[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Numerous experimental results, hitherto unpublished, have been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.