogenous food far below the standards set by Munk
and Rosenheim. The deleterious results reported by these investigators
were not due to the effects of low proteid or to diminished consumption
of non-nitrogenous foods, but are to be ascribed mainly to non-hygienic
conditions, or to a lack of care and physiological good sense in the
prescription of a narrow dietary not suited to the habits and needs
of this class of animals. Further, it is obvious that the more or
less broad deductions so frequently drawn from the experiments of
Munk and Rosenheim, especially in their application to mankind, are
entirely unwarranted and without foundation in fact. Our experiments
offer satisfying proof that not only can dogs live on quantities
of proteid food per day smaller than these investigators deemed
necessary, and with a fuel value far below the standard adopted by
them; but, in addition, that these animals are quite able on such a
diet to gain in body-weight and to lay by nitrogen, thereby indicating
that even smaller quantities of food might suffice to meet their true
physiological requirements.

The results of these experiments with dogs, which we have recorded in
such detail, are in perfect harmony with the conclusions arrived at by
our experiments and observations with man, and serve to strengthen the
opinion, so many times expressed, that the dietary habits of mankind
and the dietary standards based thereon are not always in accord with
the true physiological requirements of the body. If these views are
correct, and the facts presented seemingly indicate that they are,
it is time for enlightened people to give heed to such suggestions,
that their lives may be ordered more nearly in accord with the best
interests of the body. Physiological economy in nutrition is not a
myth, but a reality full of promise for the welfare of the individual
and of the community in general. Ignorance on dietary matters should
give place to an intelligent comprehension of the body’s needs, and an
adequate understanding of how best to meet the legitimate demands of
the system for nourishment under given conditions of life. It is said
that more than half the earnings of the working people of this country
is spent for food. Here, we have suggested another form of economy as
worthy of consideration; less important perhaps than that which relates
to health and strength, but still calling for thoughtful attention. We
cannot afford to be ignorant of these things; we must have definite
knowledge of the actual facts, and these can only be obtained by
careful research and investigation.

As a prominent writer on nutrition has well said, “The health and
strength of all are intimately dependent upon their diet. Yet most
people understand very little about what their food contains, how it
nourishes them, whether they are economical or wasteful in buying and
preparing it for use, and whether or not the food they eat is rightly
fitted to the demands of their bodies. The result of this ignorance is
great waste in the purchase and use of food, loss of money, and injury
to health” (Atwater). We all recognize the general force and truth of
this statement, but there is a surprising lack of appreciation of the
full significance of what is involved thereby. If it is true that the
demands of the body for proteid food--which of all foods is the most
expensive--are fully met by an amount equal to one-half that ordinarily
consumed, and that health and strength are more satisfactorily
maintained thereby, it is easy to see how the acquisition of dietary
habits leading to consumption of food in harmony with physiological
needs will result in a fruitful twofold economy; viz., economy in
expenditure, and of still greater moment, economy in the activities of
the body by which food and its waste products are cared for.




CHAPTER VIII

PRACTICAL APPLICATIONS WITH SOME ADDITIONAL DATA

  TOPICS: Proper application of the results of scientific research
  helpful to mankind. Dietary habits should be brought into conformity
  with the true needs of the body. The peculiar position of proteid
  foods emphasized. The evil effects of overeating. What the new
  dietary standards really involve. The actual amounts of foodstuffs
  required. Relation of nutritive value to cost of foods. The
  advantages of simplicity in diet. A sample dietary for a man of
  70 kilograms body-weight. A new method of indicating food values.
  Moderation in the daily dietary leads toward vegetable foods. The
  experiments of Dr. Neumann. The value of fruits as food. The merits
  of animal and vegetable proteids considered in relation to the
  bacterial processes in the intestine. A notable case of simplicity
  in diet. Intelligent modification of diet to the temporary needs of
  the body. Diet in summer and winter contrasted. Value of greater
  protection to the kidneys. Conclusion.


Knowledge has value in proportion to the benefit it confers, directly
or indirectly, on the human race. Every new scientific fact or
principle brought to light promises help in the understanding of
Nature’s laws, and when rightly interpreted and properly applied is
sure to aid in the advancement and prosperity of the individual and of
the community. Proper methods of living, economical adjustment of the
intake to the varying needs of the body, avoidance of excessive waste
of foodstuffs and of energy, are all desirable precepts, which rational
people presumably are inclined to follow so far as their knowledge and
understanding of the subject will permit. Here, as elsewhere, false
teaching may be exceedingly mischievous and lead to costly errors;
while blind reliance upon customs, instinct, and superstitions is
hardly in keeping with twentieth-century progress.

Modern scientific methods should give us help in dietetics, as in
other branches of hygiene and practical medicine. A few short years
ago, diphtheria was a scourge which brought misery to many a home,
for there was at hand no adequate means of combating the disease; but
scientific research has given us new light, and placed at our command a
weapon of inestimable value. Do we hesitate to use it when the occasion
arises, because it happens to be out of keeping with old-time customs
and traditions? No, we recognize the possibility of help, and as the
need is urgent we turn to it quickly, with hope and thankfulness that
scientific progress has opened up a pathway of escape from a threatened
calamity.

Not many years ago we drank freely of such water as was at hand,
without realization of danger from bacteria or disease germs, looking
on epidemics of typhoid fever perhaps as a visitation of Divine
Providence, in punishment of our many sins and to be borne meekly and
with resignation. But all this has changed through the researches of
bacteriologists and chemists; scientific facts of the utmost importance
have been clearly established; a classification of water-borne
diseases has been adopted, and we realize fully that diseases of this
order can be kept from our doors by proper precautions applied to
our water supply. To-day, epidemics of typhoid fever are traceable
solely to the ignorance or carelessness of the individual or of the
commonwealth, and the exemption which we of the present generation
have from this class of diseases is directly due to the application of
precautionary measures based on the information furnished by scientific
investigation. It is proper for us to use caution in the acceptance of
new ideas, but not that form of caution which refuses change on the
ground that what has been is sufficiently good for the present and the
future. The point of view is ever changing with advance of knowledge,
and it is not profitable to exclude opportunities for improvement
in personal hygiene and general good health, any more than in other
matters that affect the prosperity of the individual or the community.

Dietary habits should be brought into conformity with the true needs of
the body. Excessive consumption of proteid food, especially, should be
avoided on the ground that it is not only unnecessary and wasteful, but
is liable to bring penalties of its own, most undesirable and wholly
uncalled for. We may, perhaps, accept these statements at their full
value, and yet have a shadow of doubt in our minds as to whether, after
all, dietary customs do not harmonize sufficiently at least with true
nutritive requirements. All the data that we have presented in the
preceding chapters, however, have seemingly given a positive answer to
such doubts, and indicate quite clearly that the results of scientific
study are opposed to the prevailing dietary standards, especially
as regards proteid food. As the celebrated physiologist Bunge has
expressed it, “The necessity for a daily consumption of 100 grams of
proteid is incomprehensible, so long as we do not know of any function
of the body in the performance of which the chemical potential energies
of the destroyed proteid are used up.”

Perfectly trustworthy evidence is at hand showing that the needs of
the body for potential energy can be fully met, and indeed are more
advantageously met, by the non-nitrogenous foods, carbohydrates and
fats. The energy of muscle work, as we have seen, comes preferably
from the breaking down of non-nitrogenous material, so that there is
no special call for proteid in connection with increased muscular
activity. In fact, it would appear that the need for proteid food
by man is limited to the requirements of growth and development,
reinforced by the amount called for in that form of tissue exchange
which we have emphasized under the term “endogenous proteid
metabolism,” or true tissue metabolism. To be sure, there must be a
certain reserve of proteid, available in case of emergency, but this is
easily established without resorting to excessive feeding.

The peculiar position which proteid foods occupy in man’s dietary
naturally make them the central figure, around which the other foods
are grouped. No other form of food can take the place of proteid; a
certain amount is needed each day to make good the loss of tissue
material broken down in endogenous katabolism, and consequently our
choice and combination of the varied articles of diet made use of
should be regulated by the amount of proteid they contain. But while
proteid foods occupy this commanding position, it is not necessary
or desirable that they should exceed the other foodstuffs in amount,
or indeed approach them in quantity. We must be ever mindful of
the fact, so many times expressed, that proteid does not undergo
complete oxidation in the body to simple gaseous products like the
non-nitrogenous foods, but that there is left behind a residue of
non-combustible matter--solid oxidation products--which are not
so easily disposed of. In the forceful language of another, “The
combustion of proteid within the organism yields a solid ash which must
be raked down by the liver and thrown out by the kidneys. Now when
this task gets to be over-laborious, the laborers are likely to go on
strike. The grate, then, is not properly raked; clinkers form, and
slowly the smothered fire glows dull and dies” (Curtis).

Even though no such dire fate overtakes one, the penalties of excessive
proteid consumption are found in many ills, for which perhaps the
victim seeks in vain a logical explanation; gastro-intestinal
disturbance, indigestion, intestinal toxæmia, liver troubles, bilious
attacks, gout, rheumatism, to say nothing of many other ailments,
some more and some less serious, are associated with the habitual
overeating of proteid food. But excessive food consumption is by no
means confined to the proteid foodstuffs; general overfeeding is a
widespread evil, the marks of which are to be detected on all sides,
and in no uncertain fashion. One of the most common signs of excessive
food consumption is the tendency toward obesity, a condition which
is distinctly undesirable and may prove decidedly injurious. Undue
accumulation of fat is not only a mechanical obstacle to the proper
activity of the body as a whole, but it interferes with the freedom
of movement of such muscular organs as the heart and stomach, thereby
interposing obstacles to the normal action of these structures.
Further, whenever undue fat formation is going on in the body, there is
the ever present danger that the lifeless fat may replace the living
protoplasm of the tissue cells and so give rise to a condition known as
“fatty degeneration.” While a superabundance of fat in the body is a
sure telltale of overeating, the absence of obesity is by no means an
indication that excess of food is being avoided. There is here, in man
as in animal kind, much idiosyncrasy; some persons, especially those
endowed with a long and large frame, tend to keep thin even though
they eat excessively, while others grow fat much more readily. As a
well-known physician has expressed it, “In the one case, the subject
burns, instantly and mercilessly, every stick of fuel delivered at his
door, whether or not he needs the resulting hot fire roaring within,
while the other, miser-like, hoards the rest in vast piles, filling the
house from cellar to garret.”

Temperance in diet, like temperance in other matters, leads to good
results, and our physiological evidence points out plainly, like
a signpost all can read, that there is no demand on the part of
the body for such quantities of food as custom and habit call for.
Healthfulness and longevity are the prizes awarded for the successful
pursuance of a temperate life, modelled in conformity with Nature’s
laws. Intemperance, on the other hand, in diet as in other matters,
is equally liable to be followed by disaster. A physician of many
years’ experience, with opportunities for observation among different
classes of people, has written, “that overeating tends to shrink the
span of life in proportion as it expands the liver is demonstrable
both directly and indirectly. Let any actuary of life-insurance be
asked his experience with heavy-weight risks, where the waist measures
more than the chest, and the long-drawn face of the businessman, at
memory of lost dollars, will make answer without need of words. Then
let be noted the physique of the blessed ones that attain to green old
age, and, in nine cases out of ten, spry old boys--no disparagement,
but all honor in the phrase--will be found to be modelled after the
type of octogenarian Bryant or nonogenarian Bancroft--the whitefaced,
wiry, and spare, as contrasted with the red-faced, the pursy, and the
stout. It is true, as has already been mentioned, that in old age
much of an adventitious obesity is absorbed and disappears, but the
Bryant-Bancroft type is that of a subject who never has been fat at
all. And just such is preëminently the type that rides easily past the
fourscore mark, reins well in hand, and good for many another lap in
the race of life.”[76]

  [76] Edward Curtis, M.D.: Nature and Health, p. 70. Henry Holt &
  Company, New York, 1906.

