A sufficient quantity of
good and suitable food is the body's first need. Proper housing, fresh air and exercise
play their part in maintaining health, but without good food they avail little.
Famine, war and poverty interfere at times with man's normal food-supply, and
his health suffers. Far more serious is the daily use of improper food; it
insidiously undermines the constitution. A good diet must satisfy all the
nutritional requirements of the body, but the amount of food needed varies
under different conditions of life, and the kind is largely controlled by local
circumstances. Generally a variety of food-stuffs is available, and if the diet
consists of a few well-chosen natural foods of both vegetable and animal origin
it will meet all the demands of the body. Under the conditions of modern
civilisation a well-mixed and varied diet may be palatable and apparently
sustaining, but if continued for a long time it may prove to be inadequate
because it fails to supply everything that is wanted. A mixed diet of fresh
meat, milk, butter, eggs, fruits, vegetables and wholemeal bread, such as our
grandparents ate, supplied all that is necessary, but this food is now largely
replaced by canned meat, sterilised milk, margarine, egg substi­tutes; bottled,
tinned and dried fruits, vegetables and milk; white bread and other highly
milled cereals. The effect of preservatives and refining processes has been to
what may be called the vital substances in the food, substances as important as
the actual food material itself.


The Composition and Function of Food.

The natural diet of man and animals contains certain substances every
one of which is essential for the main­tenance of life and health. These
substances belong to the following groups :

(i) Proteins.         -i

I Also called the Accessory
I        Food


Carbohydrates. J- Known
as the Proximal Principles.


Fats.               J






Fat-soluble A-Vitamin.


Water-soluble B-Vitamin.


Water-soluble C-Vitamin. Except the vitamins, the relative proportions
in which

various groups are present in the food is deter­mined by chemical analysis. In
fact, the relative pro­portions of protein, carbohydrate, fat, salts and water
are the only data generally considered in judging the food-value of any diet.
It is not possible as yet to determine the quantity of vitamins in the food.
The following analyses x show the wide differences in the
composition of various foods :—








beef, hindquarter





















cod   .















Cabbage    .








boiled    .



1 "5





















1 The composition of common foods is given
in Analyses and Energy Values of Foods, by R. H. A. Plimmer, 1921, from
which these figures are taken.



The purpose of food is twofold; it is needed as a source of energy for
the warmth and activities of the body, and to supply material for the growth of
new tissues in the young animal and for maintenance in the adult, that is, for
replacing substance lost to the body by wear and tear. Only a small amount of
food is required for growth and maintenance by the adult, yet, as everyone
knows, an adult eats more food than a rapidly growing little child. This is
because the greater part of the food is used as a source of energy, and the
body of the adult requires more energy, as it is larger to keep warm and
heavier to move about. Even during the period of most rapid growth in the
child, more food is used as fuel (energy) than for increasing the size of the


The whole series of chemical interchanges undergone by substances in the
body from the time they enter it as food till their removal as waste products
is commonly referred to as metabolism. Anabolism includes all the processes
which contribute to the building up of the living tissues; the reverse, or
breaking down, processes are termed catabolism. These chemical changes are
accompanied by an evolution of heat which is treated separately as the energy
metabolism. The living organ­ism conforms to the physical laws of conservation
of mass and energy, so that any surplus output of material or energy over the
intake can only be derived from a loss of body substance. The metabolism, both
of energy and chemical material, can be expressed in the form of a balance
sheet. The income is the amount of food taken into the body less the unabsorbed
food passing out in the faeces. The expenditure of material is measured by the
amount of carbon di-oxide gas breathed out by the lungs and by an estimation of
the substances passed out in the urine, together with an allowance for the
slight output of material through the skin. The heat given out during the
chemical changes in the body is like the heat given out


the burning of a coal fire in a grate. Both are processes of oxidation, that is
of burning. All fuel consists of organic compounds. These compounds contain the
element carbon; coal and the carbohydrates, fats and proteins are organic
compounds. As the result of oxidation or burn­ing, organic compounds, whether
coal or food, combine with oxygen and are finally resolved into the gas carbon
di-oxide, water and ash—the ash consists of the incom­bustible mineral salts in
the fuel. The carbon di-oxide and some of the water formed by the combustion of
food are breathed out by the lungs. Waste material from the tissues,
incombustible (= ash) and incompletely burned material {= smoke) are excreted
in watery solution by the kidneys. Certain constituents of the food are not
digested and pass out in the faeces or stools.

The energy or heat which is liberated by the combustion of organic
substances is used by the body for warmth and for movements of all kinds.

The Constituents of the Food. The Carbohydrates.

Starch and sugar are the principal carbohydrates in the food.

Starch is the chief constituent of cereals,
various kinds of flour, peas, haricot beans, potato, tapioca, sago, etc.

There are several kinds of sugar :

Cane sugar, or sucrose, from the beet, palm, maple, sugar-cane, etc.