With these thoughts before us, we may consider briefly just what is
involved in these new dietary standards that aim to conform more
closely with actual body needs. Referring at first to proteid food, it
may be wise to again emphasize the fact that the weight of the body,
_i. e._, the weight of the proteid-containing tissues, as contrasted
with excessive fat accumulation, is one of the important factors not to
be overlooked when determining the dietary needs of a given individual.
As must be perfectly clear, from all that has been said, the man of
170 pounds’ body-weight has more proteid tissue to nourish than the
man of 130 pounds’ weight, and consequently what will satisfy the
requirements of the latter individual will not suffice for the former.
We must understand distinctly that no general statement can be made
applicable to mankind at large, but due consideration must be given to
the size and weight of the individual structure. We have found that
the average need for proteid food by adults is fully met by a daily
metabolism equal to an exchange of 0.12 gram of nitrogen per kilogram
of body-weight. This means a katabolism of three-fourths of a gram of
proteid matter daily, per kilogram.

Remembering, however, that the intake of proteid food must be somewhat
in excess of the actual proteid katabolism, since not all of the
proteid of the food is available, and as this is a variable amount
depending upon the proportion of animal and vegetable foods with their
different degrees of digestibility and availability, we may place the
required intake of proteid at 0.85 gram per kilogram of body-weight,
still keeping to maximum figures for safety’s sake. Hence, for a man
weighing 70 kilograms or 154 pounds, there would be required daily
59.5 grams--say 60 grams--of proteid food to meet the needs of the
body. These are perfectly trustworthy figures, with a reasonable margin
of safety, and carrying perfect assurance of being really more than
sufficient to meet the true wants of the body; adequate to supply all
physiological demands for reserve proteid, and able to cope with the
erratic requirements of personal idiosyncrasies. It will be observed
that such an intake of proteid food daily is equal to one-half the Voit
standard for a man of this weight, while it is still further below the
Atwater standard and far below the common practices of the majority of
mankind in Europe and America, as indicated by the published dietary
studies.

It may not be out of place to state at this point that in the writer’s
opinion the use of the terms “standard diet” and “dietary standards,”
etc., is objectionable, since such usage seems to demand a certain
degree of definiteness in the daily diet for which there is no
justification. As in the use of the term “normal diet,” there is danger
of misinterpretation, and of the assumption that dietary habits should
be regulated strictly in accord with certain set principles. This I
believe to be altogether wrong; there should be, on the contrary,
full latitude for individual freedom, but freedom governed by an
intelligence that appreciates the significance of scientific fact
and is willing to mould custom and habit into accord with them. What
is needed to-day is not so much an acceptance of the view that man
requires daily 0.85 gram of proteid per kilogram of body-weight, as a
full appreciation of the general principle, which our definite figures
have helped to work out, that the requirements of the body for proteid
food are far below the customary habits of mankind, and that there is
both economy and gain in various directions to be derived by following
the general precepts which this view leads to. In other words, there
is no advantage, but, on the contrary, much bother and worriment, in
attempting to follow out in practice the details of our more or less
exact physiological experiments.

The general teaching which they afford, however, can be adopted and
put in practice in our daily lives, without striving to follow too
closely the so-called standards which our experiments have led to.
Again, the sample dietaries adopted in our experiments have no special
virtue, aside from the general principle they teach that simple foods
are quite adequate for the nourishment of the body, and that the amount
of nitrogen or proteid they contain was sufficient to meet the demands
of the particular individuals consuming it. Broadening intelligence on
matters of food composition is called for on all sides, and as this
is acquired together with due appreciation of the relative nutritive
values of proteid, fat, and carbohydrate, there is placed at our
command the power of intelligent discrimination, with the ability
to apply the principles set forth in our own way, in harmony with
personal likes and dislikes.

To the majority of us, not very familiar with the percentage
composition of ordinary food materials, and unaccustomed to the
weighing of food in grams, the figures given from time to time may
have failed to convey a very definite impression regarding the actual
amounts of the various foods made use of. Further, our ideas concerning
the bulk of many of the common articles of food necessary to furnish
the 60 grams of proteid required daily by a man of 70 kilograms
body-weight may be somewhat hazy. The following table, however, will be
of service in this direction:


SIXTY GRAMS OF PROTEID ARE CONTAINED IN

                                                   Fuel Value[77]
  One-half pound fresh lean beef, loin              308 calories
  Nine hens’ eggs                                   720
  Four-fifths pound sweetbread                      660
  Three-fourths pound fresh liver                   432
  Seven-eighths pound lean smoked bacon            1820
  Three-fourths pound halibut steak                 423
  One-half pound salt codfish, boneless             245
  Two-and one-fifth pounds oysters, solid           506
  One-half pound American pale cheese              1027
  Four pounds whole milk (two quarts)              1300
  Five-sixths pound uncooked oatmeal               1550
  One and one-fourth pounds shredded wheat         2125
  One pound uncooked macaroni                      1665
  One and one-third pounds white wheat bread       1520
  One and one-fourth pounds crackers               2381
  One and two-thirds pounds flaked rice            2807
  Three-fifths pound dried beans                    963
  One and seven-eighths pounds baked beans         1125
  One-half pound dried peas                         827
  One and eleven-twelfths pounds potato chips      5128
  Two-thirds pound almonds                         2020
  Two-fifths pound pine nuts, pignolias            1138
  One and two-fifths pounds peanuts                3584
  Ten pounds bananas, edible portion               4600
  Ten pounds grapes                                4500
  Eleven pounds lettuce                             990
  Fifteen pounds prunes                            5550
  Thirty-three pounds apples                       9570

  [77] Fuel value of the quantity needed to furnish the sixty grams of
  proteid.

The figures in this table are instructive in many ways. First, it is
to be noted that the daily proteid requirement of sixty grams can
be obtained from one-half pound of lean meat (uncooked), of which
the loin steak is a type. Subject to some variations in content of
water, an equivalent weight of lean flesh of any variety, lamb, veal,
poultry, etc., will furnish approximately the same amount of proteid.
With fish, such as halibut steak, and with liver, three-quarters of a
pound are required; while with sweetbreads, four-fifths of a pound are
needed to furnish the requisite amount of proteid. Of salt codfish,
one-half pound will provide the same amount of proteid as an equivalent
weight of fresh beef; while with lean smoked bacon the amount rises
to seven-eighths of a pound. Among the vegetable products, it is to
be observed that dried peas and beans, almonds and pine nuts, are as
rich in proteid as the above-mentioned animal foods, essentially the
same weights being called for to provide the daily requirement of
proteid. The same is true of cheese, the variety designated having such
a composition that one-half pound is the equivalent, so far as the
content of proteid is concerned, of a like amount of fresh beef. We
must not be unmindful of the fact previously mentioned, however, that
there are differences in digestibility among these various foodstuffs
which tend to lower somewhat the availability of the vegetable
products, also of the cheese, thereby necessitating a slight increase
in the amount of these foods required to equal the value to the body of
lean meat.

Secondly, passing to the other extreme in our list, we find indicated
types of foods exceedingly poor in proteid, such as the fruits;
notably, bananas, grapes, prunes, apples, etc., also lettuce, and in
less degree potatoes. These are the kinds of food that may legitimately
attract by their palatability, but do not add materially to our intake
of proteid even when consumed in relatively large amounts. Thirdly, we
see clearly indicated a radical difference between the animal foods
and those of vegetable origin, in that with the former the fuel value
of the quantity necessary to furnish the sixty grams of proteid is
very small, as compared with a like amount of the average vegetable
product. One-half pound of lean meat, for example, with its 60 grams of
proteid, has a fuel value of only 308 calories, while two-thirds of a
pound of almonds has a fuel value of 2020 calories, and one-half pound
of dried peas 827 calories. Naturally, this is mainly a question of
the proportion of fat or oil present. With fat meat, as in bacon, the
calorific value rises in proportion to increase in the amount of fat,
the proteid decreasing in greater or less measure.

The main point to be emphasized in this connection, however, is that a
high proteid animal food, like lean meat, eggs, fish, etc., obviously
cannot alone serve as an advantageous food for man. We see at once the
philosophy of a mixed diet. Let us assume that our average man of 70
kilograms body-weight needs daily 2800 calories. On this assumption,
if he were to depend entirely upon lean beef for his sustenance, he
would require daily four and a half pounds of such meat, which amount
would furnish nine times the quantity of proteid needed by his system.
The same would be more or less true of other kindred animal products.
On the other hand, certain vegetable foods on our list, such as flaked
rice, crackers, and shredded wheat, contain proteid, with carbohydrate
and fat, in such proportion that the energy requirement would be met
essentially by the same quantity as served to furnish the necessary
proteid. Passing to the other extreme among the vegetable products, as
in potatoes and bananas, for example, we find fuel value predominating
largely over proteid content. The ideal diet, however, is found in a
judicious admixture of foodstuffs of both animal and vegetable origin.
Wheat bread, reinforced by a little butter or fat bacon to add to its
calorific value, shredded wheat with rich cream, crackers with cheese,
bread and milk, eggs with bacon, meat with potatoes, etc.; the common,
every-day household admixtures, provide combinations which can easily
be made to accord with true physiological requirements. The same may be
equally true of the more complicated dishes evolved by the high art of
modern cookery.

Lastly, our table throws light upon certain questions of household
economy. The cost of foods is regulated mainly not by the value of the
nutrients contained therein, but by other factors of quite a different
nature. Relationship between supply and demand naturally counts here
as in other directions, but our demand is liable to be based not upon
food values, but rather upon delicacy of flavor, palatability, and
other kindred fancies, some real and some imaginary. Ordinary crackers
can be purchased for ten cents a pound, but if we desire a little
stronger flavor of salt and a special box to hold them, we pay eighteen
cents a pound. Rolled very thin and thus made more delicate, they cost
twenty-five cents, while sold under a special name and perhaps tied
with a blue ribbon they cost thirty-five cents a pound. Their nutritive
value per pound is the same in all cases, but we pay something for
the increased labor of preparation and a good deal for the added
attractiveness to eye and palate. We pay twenty-two cents a pound for
round steak, thirty-two cents for loin steak, and seventy-five cents a
pound for sweetbreads, the high price of the latter being regulated by
the relative scarcity of the article and not by its food value. As our
table indicates, the real value of sweetbread as a source of proteid
is only a little more than half that of lean beef. Its fuel value,
however, is somewhat more than that of beef, but a little fat added
to the latter will more than compensate and at a trifling cost. When
we can afford it, we pay the increased price for sweetbreads simply
because their delicacy and flavor are attractive to us. We should
not do it under the mistaken idea that we are indulging in a highly
nutritive article of food, for as a matter of fact it is not only less
nutritive than a corresponding weight of lean beef, but in addition
it possesses certain qualities, in its high purin-content, that are a
menace to good health if indulged in too freely.

Where expense must be carefully guarded, or where the condition of the
family purse is such that conflicting demands must be intelligently
considered in order to insure wise expenditure and the greatest
permanent good of the many, it is well to remember that price is no
guarantee whatever of real nutritive value. Two quarts of milk will
furnish half the daily fuel requirement of our average man and the
entire proteid requirement, while its cost is only sixteen cents.
Reinforced by a pound loaf of wheat bread, the energy requirement for
the day would be fully met, with surplus nitrogen to store up for
future needs, and at an additional cost of only ten cents. A mixture in
this proportion, however, would not be strictly physiological, since
it is wasteful of proteid, but it may serve to illustrate the point.
A better illustration is found in an admixture, quite adequate to
supply the daily needs of our average man, both for proteid and energy,
composed of one-quarter of a pound of lean beef, two-thirds of a pound
of bread, and half a pound of butter, and at a total cost not to exceed
thirty cents. The contrast of such prices with what is so commonly
paid for table delicacies is somewhat striking; it could be made still
more so by drawing upon many common vegetable foods, rich alike in
proteid and in fuel value, the cost of which is even less than the
simple food mixtures just referred to. It is not necessary, however, to
enlarge upon this question; it is sufficient to merely emphasize the
fact that the exaggerated demand of our present generation for dietetic
luxuries is leading us far away from the proverbially simple life of
our forefathers, and without adding in any way to the effectiveness of
the daily diet. On the contrary, it is in part responsible for the high
proteid consumption of the present day, with its attendant evils, and
involves a large and unnecessary expenditure without adequate return.
The wants of the body for food are far more advantageously met by a
simple dietary, moderate in amount and at an expense comparatively
slight.

A recent writer,[78] in the “British Medical Journal,” a practitioner
of medicine in the Highlands of Scotland, has said that these are
“facts of common experience in the Highlands of Scotland, and probably
among the peasantry of other countries also, where the old beliefs and
customs have not too readily given way to the luxuries of civilization.
Oatmeal in one form or another is a daily ingredient in the diet of a
Highland peasant. The potato also is a staple food, and is consumed in
large quantities with salt herring or other fish, and perhaps in some
cases salt mutton or pork. Milk and eggs are used by most. The growing
consumption of tea, however, and the increasing relish for sweets,
candy, pastry, and biscuits, threaten to destroy the old way of living.
A typical day’s diet for a crofter or fisherman who still believes in
the traditional diet would be somewhat like this:

  Breakfast.--Oatmeal porridge or brose with milk; bread, butter, and tea.
  Dinner.--Potatoes galore and herrings, or other salt fish.
  Supper.--Porridge and milk, or oat bread and cheese, and tea.

  [78] Aran Coirce: British Medical Journal, April 7, 1906, p. 829.

“I have often been assured by shepherds that they could work all
day ‘on the hill’ after a breakfast of oatmeal brose and milk,
without fatigue and without feeling hungry, returning in the evening
to partake of a dish of broth, potatoes, and salt mutton. In these
diets, proteid forms a very small proportion, and yet a hardier race
than these shepherds and fishermen cannot be found.” It should be
added that “brose” consists of a few handfuls of oatmeal, to which
is added boiling water, the mixture being stirred vigorously and
placed for a few minutes near the fire. It is then eaten with milk, or
better, with cream. In the absence of positive data, it can only be
asserted that the above dietary stands for simplicity and frugality.
Its proteid-content may be low, but the amount of proteid taken per
day by these Highlanders will obviously depend upon the _quantity_
of food consumed. Oatmeal is fairly rich in proteid, and it is quite
conceivable that the amount eaten daily may be such as to result in a
high proteid exchange.

It will be profitable for us to gain, if possible, a fairly clear idea
of the quantities of food requisite for our average man of 70 kilograms
body-weight; _i. e._, the amounts necessary to provide 60 grams of
proteid and 2800 calories. With this end in view, we may outline a
simple dietary, expressed in terms that will convey a clear impression,
showing what may be eaten without overstepping the required limits of
proteid or total calories:


BREAKFAST

                                           Proteid       Calories
  One shredded wheat biscuit               3.15 grams      106
      30 grams
  One teacup of cream                      3.12            206
     120 grams
  One German water roll                    5.07            165
      57 grams
  Two one-inch cubes of butter             0.38            284
      38 grams
  Three-fourths cup of coffee              0.26            ...
     100 grams
  One-fourth teacup of cream               0.78             51
      30 grams
  One lump of sugar                         ...             88
      10 grams                            -----            ---
                                          12.76            850


LUNCH

                                           Proteid       Calories
  One teacup homemade chicken soup         5.25 grams       60
      144 grams
  One Parker-house roll                    3.38            110
      38 grams
  Two one-inch cubes of butter             0.38            284
      38 grams
  One slice lean bacon                     2.14             65
      10 grams
  One small baked potato                   1.53             55
      2 ounces, 60 grams
  One rice croquette                       3.42            150
      90 grams
  Two ounces maple syrup                    ...            166
      60 grams
  One cup of tea with one slice lemon       ...             ..
  One lump of sugar                         ...             38
      10 grams                            -----            ---
                                          16.10            928


DINNER

                                                      Proteid   Calories
  One teacup cream of corn soup                         3.25       72
      130 grams
  One Parker-house roll                                 3.38      110
      38 grams
  One-inch cube of butter                               0.19      142
      19 grams
  One small lamb chop, broiled                          8.51       92
      lean meat, 30 grams
  One teacup of mashed potato                           3.34      175
      167 grams
  Apple-celery lettuce salad with mayonnaise dressing   0.62       75
      50 grams
  One Boston cracker, split                             1.32       47
      2 inches diameter, 12 grams
  One-half inch cube American cheese                    3.35       50
      12 grams
  One-half teacup of bread pudding                      5.25      150
      85 grams
  One demi-tasse coffee                                  ...       ..
  One lump of sugar                                      ...       38
      10 grams                                         -----      ---
                                                       29.21      951

The grand totals for the day, with this dietary, amount to 58.07 grams
of proteid and 2729 calories. It is of course understood that these
figures are to be considered as only approximately correct, but the
illustration will suffice, perhaps, to give a clearer understanding of
the actual quantities of food involved in a daily ration approaching
the requirements for a man of 70 kilograms body-weight. Further, there
may be suggested by the figures given for proteid and fuel value of
the different quantities of foods, a clearer conception of how much
given dietary articles count for in swelling the total values of a
day’s intake. Moreover, it is easy to see how the diet can be added to
or modified in a given direction. If a little more proteid is desired
without changing materially the fuel value of the food a boiled egg can
be added to the breakfast, for example. An average-sized egg (of 53
grams) contains 6.9 grams of proteid, while it will increase the fuel
value of the food by only 80 calories. Or, if more vegetable proteid is
wished for, a soup of split-peas can be introduced, without changing
in any degree the calorific value of the diet. Thus, one teacup of
split-pea soup (144 grams) contains 8.64 grams of proteid, while the
fuel value of this quantity may be only 94 calories. The addition of
one banana (160 grams) will increase fuel value 153 calories, but will
add only 2.28 grams of proteid. If it is desired to increase fuel
value without change in the proteid-content of the food, recourse can
always be had to butter, fat of meat, additional oil in salads, or to
syrup and sugar.

Such a menu as is roughly outlined, however, has perhaps special value
in emphasizing how largely the proteid intake is increased by foods
other than meats, and which are not conspicuously rich in proteid
matter. All wheat products, for example, while abounding in starch,
still show a large proportion of proteid. Thus, shredded wheat biscuit
(1 ounce), which is a type of many kindred wheat preparations, from
bread and biscuit to the various so-called breakfast foods, yields
about 3 grams of proteid per ounce and approximately 100 calories.
Even potato, which is conspicuously a carbohydrate food owing to its
large content of starch, yields of nitrogen the equivalent of at least
three-fourths of a gram of proteid per ounce. If larger volume is
desired without much increase in real food value, there are always
available green foods, such as lettuce, celery, greens of various
sorts, fruits, such as apples, grapes, oranges, etc. Too great reliance
on meats as a type of concentrated food, on the other hand, augments
largely the intake of proteid, and adds a relatively small amount to
the fuel value of the day’s ration.

An ingenious method of indicating food values, which promises to
be of service in sanatoria and under other conditions where it is
desirable to record or correct the diet of a large number of persons,
has been devised recently by Professor Fisher.[79] The method aims to
save labor, and is likewise designed to visualize the magnitude and
proportions of the diet. The food is measured by calories instead of
by weight, a “standard portion” of 100 large calories being the unit
made use of. In carrying out the method, foods are served at table in
“standard portions,” or multiples thereof. In the words of Fisher, the
amount of milk served, for example, “instead of being a whole number of
ounces, should be 4.9 ounces--the amount that contains 100 calories.
This ‘standard portion’ constitutes about two-thirds of an ordinary
glass of milk. Of the 100 calories which it contains 19 will be in the
form of proteid, 52 in fat, and 29 in carbohydrate.” In the carrying
out of this plan, it is evident that the weight of any food yielding
100 calories becomes a measure of the degree of concentration. From
the standpoint of fuel value, olive oil is probably one of the most
concentrated of foods, approximately one-third of an ounce containing
100 calories. The following table, taken from Fisher’s description
of his method, will serve to show the amounts of several foods
constituting a “standard portion,” and also the number of calories in
the form of proteid, fat, and carbohydrate:

  [79] Irving Fisher: A new method for indicating food values. American
  Journal of Physiology, vol. 15, p. 417, 1906.

  +-----------------------+-------------+--------+--------+--------+--------+
  |     Name of Food      |   Weight    |        |        |        |        |
  |     and “Portion”     | containing  |Proteid.|  Fat.  | Carbo- | Total. |
  |   roughly estimated.  |100 Calories.|        |        |hydrate.|        |
  +-----------------------+------+------+--------+--------+--------+--------+
  |                       |ounces|grams |calories|calories|calories|calories|
  |Almonds, a dozen       | 0.53 |  15  |  13.0  |  77.0  |    10  |   100  |
  |Bananas, one large     | 3.50 |  98  |   5.0  |   5.0  |    90  |   100  |
  |Bread, a large slice   | 1.30 |  37  |  13.0  |   6.0  |    81  |   100  |
  |Butter, an ordinary pat| 0.44 |  13  |   0.5  |  99.5  |    ..  |   100  |
  |Eggs, one large        | 2.10 |  60  |  32.0  |  68.0  |    ..  |   100  |
  |Oysters, a dozen       | 6.80 | 190  |  49.0  |  22.0  |    29  |   100  |
  |Potatoes, one          | 3.60 | 100  |  10.0  |   1.0  |    89  |   100  |
  |Whole milk,            |      |      |        |        |        |        |
  |  two-thirds glass     | 4.90 | 140  |  19.0  |  52.0  |    29  |   100  |
  |Beef sirloin,          |      |      |        |        |        |        |
  |  a small piece        | 1.40 |  40  |  31.0  |  69.0  |    ..  |   100  |
  |Sugar, five teaspoons  | 0.86 |  24  |  ....  |  ....  |   100  |   100  |
  +-----------------------+------+------+--------+--------+--------+--------+

Obviously, to make use of the “calories per cent” method a table such
as the above, covering all common foodstuffs and showing the weight of
each food constituting a standard portion, together with the calories
of proteid, fat, and carbohydrate in this portion, is necessary. The
chief advantage of the method, however, is that it lends itself readily
to geometrical representation and affords an easy means of determining
the constituents of combinations of different foods by use of a simple
mechanism, for a description of which reference must be made to the
original paper.

Any attempt to follow a daily routine which accords with the true
needs of the body leads necessarily toward foods derived from the
plant kingdom, with the adoption of simple dietary habits, and with
greater freedom from the exciting influence of the richer animal
foods. There is, however, virtue in a simple dietary that appeals and
satisfies, and in so doing testifies to the completeness with which
it meets the physiological requirements of the body. A physician,[80]
writing in the “British Medical Journal,” says: “I determined to give
the minimum-of-proteid diet a fair trial in my own case. The result
was that I was relieved of a life-long tendency to acid dyspepsia and
occasional sick headache; my fitness for work, my appetite and relish
for food, were increased, without any diminution, but rather a slight
increase, in my weight. My practice extends over a wide area of rough
mountainous country involving long journeys on cycle, on foot, driving,
and in open boats, in fair and foul weather. The muscular exertion
and endurance necessary for the work would seem to require a large
proportion of proteid and a generous diet altogether, but since I began
to experiment I have suffered less than formerly from fatigue, and seem
to eat in all a smaller quantity of food. My diet consists of:

  [80] Aran Coirce: British Medical Journal, April 7, 1906, p. 829.

  Breakfast, 8.30 A.M.--Oatmeal cakes, bread and butter, about 1 cubic
  inch of cheese or bloater paste, marmalade, and one breakfast cup of
  tea.

  Lunch, 1.30 P.M.--Same as breakfast, with occasionally a boiled egg,
  and sometimes coffee instead of tea.

  Dinner, 7 P.M.--Thick soup containing vegetables, with bread,
  followed by suet pudding or fruit tart; or vegetable stew, containing
  2 or 3 ounces of meat, with boiled potatoes, followed by milk pudding
  and jam, and occasionally a cup of black coffee.”

This statement of personal experience is in close accord with
statements that have come to the writer in hundreds of letters during
the past two or three years, from persons who have for some reason
chosen to follow a more abstemious mode of life. Such testimony has a
certain measure of value in that it offers corroborative evidence of
the beneficial effects of a moderate diet, more closely in accord with
the actual demands of the body for food. It does not, however, carry
quite that degree of assurance that scientific evidence, gathered by
careful observers and controlled by weights and measures that hold the
imagination in check, affords; and so we may turn to a different type
of testimony, presented in an elaborate research by Dr. Neumann,[81]
of the Hygienic Institute at Kiel, an experiment on himself extending
through a total of 746 days.

  [81] Dr. med. et phil. R. O. Neumann: Experimentelle Beiträge zur
  Lehre von dem täglichen Nahrungsbedarf des Menschen unter besonderer
  Berücksichtigung der notwendigen Eiweissmenge. Archiv für Hygiene,
  Band 45, p. 1, 1902.

The experiment was divided into three periods. In the first period of
ten months the subject, with a body-weight of 66.5 kilograms, consumed
daily on an average the amounts of food indicated in the following
table. In this same table are also included the daily values, based on
the preceding data, for a body-weight of 70 kilograms. Thirdly, the
table likewise shows the amounts of utilizable food contained in the
foodstuffs actually eaten, on the basis of 70 kilos body-weight.


AVERAGE DAILY FOOD FOR TEN MONTHS

  +------------+-----------------+----------------+-----------------+
  |            |Actually consumed|Calculated for a| Utilizable Food |
  |            | by the Subject, | Body-weight of |for a Body-weight|
  |            |   66.5 Kilos    |   70 Kilos     |   of 70 Kilos   |
  +------------+-----------------+----------------+-----------------+
  |Proteid     |    66.1 grams   |   69.1 grams   |   57.3 grams    |
  |Fat         |    83.5         |   90.2         |   81.2          |
  |Carbohydrate|   230.0         |  242.0         |  225.0          |
  |Alcohol     |    43.7         |   45.6         |   41.0          |
  |Fuel value  |  2309 calories  | 2427 calories  |  2199 calories  |
  +------------+-----------------+----------------+-----------------+

During this period of ten months, the body-weight of the subject
remained practically constant, or indeed showed a slight gain up to 67
kilograms. All the functions of the body, and the general condition
of good health, were in no wise impaired; so that in the words of the
subject, the amount of food eaten must have been sufficient for the
needs of the body. Somewhat striking is the fact that of the 2309
calories in the daily food, more than one-fourth was derived from the
beer consumed daily (1200 c.c.). Also noticeable is the relatively
small amount of carbohydrate taken daily, only about one-half the
quantity designated by Voit as the average requirement of German
laborers. Finally, it is to be observed that during this period of
ten months, the daily consumption of food as calculated for a man of
70 kilograms body-weight, based on the actual food consumption of the
subject with a weight of 66.5 kilos, was not widely different from our
own statement of 60 grams of proteid and 2800 calories. The tendency,
however, in Dr. Neumann’s experiment was toward lower fuel values and
somewhat higher proteid consumption.

In a second period of 50 days, with a slightly larger daily intake,
Dr. Neumann observed that his body was laying by nitrogen, _i. e._,
storing up proteid on a daily diet of 76.5 grams of proteid and with
sufficient fat and carbohydrate to furnish a total fuel value of 2658
calories. In the final period of 8 months, the following data were
obtained:


AVERAGE DAILY FOOD FOR EIGHT MONTHS

  +------------+-----------------+----------------+-----------------+
  |            |Actually consumed|Calculated for a| Utilizable Food |
  |            | by the Subject, | Body-Weight of |for a Body-Weight|
  |            |   71.5 Kilos.   |   70 Kilos.    |   of 70 Kilos.  |
  +------------+-----------------+----------------+-----------------+
  |Proteid     |   76.2 grams    |   74.0 grams   |    61.4 grams   |
  |Fat         |  109.0          |  106.1         |    95.5         |
  |Carbohydrate|  168.9          |  164.2         |   152.7         |
  |Alcohol     |    5.5          |    5.3         |     4.7         |
  |Fuel value  |  2057 calories  |  1999 calories |   1766 calories |
  +------------+-----------------+----------------+-----------------+

During this period, it is to be noted that the fuel value of the
day’s food averaged only 2057 calories, which for a body-weight of
70 kilograms would amount to less than 2000 calories. The proteid
consumption, however, was larger than we have found to be necessary
for a man of the above weight. Still, the facts are in harmony with
the general principle that there is no necessity for a daily intake
of food such as common usage dictates, there being obviously a wide
difference between a minimal daily consumption of 118 grams of proteid
and 3000 or more calories, such as is assumed to be needed by a man of
70 kilos, and 74 grams of proteid with 1999 calories. Under the latter
conditions, the subject gained a kilogram in weight during the eight
months, while the establishment of nitrogen equilibrium testifies to
the now generally accepted view that it is quite possible for the body
to establish nitrogen equilibrium at different levels, _i. e._, with
different quantities of proteid food and different fuel values.

The diet made use of by Neumann was a mixed one, containing a great
variety of animal and vegetable foods, but withal simple and moderate
in quantity. Calculated per kilogram of body-weight, the average
consumption of food material per day during the three periods was as
indicated in the following table:


DAILY FOOD CONSUMPTION PER KILOGRAM OF WEIGHT

  +-------------+--------+-----+-------------+--------+--------+
  |             |Proteid.| Fat.|Carbohydrate.|Alcohol.|Calories|
  +-------------+--------+-----+-------------+--------+--------+
  |             |  grams |grams|    grams    | grams  |        |
  |First Period |  0.99  | 1.3 |     34.5    |  0.56  |  34.7  |
  |Second Period|  1.10  | 2.3 |     33.4    |  . .   |  59.7  |
  |Third Period |  1.00  | 1.5 |     23.4    |  0.07  |  28.5  |
  +-------------+--------+-----+-------------+--------+--------+

The average of daily food consumption for the total of 746 days was as
follows: 74.2 grams proteid, 117 grams fat, 213 grams carbohydrate, and
2367 calories. On such a diet, during this long period, equilibrium was
satisfactorily maintained, thereby furnishing additional evidence that
quantities of food way below the so-called normal amounts are quite
adequate to meet the needs of the body. There is no conflict whatever
between these results and our own; they both point in the same general
direction. Perhaps the one thing that needs to be again emphasized,
however, in view of the low fuel values used by Neumann, is that while
they proved quite adequate in his case, the demand in this direction is
governed largely by the degree of bodily activity. In fact, Neumann’s
results with fuel values are in perfect harmony with the data obtained
by us with professional men, but the writer is inclined to believe that
for the majority of mankind, with the varying degrees of activity and
muscular exertion called for, a somewhat larger number of heat units is
desirable, and indeed on many occasions demanded.

Still, it is perfectly obvious that custom has greatly exaggerated
the fuel values required in ordinary muscular work, and such results
as are here presented tend to emphasize the true relationship between
actual requirements and fuel intake. Further, it must not be overlooked
that the rate of proteid katabolism is governed in large measure by the
amount of non-nitrogenous food, and consequently a too narrow margin in
the consumption of the latter will obviously result in a higher rate
of proteid exchange. We are inclined to the belief that a satisfactory
degree of bodily efficiency is more liable to be maintained with a
somewhat larger consumption of carbohydrate food, combined with a
reduction in proteid food to a level nearer our own figures. It will
be observed that the average amount of carbohydrate taken daily by
Neumann, during the 746 days, was only 213 grams, while the daily
consumption of fat averaged 117 grams. These figures are interesting
and instructive in many ways, especially as indicating the ease with
which the body accommodates itself to a relatively low intake of
proteid food, combined with a small proportion of starches and sugars.
This relationship between carbohydrate and fat might well occur at
times as a natural result of personal taste, but as a general rule it
is probably better, from the standpoint of digestibility and general
availability, for the daily food to contain a larger proportion of
carbohydrate.

Under this head, I would lay special stress upon the value to the
body of the natural sugars as well as of starch. We are inclined to
deprecate the widespread use of candy, especially among children, and
there is no doubt that the too lavish use of sugar in such concentrated
form does at times do harm; but when eaten as an integral part of the
many available fruits its use cannot be too highly lauded, for both
young and old. Oranges, grapes, prunes, dates, plums, and bananas are
especially to be commended, and in lesser degree peaches, apricots,
pears, apples, figs, strawberries, raspberries, and blueberries. In
all of these fruits, it is the sugar especially that gives food value
to the article, while the mild acids and other extractives, together
with the water of the fruit, help in other ways in the maintenance
of good health. Where personal taste and inclination are favorably
disposed, the first six fruits named can be partaken of freely, and the
diet of the young, especially, can be advantageously modified by the
liberal use of such articles of food.

Of the other fruits, apples when thoroughly ripe are above reproach
if properly masticated, but the raw fruit is somewhat indigestible
when swallowed in too large pieces, and may cause trouble to a
delicate stomach. A baked apple, on the other hand, is both savory and
wholesome, and if served with sugar and cream, for example, constitutes
a most healthful and satisfying article of food. Peaches, apricots,
and strawberries as ripe fruits are likewise exceedingly valuable, but
here personal idiosyncrasy frequently comes to the fore, especially
with strawberries, and prohibits their free use. The peculiar acidity
of these latter fruits is occasionally a source of trouble, which
leads to their avoidance; but this is far less liable to happen with
people living on a low proteid diet with its greater freedom from purin
derivatives, or uric acid antecedents. Further, there is a tendency
on the part of some individuals to suffer from acid fermentation with
too liberal use of starches and sugar, but as a rule the advantages
of ordinary starchy and natural sugar-containing foods cannot be
overestimated. It is certainly wise to give them a conspicuous place in
the daily dietary and to encourage their use, especially by children.

As has been stated in several connections, a diet which conforms to the
true nutritive requirements of the body must necessarily lead toward
vegetable foods. In no other satisfactory way can excess of proteid be
avoided, and at the same time the proper calorific value be obtained.
This, however, does not mean vegetarianism, but simply a greater
reliance upon foods from the plant kingdom, with a corresponding
diminution in the typical animal foods. This raises the question of the
possible relation of diet to the bacterial processes of the intestine,
knowing, as we do, that the latter are of primary importance in the
causation of certain forms of auto-intoxication, etc. Recent studies
have indicated that the bacterial flora of carnivorous animals is
quite different from that of herbivorous animals, and this being so,
it is easy to see how a predominance of vegetable or animal food may
modify the bacterial conditions of the intestinal tract in man. Dr.
Herter[82] has reported the presence in the intestines of cats, dogs,
tigers, lion, and wolf of many spore-holding bacilli, as well as free
spores and vegetative forms of anærobic organisms; some of which at
least are decidedly pathogenic when injected into the subcutaneous
connective tissue, leading to serious and even fatal results within
twenty-four hours. With herbivorous animals, on the other hand, such as
the buffalo, goat, horse, elephant, etc., the predominating organisms
are of a different order from those found in the intestines of the
carnivora; proving practically non-pathogenic, or only slightly so,
when injected subcutaneously, and less disposed to produce putrefactive
changes or other chemical decompositions.

  [82] C. A. Herter: Character of the Bacterial Flora of Carnivorous
  and Herbivorous Animals. Science, December 28, 1906, p. 859.

In the words of Dr. Herter, “These differences in the appearance
and behavior of the bacteria derived from typical carnivora and
herbivora suggest that the habit of living upon a diet consisting
exclusively of raw meat entails differences in the types of bacteria
that characterize the contents of the large intestine. The occurrence
of considerable numbers of spore-bearing organisms in the carnivora
points to the presence of anærobic putrefactive forms in great
numbers. The results of subcutaneous inoculations into guinea-pigs
bear out this view and indicate that the numbers of organisms capable
of producing a hemorrhagic œdema with tissue necrosis, with or without
gas-production, are very considerable.... The observations recorded are
of much interest in relation to the bacterial processes and nutrition
of herbivorous as distinguished from carnivorous animals, and are
significant furthermore for the interpretation of bacterial conditions
found in man. The question arises whether the abundant use of meat over
a long period of time may not favor the development of much larger
numbers of spore-bearing putrefactive anærobes in the intestinal tract
than would be the case were a different type of proteid substituted for
meat.” While it may be said truly that observations of this character
are as yet not sufficiently numerous or conclusive to warrant positive
or sweeping statements, yet there is a suggestion here well worthy of
thoughtful consideration in its general bearing on the nutrition of
mankind.

Simplicity in diet, with or without complete abstinence from meat,
is often resorted to as a means of relief from bodily ailments, and
such cases sometimes afford striking illustrations of the adequacy
and benefits of a relatively low intake of food. Cases of this sort,
perhaps, are more frequently observed among elderly people, where the
daily requirements are not so great as with younger and more active
persons, but they offer evidence in support of our main thesis that
dietary habits are no guarantee of bodily requirements. I have in mind
the details of an exceedingly interesting case reported with much care
by Dr. Fenger;[83] the case of a man who at 61 years of age, after a
long period of poor health, brought himself quickly into a condition of
sound health by a daily diet characterized by extreme simplicity and
with an exceedingly low fuel value. The daily diet made use of during
the fifteen years the subject was under examination consisted of the
following articles:

  [83] Dr. S. Fenger: Beiträge zur Kenntniss des Stoffwechsels im
  Greisenalter. Skandinavisches Archiv für Physiologie, Band 16, p.
  222, 1904.

  1889–1892: 1 egg, 1 quart of oatmeal soup, 2 quarts of skim milk,
  1-1/2 ounces of red wine, 1/4 ounce of sugar.

  1892–1894: 2 eggs, 1 quart of oatmeal soup, 2 quarts of skim milk,
  1-1/2 ounces of red wine, 1/4 ounce of sugar.

  1894–1900: 3 eggs, 1 pint of oatmeal soup, 2 quarts of skim milk,
  1-1/2 ounces of red wine, 1/4 ounce of sugar, 2 ounces of plum and
  raspberry juice.

  1900–1903: 3 eggs, 1 pint of barley soup, 3 pints of sweet milk, 1
  pint of buttermilk, 1-1/2 ounces of red wine, 1/4 ounce of sugar, 2
  ounces of plum and raspberry juice.

It will be observed that during these fifteen years the subject
partook of no meat whatever, and further, that the diet was wholly
in fluid form. At the close of this long period, the subject, being
then 75 years of age, was reported as well and in good health, with
satisfactory physical condition for a person of his years. He was a
man of small body-weight, only 42 kilograms, but during this period
of voluntary restriction in diet, he suffered no loss. It is perhaps
worthy of comment also that all through this lengthy period no salt
was taken other than what was naturally present in the simple foods
made use of. The point to attract our attention especially, however, is
that for fifteen years, during which the quality and quantity of this
man’s food was carefully observed, body-weight, general good health,
and physical vigor were all maintained, together with freedom from the
ills of previous years and with a daily diet characterized by extreme
simplicity. The chemical composition of the diet was likewise peculiar,
particularly in its exceedingly low fuel value. The following table
shows the amounts of proteid, fat, and carbohydrate consumed daily
during the four periods designated:

  +---------+--------+-----+--------+---------+---------+---------+
  |         |        |     | Carbo- |         |Calories | Proteid |
  | Period. |Proteid.|Fat. |hydrate.|Calories.|   per   |   per   |
  |         |        |     |        |         |Kilogram.|Kilogram.|
  +---------+--------+-----+--------+---------+---------+---------+
  |         | grams  |grams|  grams |         |         |  grams  |
  |1889–1892|  79.8  | 21.7|  152.0 |   1125  |    26   |  1.90   |
  |1892–1894|  85.2  | 27.0|  152.0 |   1200  |    28   |  2.03   |
  |1894–1900|  87.0  | 30.1|  150.1 |   1230  |    29   |  2.07   |
  |1900–1903|  84.4  | 73.7|  148.3 |   1600  |    38   |  2.00   |
  +---------+--------+-----+--------+---------+---------+---------+

Especially noticeable here is the low intake of fat and carbohydrate,
with the corresponding low fuel value, and also the relatively high
consumption of proteid, averaging 2.0 grams daily per kilogram of
body-weight. Dr. Fenger concludes that for a man of this age and
weight, with the relative inactivity characteristic of old age, a heat
value in the intake of 30 calories per kilogram of body-weight is quite
sufficient for the needs of the body. This may be quite true, but to
maintain nitrogen equilibrium under such conditions requires a larger
intake of proteid food than is desirable. It will be observed that in
the last period of four years a very decided change in the diet was
instituted; proteid was diminished somewhat, but the noticeable change
was the decided increase in fat, produced in large measure by the
substitution of whole milk, with its contained cream, for skim milk. In
the words of Dr. Fenger, this change was necessitated by the appearance
of gout in the subject. From superficial examination of the dietary of
the preceding eleven years there would seem no occasion for criticising
the subject for high living, and yet I believe we are quite within the
limits of reason in saying that the proteid exchange for a subject of
this body-weight was altogether too high. The heat requirements of the
body were being met in an unnecessarily large degree from the breaking
down of proteid material, with consequent formation of excessive
nitrogenous waste, among which uric acid was plainly conspicuous.

One comment to be made here is that meat and other rich
purin-containing foodstuffs are not the only source of gout and uric
acid. Excessive proteid katabolism, both exogenous and endogenous, is
a possible source of danger in this respect, and the above subject,
though living on an exceptionally simple diet, was consuming far more
proteid per kilogram of body-weight than was necessary or desirable.
Increase of fatty food naturally served to diminish the rate of proteid
katabolism, and this could have been advantageously accompanied by
a still greater reduction in the amount of proteid ingested, and a
larger addition of non-nitrogenous foodstuffs. In old age, there is
naturally a slowing down of the metabolic processes, and both nitrogen
equilibrium and body equilibrium can be satisfactorily maintained by a
relatively small intake of food and with gain to the body; but there
is every reason to believe that economy in proteid food can be more
advantageously adopted than economy in non-nitrogenous foodstuffs.

Finally, we may call attention to the many possibilities of an
intelligent modification of the daily diet to the temporary needs
of the individual. The season of the year, summer and winter, the
climate, the degree of activity of the body, the state of health,
temporary ailments, etc., all present special conditions which admit
of particular dietetic treatment. In hot summer weather, for example,
there is plainly less need for food than in the cold winter season,
especially for fat with its high calorific value. During the cold part
of the year, the lower temperature of the surrounding air, with the
tendency toward greater muscular activity, calls for more extensive
chemical decomposition in order to meet the demand for heat, and the
energy of muscular contraction. There is perhaps no special reason for
any material change in the amount of proteid food consumed in the two
seasons, except in so far as it may seem desirable at times to take
advantage of the well-known stimulating properties of proteid to whip
up the general metabolism of the body, in harmony with the principle
that all metabolic processes may need spurring to meet the demands of a
greatly lowered temperature in the surrounding air.

Fuel value, however, should be increased somewhat during the winter
months in our climate. Fat promises the largest amount of energy,
but there is more of a tendency to store up excess of fat than of
carbohydrate, hence the latter foods have certain advantages as a
source of the additional energy needed during cold weather. In warm
weather, it should be our aim to diminish unnecessary heat production
as much as possible, though it must be remembered that the body is
to be maintained approximately at least in equilibrium, and this
calls for an adequate amount of food. Lighter foods, however, may be
advantageously employed, such as fruits, vegetables, fresh fish, etc.
Fats and fat meats especially are to be avoided, not only because
there is no specific need for them, but particularly on account of a
greater sensitiveness of the gastro-intestinal tract during the hot
seasons of the year, that is liable to result in a disturbance whenever
undue quantity of rich or heavy food is taken. Further, in hot summer
weather we may advantageously live more largely on foods served cold,
and thereby avoid the heat ordinarily introduced into the body by
hot fluids and solids. These, however, are all obvious physiological
truths, constituting a form of physiological good sense the application
of which calls for no special expert knowledge.

Less obvious, though no less important, is the partial protection
that can be afforded to weakened or disabled kidneys by judgment and
discrimination in the matter of diet. In acute or chronic nephritis,
forms of so-called Bright’s disease, is there not danger of overtaxing
organs already weakened by placing upon them the daily duty of
excreting large amounts of solid nitrogenous waste, as well as of the
various inorganic salts which are so intimately associated with many of
the organic foodstuffs? The consumption of excessive and unnecessary
amounts of proteid food simply means the ultimate formation of just
so much more urea, uric acid, etc., which must be passed out through
the kidneys. In the words of Bunge, “There is no organ in our body so
mercilessly ill treated as the kidneys. The stomach reacts against
overloading. The kidneys are obliged to let everything pass through
them, and the harm done to them is not felt till it is too late to
avoid the evil consequences.” It would seem the part of wisdom,
therefore, to adjust the daily intake of proteid food to as low a
level as is consistent with the true needs of the body, in those cases
where the kidneys are at all enfeebled, or where it seems desirable to
exercise due precaution as a possible means of prevention.

Equal care is frequently called for in connection with the mineral
matters which enter so largely into many natural foodstuffs, or which
are introduced as condiments. As an illustration, we may note one or
two peculiarities in the distribution of sodium and potassium salts
in the tissues of the body. Potassium is an indispensable constituent
of every living cell, and the latter has the power of absorbing and
holding on to such amounts of this particular element as may be
necessary for the functional activity of the tissue of which it is a
part. Sodium, on the other hand, stands in a different relationship
to living structures. It is widely distributed, but in the higher
animals, as in man, sodium salts are most abundant in the fluids of
the body, notably in the plasma of the blood. Herbivorous animals have
a strong liking for sodium chloride or common salt, but this is not
true of carnivorous animals; indeed, the latter animals have a great
dislike for salty articles of food. Vegetable products are all rich in
potassium salts, whereas ordinary animal foods, such as meat, eggs,
milk, and blood, are relatively poor in this element.

It is claimed that the abundance of potassium salts in vegetable foods
is the cause of the apparent need for sodium chloride by herbivorous
animals, and in lesser degree by man. This is explained by supposing
that when the salts of potassium reach the blood by absorption of the
vegetable foods, an interchange takes place with the sodium chloride
of the blood plasma. “Chloride of potassium and the sodium salt of the
acid which was combined with the potassium are formed. Instead of the
chloride of sodium, therefore, the blood now contains another sodium
salt, which did not form part of the normal composition of the blood,
or at any rate not in so large a proportion. A foreign constituent
or an excess of a normal constituent, _i. e._, sodium carbonate,
has arisen in the blood. But the kidneys possess the function of
maintaining the same composition of the blood, and of thus eliminating
every abnormal constituent and any excess of a normal constituent. The
sodium salt formed is therefore ejected by the kidneys, together with
the chloride of potassium, and the blood becomes poorer in chlorine
and sodium. Common salt is therefore withdrawn from the organism by
the ingestion of potassium salts. This loss can only be made up from
without, and this explains the fact that animals which live on a diet
rich in potassium, have a longing for salt” (Bunge). It is certainly a
fact worthy of note that man takes only one salt as such in addition
to those that are naturally present in his food, and it is equally
significant that sodium chloride is by no means lacking in ordinary
foodstuffs. If the individual lives entirely on animal foods, he has no
desire for salt, but as soon as he adopts a vegetable diet the craving
for salt shows itself. Vegetable foods, however, are not all alike in
their content of potassium salts; some, like rice, contain relatively
little, while others, like potatoes, peas, and beans, are comparatively
rich in this element.

We may recognize in these statements a physiological demand for a
certain amount of salt, especially when vegetable foods enter into
the daily dietary, but there is no justification for the employment
of such quantities as are generally made use of. Where the vegetable
food is largely rice, a small fraction of a gram of salt is really
sufficient for all physiological purposes; and in those cases where
ordinary cereals, legumes, potatoes, etc., constitute the chief part
of the dietary, a few grams of salt, at the most, will suffice to meet
the daily needs. Common usage, however, frequently raises the amount
consumed to 25 grams or more per day, the bulk of which is at once
eliminated through the kidneys; thereby entailing a certain amount
of renal activity, which must, it would seem, constitute something
of a strain upon organs ordinarily hard worked at the best. “Do we
not impose too great a task upon them, and may it not be fraught
with serious consequences? When on a diet of meat and bread, without
salt, we excrete not more than from 6 to 8 grams of alkaline salts
in twenty-four hours. With a diet of potatoes, and a corresponding
addition of salt, over 100 grams of alkaline salts pass through the
kidneys in the day. May not there be danger in this? The habit of
drinking spirituous liquors, which moreover is reckoned one of the
causes of chronic nephritis, also brings about the immoderate use of
salt, and thus one sin against nature leads to another” (Bunge).

The moral we would draw (from these observations) is that in weakened
conditions of the kidneys there is reason in reducing the rate of
proteid exchange to the lowest level consistent with the maintenance
of equilibrium and the preservation of strength and vigor, thereby
diminishing the amount of nitrogenous waste to be eliminated and the
consequent strain upon these organs. Further, there is suggested
moderation in the amount of salt to be used daily, and some
circumspection in the amount and quality of vegetable foods consumed
in order to regulate more effectually the quantity of saline waste to
be handled by the kidneys. These conclusions are just as worthy of
consideration as the more obvious rule that in diabetes or glycosuria
proper precaution must be observed in the eating of carbohydrate
foods. In gout and rheumatism, accumulated physiological knowledge
teaches plainly the necessity of avoiding those foods that are rich in
purin-containing compounds. Uric acid owes its origin in part at least
to substances of this class; and as an ounce of prevention is worth
more than a pound of cure, we may by proper moderation in the use of
such foods save ourselves from the disagreeable effects of accumulated
uric acid deposits.

In conclusion, the nutrition of man, if it is to be carried out by the
individual in a manner adapted to obtaining the best results, involves
an intelligent appreciation of the needs of the body under different
conditions of life, and a willingness to accept and put in practice the
principles that scientific research has brought to light, even though
such principles stand opposed to old-time traditions and customs. The
master words which promise help in the carrying out of an intelligent
plan of living are moderation and simplicity; moderation in the amount
of food consumed daily, simplicity in the character of the dietary, in
harmony with the old saying that man _eats to live_ and not lives to
eat. In so doing there is promise of health, strength, and longevity,
with increased efficiency, as the reward of obedience to Nature’s
laws.




INDEX


  A

  Abderhalden, Emil, 35

  Absorption, a physiological process, 41
    diffusion as a factor in, 41
    from the stomach, 31
    in intestine, 37
    of fats, 43, 49
    of fats, in dogs on low proteid diet, 233, 261
    of food products, by blood, 44
    of peptones, 41
    of proteid in dogs on low proteid diet, 233, 262
    of proteid products, 47
    of proteoses, 41
    osmosis, as factor in, 41
    paths of, 44
    reconstruction of proteid during, 42
    selective action, of sugars, 47

  Acid, aspartic, 34, 67, 259
    glutaminic, 34, 259
    hydrochloric, 25, 26
    uric, 73
    uric, excretion of, as influenced by diet, 144

  Acids, amino, 34
    diamino, 34

  Adenase, 71

  Adenin, 72

  Aldehydase, 64

  Amino acids, 34, 67

  Ammonia, 70, 259

  Amylopsin, 32

  Anabolism, 50

  Animals, influence of low proteid diet on high proteid, 231, 233, 243

  Animal starch, _see_ Glycogen

  Appetite, in relation to food requirements, 162

  Arginin, 34, 68, 70, 259

  Argutinsky, views on muscle work, 123

  Aspartic acid, 34, 67, 259

  Assimilation limits of sugars, 47

  Athlete, photograph of, 190

  Athletes, fuel value of food of, on low proteid diet, 198
    strength tests of, on low proteid diet, 206
    true proteid requirement of, 186

  Atwater and Benedict, 109, 111

  Autodigestion (_see_ Autolysis), 63

  Autolysis, 12

  Availability, of foods, 12
    of carbohydrates, as source of energy, 45


  B

  Bacterial flora in intestine, of carnivora, 292
    of herbivora, 292

  Bacterial processes in intestine, in relation to food, 292

  Balance, nutritive, as affected by various factors, 117, 118

  Basal energy exchange, 104

  Beaumont, William, on movements of stomach, 27

  Benedict, F. G., _see_ Atwater and Benedict

  Bergell and Lewin, 36

  Beriberi, and diet, 224

  Blood, absorption of food products by, 44
    behavior of disaccharides when introduced into, 39
    effects of injection of proteoses and peptones into, 41
    relation of sugar in, to glycogen, 46
    sugar in, 45

  Body, amounts of food required to furnish proteid needs of, 274
    efficiency of, as a machine, 111
    equilibrium, 78
    nature of oxidation in the, 60
    needs of nitrogen by, 4
    needs for food by, 169
    needs and dietary habits, 268
    needs of proteid by, 268, 272
    relation of oxygen to decompositions in, 61
    resistance, _see_ Resistance
    sample dietary supplying needs of, 280
    site of oxidation in, 62
    surface, relation to energy exchange, 104, 105
    surface, relation to nitrogen requirement in dogs, 248

  Body-weight, on low proteid diet, 175, 181, 185, 190, 199, 245–255
    relation to proteid requirement, 184, 188, 198, 227

  Bright’s disease, _see_ Nephritis

  Breisacher, L., on minimum proteid requirement, 172

  Bunge, 124


  C

  Calorie, 14

  Calorimeter, respiration, 102

  Cane sugar, assimilation limit of, 47
    behavior when introduced into blood, 39
    utilization of, 40

  Cannon, W. B., on muscular movements of stomach, 28, 29

  Carbon dioxide, output in rest, 111, 112
    dioxide, output during work, 111, 123
    equilibrium, 84
    excretion, during fasting, 84
    moiety of proteid, 129

  Carnivora, bacterial flora in intestine of, 292

  Carbohydrates, as food, 6
    as fuel, 6
    as heat producers, 58
    as proteid sparers, 92
    as source of energy, 128
    as source of energy in fasting, 81
    as source of energy in work, 58
    availability of, 13
    availability of, as source of energy, 45
    composition of, 5
    formation from proteid, 129
    fuel value of, 15
    in foodstuffs, 7
    liver as regulator of, 45
    respiratory quotient of, 107

  Casein, cleavage products of, 70

  Caspari and Glässner, on minimum proteid requirement in man, 172

  Cellulose, in vegetables, influence on digestion, 263

  Chemical character of proteid, influence on nutrition, 256
    composition of foodstuffs, 7

  Circulating proteid, 134

  Clapp, S. H.
        (_see_ Osborne and Clapp, on proteid cleavage products), 258

  Cleavage, oxidative, 61

  Climbing, oxygen consumption in, 116

  Cogan, Thomas, on temperance in food, 166

  Cohnheim, Otto, on proteid decomposition, 36

  Composition, of proteid, 3
    of carbohydrate, 5
    of fat, 6

  Cornaro, Louis, on temperance in food, 168

  Cost of foods in relation to nutritive value, 277

  Creatin, 74

  Creatinin, 74
    excretion, as influenced by diet, 144

  Curtis, Edward, Nature and Health, 2, 5, 214


  D

  Dapper, Max, 99

  Dangers of underfeeding, 214

  Degeneration, fatty, 270

  Deuteroproteose, 67, 69

  Dextrins, 21, 37

  Dextrose, 37
    assimilation, limit of, 47
    utilization of, 40

  Diabetes, phloridzin, 130

  Diamino acids, 34

  Diet, and beriberi, 224
    and renal activity, 297
    effects of exclusive proteid, upon rats, 239
    effects of intemperance in, 270
    effects of rice, on rats, 240
    fat absorption in dogs on low proteid, 233, 261
    influence of, on creatin in excretion, 144
      exclusive proteid, on progeny in rats, 240
      on growth in rats, 239
      monotony in, 242
      on oxygen consumption in man at rest, 126
      on oxygen consumption in man at work, 126
      on respiratory quotient in man at rest, 126
      on respiratory quotient in man at work, 126
      rice, on growth in rats, 240
      on urea excretion, 144
      on uric acid excretion, 144
      vegetable, upon dogs, 254, 256
    in relation to nephritis, 297
    in relation to nitrogen distribution in urine, 144
    in relation to seasons of the year, 296
    of Highlanders, 279
    low proteid, influence on body-weight in dogs, 245, 249, 250,
        251, 252, 255
    nitrogen excretion during severe work on exclusive proteid, 123, 124
    philosophy of a mixed, 92, 276
    relation of endurance to low proteid, 210, 212
    relation of inorganic salts to, 299, 300
    relation of work to, 126
    relation of vegetable food to low proteid, 291
    sample, of soldiers, 194
    sample, in experiments on true proteid requirement in man, 178,
        182, 189, 195
    simplicity in, advantages of, 279, 293
    temperance in, 270
    utilization of fat in dogs on low proteid, 261
    utilization of nitrogen in dogs on low proteid, 262
    variety in, 229, 242

  Diets, normal, _see_ Standard diets
    standard, 155

  Dietary habits, in relation to needs of body, 268
    of fruitarians, 215
    of Japanese, 225
    sample, supplying needs of body, 280
    standards, use of the term, 272

  Dietetic customs of mankind, 154

  Dietetics, habit in, 159

  Diffusion, as factor in absorption, 41

  Digestibility, _see_ Availability

  Digestion, gastric, of proteids, 26
    importance of gastric, 30
    influence of cellulose in vegetables on, 263
    in the stomach, 25
    object of gastric, 30
    of fat, in intestine, 36
    of fat, in stomach, 36
    of starch, 21
    products of pancreatic, of fats, 36
    products of pancreatic, of proteids, 34, 67
    products of pancreatic, of starch, 37
    products of salivary, 21
    salivary, in stomach, 23

  Digestive products, reconstruction of proteid from, 42

  Disease, relation of excessive proteid consumption to, 269

  Dogs, effects of low proteid diet on, 232–236, 245–255
    fasting experiments on, 82
    fat absorption in, on low proteid diet, 233, 261
    fuel value requirement of, 234, 236, 245–255
    influence of low proteid diet upon body-weight in, 245–255
    influence of vegetable diet on, 254, 256
    nitrogen requirement of, 234, 235, 236, 245–255
    photographs of, 248
    proteid absorption in, on low proteid diet, 233, 262
    proteid requirement, experiments by Munk, 232
    proteid requirement, experiments by Rosenheim, 234
    proteid requirement, experiments by Jägerroos, 236
    proteid requirement, experiments by author, 243
    utilization of fat in, on low proteid diet, 261
    utilization of nitrogen in, on low proteid diet, 262

  Disaccharides, utilization of, 40


  E

  Edestin, cleavage products of, 70

  Efficiency of body, as a machine, 111

  Egg albumin, cleavage products of, 70

  Endogenous metabolism, 145, 146

  Endurance, relation of, to low proteid diet, 210, 212

  Energy, availability of carbohydrates, as source of, 45
    basal exchange, 104
    carbohydrate as source of, 128
    carbohydrate as source of, in fasting, 81
    conservation of, in man, 103
    exchange, effect of muscular work, 109, 110, 113, 115
    exchange, factors modifying, 105, 106
    exchange, in relation to work, 119
    exchange proportional to body surface, 104, 105
    fat as source of, 128
    fat as source of, in fasting 81
    foods as source of, 15
    metabolism of, in man, 103
    of muscle contraction, 121
    origin of, in fasting, 81
    output, in man, 103
    produced by man, 106
    proteid as source of, 122, 123, 124, 129
    proteid as source of, in fasting, 81
    source of, in body, 21, 121
    source of, during fasting, in work, 125

  Enterokinase, 33

  Enzymes, deamidizing, 71, 72
    in gastric juice, 25
    in pancreatic juice, 32
    in saliva, 20
    intracellular, 63, 71, 72, 75
    reversible action of, 21
    specificity of, 21

  Equilibrium, carbon, 84
    nitrogenous, 78
    of body, 78

  Erepsin, 34

  Exchange, basal energy, 104
    of energy, as affected by work, 109, 110, 113, 115, 119
    of energy, factors modifying, 105, 106
    of energy, relation to body surface, 104, 105

  Exogenous metabolism, 145, 146


  F

  Fasting, carbohydrates as source of energy in, 81
    excretion of carbon during, 84
    excretion of nitrogen during, 80, 82, 84
    experiments on dogs, 82
    experiments on man, 80, 84
    fat as source of energy in, 81
    fuel value during, 86
    fuel value of fat, metabolized during, 86
    metabolism of fat during, 84
    nitrogen excretion during, 80, 82, 84
    origin of energy in, 81
    proteid as source of energy in, 81
    proteid metabolism during, 83
    relation of nitrogen excretion to work during, 125
    source of energy for work during, 125

  Fat, absorption, 43, 49
    absorption in dogs on low proteid diet, 233, 261
    as food, 6
    as fuel, 6
    as source of energy, 128
    as source of energy during work, 58
    as source of energy in fasting, 81
    composition of, 6
    digestion of, in intestine, 36
    digestion of, in stomach, 36
    fuel value of, 15
    fuel value of, metabolized during fasting, 86
    hydrolysis of, 36
    influence of feeding, on body fat, 44
    in foodstuffs, 7
    laying on of, from overfeeding, 98, 99
    metabolism during fasting, 84, 86
    respiratory quotient of, 107
    saponification of, 36
    specificity of body, 44
    synthesis of, 43
    utilization of, in dogs on low proteid diet, 261

  Fats, availability of, 13
    as heat producers, 58
    as proteid sparers, 92

  Fatty degeneration, 270

  Fatigue, relation to low proteid diet, 208

  Fenger, S., 293

  Fick and Wislicenus, on source of muscular energy, 121

  Fischer, Emil, 21

  Fisher, Irving, on endurance and low proteid diet, 210
    on method of indicating food values, 283

  Folin, Otto, theory of proteid metabolism, 144

  Food, absorption and utilization of, in dogs on low proteid diet, 261, 262
    amounts, required for proteid needs of body, 274
    as fuel, 6
    as source of energy, 15
    availability of, 12
    carbohydrates as, 6
    character of, in relation to bacterial processes in intestine, 292
    consumption and obesity, 270
    consumption, relation to prosperity, 160
    fats as, 6
    fuel value of, 274
      of fruitarians, 217
      in experiments on proteid requirement, athletes, 198
      in experiments on proteid requirement, professional men, 178, 180, 185
      in experiments on proteid requirement, soldiers, 198
      of Japanese, 219, 221
    fuel value requirement of, in dogs, 234, 236, 245–255
    influence of, on respiratory quotient, 107
    needs of body for, 169
    of man, 2
    proteids as, 3, 5
    real need of body for proteid, 272
    relation of appetite to, 162
    relation of nutritive value and cost of, 277
    requirements, factors modifying, 165
    temperance in, 166, 168
    value of fruits as, 290
    values of, method of indicating, 283

  Foods, respiratory, 58
    time, remain in stomach, 29, 30

  Foodstuffs, carbohydrate in, 7
    composition of, 7
    fat in, 7
    fuel value of, 7
    inorganic salts in, 7
    organic, 3
    plastic, 58
    proteid in, 7
    water in, 7

  Fritz, photograph of, 199

  Fruitarians, dietary of, 215
    fuel value of food of, 217
    proteid consumption of, 217

  Fruits, value of, as food, 290

  Fuel, carbohydrate as, 6
    fat as, 6
    proteid as, 6

  Fuel value, in fasting, 86
    of carbohydrate, 15
    of fat, 15
    of fat metabolized during fasting, 86
    of food, in experiments on proteid requirement, athletes, 188
    of food, in experiments on proteid requirement, professional men,
        178, 180, 185
    of food, in experiments on proteid requirement, soldiers, 198
    of food of fruitarians, 217
    of food of Japanese, 219, 221
    of foods, 274
    of foodstuffs, 7
    of proteid, 15
    of proteid metabolized during fasting, 86
    requirement in the dog, experiments by Munk, 234
    requirement in the dog, experiments by Rosenheim, 236
    requirement in the dog, experiments by Jägerroos, 236
    requirement in the dog, experiments by author, 245–255


  G

  Gastric digestion, importance of, 30
    object of, 30
    products of, 26

  Gastric juice, action on milk, 26
    composition of, 25, 26
    functions of, 25, 27
    hydrochloric acid in, 25, 26
    influence of diet upon flow of, 25
    pepsin in, 25
    psychical stimulation of, 24

  Gastric secretion, 24

  Gelatin, as food, 4, 5

  Glässner, _see_ Caspari and Glässner

  Gliadin, cleavage products of, 70, 259

  Glutaminic acid, 34, 67, 70, 259

  Glutenin, cleavage products of, 259

  Glycerin, 36

  Glycocoll, 67

  Glycogen, formation from proteid, 130
    in liver, 46
    relation to sugar of blood, 46

  Growth, influence of diet on, in rats, 239

  Guanase, 71

  Guanin, 72


  H

  Habit, in dietetics, 159

  Heat, furnished by fats and carbohydrates, 58
    production during sleep, 104, 105
    production in work, 110

  Herbivora, bacterial flora in intestine of, 292

  Herter, C. A., on bacterial flora, 292

  Hirschfeld, Felix, on minimum proteid requirement, 170

  Histidin, 34, 68, 70

  Hofmeister, Franz, on sugar assimilation, 47

  Hunt, Reid, on low proteid diet and body resistance, 226

  Hunter, Andrew, _see_ Watson and Hunter

  Hydrochloric acid, in gastric juice, 25, 26

  Hydrolysis, of fats, 36

  Hypoxanthin, 72


  I

  Indol, 37

  Inorganic salts, and renal activity, 298, 300
    in foodstuffs, 7
    in nutrition, 2
    relation to diet, 299, 300

  Intemperance in diet, effects of, 270

  Intermediary metabolism, _see_ Exogenous metabolism

  Intestine, absorption in, 37
    chemical changes in, 33
    putrefaction in, 37
    bacterial flora of, 292

  Invertase, 40


  J

  Jägerroos, B. H., on proteid requirement in the dog, 236

  Japanese Army and Navy, rations of, 224

  Japanese, dietary of, 225
    fuel value of food of, 219, 221
    proteid consumption by, 219, 221


  K

  Katabolism, 50
    nature of proteid, 75
    oxygen in, 62
    relation to intracellular enzymes, 75

  Klemperer, on proteid requirement, 171


  L

  Lactase, 40

  Lavoisier, views on oxidation, 56

  Leucin, 34, 67, 70, 259

  Leucosin, cleavage products of, 259

  Levulose, assimilation limits of, 47

  Lewin, _see_ Bergell and Lewin

  Liebig, views on oxidation, 57, 120

  Lipase, 32

  Lipolysis, by pancreatic juice, 36

  Liver, function of, as regulator of carbohydrate, 45
    glycogen in, 46
    synthesis of proteid by, 48

  Luxus consumption, of proteid, 59

  Lüthje, 101

  Lymphatics, absorption of food products by, 44

  Lysin, 34, 68, 70, 259


  M

  Maltose, 21, 37
    behavior when introduced into blood, 39

  Man, conservation of energy in, 103
    energy produced by, 106
    experiments on oxygen consumption in, 126
    fasting experiments on, 80, 84
    food of, 2
    metabolism of energy in, 103
    minimum proteid requirement in, 170, 171, 172, 174–208
    work experiments on, 110–116

  Mastication, importance of, 23

  Meat, influence on growth in rats, 239

  Metabolic changes as influencing respiratory quotient, 108

  Metabolism, 51
    and old age, 296
    endogenous, 145
    exogenous, 145
    Folin’s theory of proteid, 144
    influence of proteid on, 83
    influence of carbohydrates on proteid, 92, 94, 95, 96, 97
    influence of fat on proteid, 92, 93, 96, 97
    influence of proteid on proteid, 88
    of energy in man, 103
    of fat during fasting, 84, 86
    oxidation in, 60
    of proteid during fasting, 83, 86
    Pflüger’s theory of proteid, 138
    processes of, 51
    significance of exogenous and endogenous proteid, 49
    significance of proteid, 131
    Voit’s theory of proteid, 134

  Methyl glycocoll, _see_ Sarcosin

  Methyl guanidin, 74

  Milk sugar, assimilation limit of, 47
    behavior when introduced into blood, 39
    utilization of, 40

  Mineral matter, _see_ Inorganic salts

  Minimum proteid requirement, 59

  Mixed diet, philosophy of a, 92, 276

  Monotony of diet, influence of, 242

  Morphotic proteid, 134

  Munk, Immanuel, on proteid requirement in the dog, 232

  Muscular movements of stomach, 27–30


  N

  Needs of body for food, 169

  Nephritis, in relation to diet, 297

  Neumann, R. O., on low proteid diet, 286

  Nitrogen, distribution of, in the urine in relation to diet, 144
    needs by body, 4
    utilization of, in dogs on low proteid diet, 262

  Nitrogen excretion, as influenced by proteid, 59, 87, 90
    during fasting, 80, 84
    during work in fasting, 125
    during excessive work, 114, 127
    during hard work on proteid diet, 123, 124
    in experiments on proteid requirement, in dogs, 245, 249, 250, 251,
        252, 255
    in experiments on true proteid requirement, athletes, 187, 188
    in experiments on true proteid requirement, professional men, 176,
        177, 181, 185
    in experiments on true proteid requirement, soldiers, 199, 200, 201
    relation to work, 122, 123, 124

  Nitrogen equilibrium, on low proteid diet, 176, 177, 181, 188, 200,
        201, 249, 250, 251, 252, 255

  Nitrogen requirement, in dogs, 234–236, 245–255
    in man, 180, 184, 185, 187, 198, 227
    relation to body-weight, 184, 248

  Nitrogenous equilibrium, 78

  Nitrogenous metabolism, theory of Folin, 144
    theory of Pflüger, 138
    theory of Voit, 134

  Normal diets, 155

  Nutrition, factors in, 16, 17
    influence of chemical character of proteid on, 256
    inorganic salts, as aids in, 2
    physiological economy in, 264
    purpose of, 2

  Nutritive balance, as affected by various factors, 117, 118

  Nuclease, 71

  Nucleoproteid, character of, 3
    cleavage products of, 71


  O

  Obesity, relation to food consumption, 270

  Old age, metabolism in, 296

  Osborne and Clapp, on chemistry of proteids of wheat kernel, 258

  Osmosis, as factor in absorption, 41

  Overeating, evil effects of, 270

  Overfeeding, in laying on of fat, 98, 99

  Oxidase, xanthin, 73

  Oxidases, 64

  Oxidation, in metabolism, 60
    nature of, in the body, 60
    older views regarding, 52
    relation to enzymes, 75
    site of, in the body, 62
    value of respiratory quotient in determination of substances
        undergoing, 125
    views of Lavoisier on, 56
    views of Liebig on, 57, 120

  Oxidative cleavage, 61

  Oxygen, in katabolism, 62
    relation to decompositions in the body, 61
    relation to proteid decomposition, 59

  Oxygen consumption, in climbing, 116
    in relation to work, 123
    in standing at rest, 116
    in walking, 116


  P

  Pancreatic digestion, of proteids, 34
    products of, 34, 67
    products of, of starch, 37

  Pancreatic juice, composition of, 32
    condition of trypsin in, 33
    enzymes in, 32
    secretion of, 31, 32
    sodium carbonate in, 32

  Paths of absorption, 44

  Pawlow, on adaptation of saliva, 18

  Pepsin, in gastric juice, 25, 26

  Peptones, 67
    absorption of, 41
    cleavage by erepsin, 34
    effects when injected into blood, 41
    formed in gastric digestion, 26

  Pflüger, E., theory of proteid metabolism, 138
    views on muscle work, 123

  Phenol, 37

  Phloridzin diabetes, 130

  Phosphorus, excretion of, in relation to work, 123

  Photograph, of athlete, 190
    of Fritz, 199

  Photographs, of dogs, 248
    of soldiers, 193

  Physical endurance, _see_ Endurance

  Physiological economy in nutrition, 264

  Plastic foodstuffs, 58

  Poisons, relation of body resistance to, on low proteid diet, 226

  Polypeptid, 35

  Portal vein, absorption of food products by, 45

  Processes of metabolism, 51

  Products, of cleavage of wheat kernel proteids, 259
    of gastric digestion, 26
    of pancreatic digestion, 37, 67
    of proteid cleavage, 70
    of putrefaction in intestine, 38
    of salivary digestion, 21

  Products of digestion, absorption of, 44

  Professional men, fuel value of food on low proteid diet, 178, 180, 185
    nitrogen equilibrium of, on low proteid diet, 176, 177, 181
    true proteid requirement of, 174

  Progeny, influence of meat diet on, in rats, 240

  Prosperity, relation to food consumption, 160

  Proteid, absorption of, in dogs on low proteid diet, 233, 262
    absorption of cleavage products, 47
    amounts of food required to supply needs of body for, 272
    as food, 3
    as fuel, 6
    as glycogen former, 130
    as source of energy, 122, 123, 124, 129
    as source of energy, in fasting, 81
    availability of, 12
    body-weight on diet low in, 170–175, 181, 185, 190, 199, 245, 249,
        250, 251
    carbon moiety of, 129
    chemical basis of protoplasm, 51
    circulating, 134
    cleavage products of, 70
    composition of, 3, 69
    consumption by fruitarians, 217
    consumption by Japanese, 219, 221
    decomposition by oxygen, 59
    decomposition in work, 58
    excessive consumption of, relation to disease, 269
    effect of diet exclusively of, on rats, 239
    effect on dogs of diet low in, 233, 234, 237, 245–255
    fat absorption in dogs on diet low in, 261
    food, real need of body for, 272
    formation of carbohydrate from, 129
    fuel value of, 15
    fuel value of, metabolized during fasting, 86
    influence of chemical character of, on nutrition, 256
      diet exclusively of, upon progeny of rats, 240
      diet low in, on high proteid animals, 231, 233, 243
      on excretion of nitrogen, 59, 87, 90
      on metabolism, 83
      on metabolism of, 88
    in foodstuffs, 7
    katabolism, 75
    luxus consumption of, 59
    metabolized during fasting, 86
    minimum requirement, 59
    morphotic, 134
    need of body for, 268
    nitrogen equilibrium on diet low in, 176, 177, 181, 200, 201, 245,
        249, 250, 251, 252, 255
    overfeeding with, 98
    reconstruction of, during absorption, 42
    relation of endurance to diet low in, 210, 212
    relation of fatigue to diet low in, 208
    respiratory quotient of, 107
    resistance of body to poisons on diet low in, 226
    safety in relation to diet low in, 231
    significance of complete cleavage of, 35
    storing of, 92, 98, 99, 100
    strength tests on diet low in, 203, 206
    synthesis, 48, 49, 68
    utilization of fat in dogs on diet low in, 261
    utilization of nitrogen in dogs on diet low in, 262
    work done at expense of, 58

  Proteid diet, experiments of Neumann on low, 286
    body-weight of dogs on low, 245, 249, 250, 251
    body-weight of men on low, 170–175, 181, 185, 190, 199
    in relation to nitrogen excretion during hard work, 123, 124
    vegetable foods in relation to, 291

  Proteid metabolism, influence of carbohydrate on, 92, 94, 95, 96, 97
    influence of fat on, 92, 93, 96, 97
    influence of proteid on, 59, 87, 90
    Folin’s theory of, 144
    Pflüger’s theory of, 138
    significance of, 131
    Voit’s theory of, 134

  Proteid requirement, fuel value of food in experiments on, athletes, 188
    fuel value of food in experiments on, professional men, 178, 180, 185
    fuel value of food in experiments on, soldiers, 198
    in dogs, experiments of Jägerroos, 236
    in dogs, experiments of Munk, 232
    in dogs, experiments of Rosenheim, 234
    in dogs, experiments of author, 243
    in man, 169, 170, 171, 172, 174–202
    nitrogen excretion in experiments on, athletes, 186, 187, 188
    nitrogen excretion in experiments on, in dogs, 245, 249, 250, 251,
        252, 255
    nitrogen excretion in experiments on, professional men, 177, 180, 185
    nitrogen excretion in experiments on, soldiers, 197, 200, 201
    relation to body-weight, 184, 188, 198, 227
    sample diets in experiments on, 178, 182, 189, 195

  Proteids, as tissue formers, 58
    of wheat kernel, cleavage products of, 259

  Proteoses, 26, 67, 69
    absorption of, 41
    cleavage by erepsin, 34
    effects when injected into blood, 41
    primary, 67, 69
    secondary, 67, 69

  Protoplasm, 51

  Protoproteose, 67, 69

  Ptyalin, 20

  Purin bases, 71, 72
    relation to uric acid, 73

  Putrefaction, in intestine, 37
    products of, 38


  R

  Rats, effects of exclusive proteid diet on, 239
    effects of rice on, 240
    influence of meat diet on progeny of, 240

  Renal activity, and diet, 297
    and inorganic salts, 298, 299, 300

  Rennin, in gastric juice, 26

  Resistance of body to poisons, relation to low proteid diet, 226

  Respiration calorimeter, 102

  Respiratory foods, 58

  Respiratory quotient, 107
    influence of foods on, 107, 126
    influence of metabolic change on, 108
    of foodstuffs, 107
    relation to work, 125
    value of, in determination of substances oxidized, 125

  Rest, carbon dioxide output during, 111
    influence of, on oxygen consumption, 126
    influence of, on respiratory quotient, 126

  Rice, influence of, on growth in rats, 240

  Rosenheim, Theodor, on proteid requirement in the dog, 234


  S

  Safety of low proteid standards, 231

  Saliva, adaptation of, 18, 19
    function of, 20
    psychical secretion of, 18
    secretion of, 17, 18

  Salivary digestion, in stomach, 23
    products of, 21

  Salts, _see_ Inorganic salts

  Saponification of fats, 36

  Sarcosin, 74

  Schnyder, 115

  Scientific research and typhoid fever, 267

  Seasons of the year, relation to diet, 296

  Secretin, 32

  Secretion, of gastric juice, 24
    of pancreatic juice, 31, 32
    of saliva, 17, 18

  Sivén, on proteid requirement, 89

  Skatol, 38

  Sleep, heat production during, 104, 105

  Soaps, 36

  Sodium carbonate, in pancreatic juice, 32

  Soldiers, fuel value of food in experiments on proteid requirement
        of, 198
    nitrogen equilibrium in experiments on proteid requirement
        of, 200, 201
    photographs of, 193
    proteid requirement of, 192
    sample diet in experiments on proteid requirement of, 195
    strength tests in experiments on proteid requirement of, 203

  Specificity of body fat, 44

  Standard diets, 155

  Standing at rest, oxygen consumption in, 116

  Starch digestion, products of, 21, 37

  Steapsin, 36

  Stomach, absorption from the, 31
    as a reservoir, 31
    digestion in the, 25–31
    fat digestion in the, 36
    muscular movements of the, 27–30
    salivary digestion in the, 23
    time foods remain in the, 29, 30

  Storing of proteid, 92, 98, 99, 100

  Strength tests, on low proteid diet, athletes, 206
    on low proteid diet, soldiers, 203

  Sugar, in blood, 45
    in blood, relation to glycogen, 46

  Sugars, behavior when introduced into blood, 39
    selective action in absorption of, 47

  Sulphur, excretion of, relation to work, 123

  Synthesis, of fat, 43
    of proteid, 48, 49, 68


  T

  Temperance in diet, 166, 168, 270

  Tissue formers, 58

  Tissue metabolism, _see_ Endogenous metabolism

  Trypsin, 32
    condition in pancreatic juice, 33

  Tryptophan, 67

  Typhoid fever and scientific research, 267

  Tyrosin, 34, 67, 70, 259


  U

  Underfeeding, dangers of, 214

  Urea, 74
    excretion of, influence of diet on, 144
    relation of, to creatin and creatinin, 74

  Uric acid, 73
    excretion of, as influenced by diet, 144
    relation of, to xanthin bases, 73

  Urine, relation of diet to nitrogen distribution in the, 144

  Utilization, of dextrose, 40
    of disaccharides, 40
    of fat in dogs on low proteid diet, 261
    of nitrogen in dogs on low proteid diet, 262


  V

  Variety in diet, 229, 242

  Vegetable diet, influence upon dogs, 254, 256

  Vegetable foods, relation to low proteid dietary, 291

  Vegetables, cellulose in, influence on digestion, 263

  Voit, Carl, on minimum proteid requirement, 171
    theory of proteid metabolism, 59, 134


  W

  Walking, oxygen consumption in, 116

  Water in foodstuffs, 7

  Watson and Hunter, influence of diet on growth in rats, 239

  Wheat kernel proteids, cleavage products of, 259

  Weight, _see_ Body-weight

  Wislicenus, _see_ Fick and Wislicenus

  Work, carbon dioxide excretion in relation to, 123
    carbon dioxide excretion during, 111, 112
    due to proteid decomposition, 58
    effect of, on energy exchange, 109, 110, 113, 115
    experiments on man, 110, 111, 112, 113, 114, 115, 116
    heat production in, 110
    influence of, on oxygen consumption, 126
    influence of, on respiratory quotient, 126
    nitrogen excretion during excessive, 127
    nitrogen excretion during fasting in, 125
    proteid decomposition in, 58
    relation of diet to, 126
      to energy exchange, 119
      fats and carbohydrates to, 58
      nitrogen excretion on proteid diet to hard, 123, 124
      nitrogen excretion to proteid diet to hard, 122, 123, 124
    relation of oxygen consumption to, 123
      phosphorus excretion to, 123
      sulphur excretion to, 123
    respiratory quotient in relation to, 125
    source of energy during fasting in, 125
    views of Argutinsky on muscle, 123
    views of Pflüger on muscle, 123
    views of Voit on muscle, 59, 134


  X

  Xanthin, 72

  Xanthin oxidase, 73