Glucose, or grape sugar, the sugar in
grapes and other fruits; also manufactured from starch and sold as a honey-like

Fructose, or fruit sugar, another kind of sugar in fruits.

Invert sugar, a mixture of glucose and
fructose, in fruits, honey, jam, etc.

Lactose, or milk sugar, the natural
sugar in milk; not sweet like other sugars to the taste.1

1 Saccharin, although 550 times as sweet as
cane sugar, is not chemically a sugar and has no food value.



Other carbohydrates are:

Glycogen, or animal starch, in
small amounts in the muscles and liver of animals.

Malt and Dextrin are digestion
products of starch. They can be prepared from barley.

Cellulose is present in all
vegetable foods. It is not digestible and is passed out in the faeces.

All the carbohydrates, except cellulose, may be con­sidered as identical
for nutritional purposes. In what­ever form the carbohydrate is eaten it is
converted during digestion into a simple sugar, such as glucose. The
carbohydrates function as fuel. A small reserve of carbohydrate is always
retained in the body as glycogen, and any excess of carbohydrate is converted
into fat and stored as a reserve of fuel.


The edible fats, whether in the form of oil or solid fat, consist almost
entirely of three fats—palmitin, stearin and olein. An oily fat contains more
olein and a hard fat more palmitin and stearin. An oily fat is more easily
assimilated than a hard fat. As a group the fats are practically alike,
although there are further chemical differences which need not be discussed

Certain fats, such as butter, cod-liver oil, the fat of egg yolk,
contain the fat-soluble A-vitamin, but leaving this vitamin out of
consideration, the fats by themselves are of equal value in nutrition. The
function of the fats is, like that of the carbohydrates, to supply heat and
energy. Within certain limits fats and carbohydrates can be used alternatively
in the diet. If all the carbohydrate in the food be replaced by fat, the fat is
incompletely oxidised; in other words, carbohydrate burns more easily than fat,
and for the complete combustion of fat in the body carbohydrate must be burned
along with it. Mixed with carbohydrate it is as if fat burns with a clear
flame, but if there is too little carbohydrate it burns smokily. The
half-burned products of fat are poisonous to the body and


coma. This condition occurs in starvation, in diabetes and in children over-fed
with fat, that is, in all cases in which fat is burned with an insufficient
quantity of carbohydrate. Very little carbohydrate and a great deal of fat are
eaten by the Eskimos, but they eat an abnormal amount of protein; in this case
the protein acts like carbohydrate, assisting in the complete combustion of

The body works more economically upon carbohydrate than upon fat, and
severe work occasions greater fatigue if fat be the source of energy. If there
is more fat than carbohydrate in the food, about n per cent, of the fat is
wasted. The main difference between carbohydrate and fat is that carbohydrate
is burned up quickly, while fat is a more sustaining fuel. The extreme cases of
the athlete, who takes sugar whilst making great muscular effort, and the
hibernating animal, which burns up slowly the fat stored in the tissues, may be

Fats can be built up in the body from the carbohydrates in the food (p.
5), and some recent feeding experiments on rats indicate that fat as such can
be omitted from the diet if the A-vitamin be supplied in a specially prepared
fat-free form. The special value of fats in nutrition thus depends on the
A-vitamin associated with them, and not on variations in their chemical


Animal tissues consist for the most part of nitrogenous substances or
albuminoids, which have received the name protein from the Greek word Trpwreva,
meaning " I am the first," that is the most important.

Proteins during digestion are converted into simpler compounds, the
amino acids, which differ greatly among themselves. The number of amino acids
is large; about eighteen to twenty have so far been recognised, and different
amounts of them are contained in different proteins. We can thus speak of the
quality of protein, but not of the quality of carbohydrate or fat. The
quality   of   protein  
is  of   such  
vital   significance  in




that the subject is considered in a separate chapter.

Protein enters into the intimate structure of the living matter or
protoplasm of the tissues, and from protein are probably derived some of the
special glandular secretions, which control and co-ordinate the vital processes
of the body.

Like carbohydrate and fat, protein is burned as a source of energy, but
for this purpose it is wasteful. Theoretic­ally there is no reason why protein
could not be used to replace both carbohydrate and fat in the food as a source
of energy, but from a practical point of view carbohydrate is a cheaper food
than protein, and such a large quantity of protein as would then be required is
not suitable to the digestion.

The Energy or Heat Value of the Food.

The energy value of fuel or food is measured in Calories. A Calorie is
the amount of heat required to raise the temperature of i kilo, of water i° C.
Every combustible substance can be burned in a special apparatus called a
calorimeter (= a heat-measurer), and the heat given out on complete combustion
measured. Food burned in the body does not give quite such high values as when
burned in the calorimeter, partly because cellulose, which is not burned in the
body, is present, and partly because pro­teins are not completely oxidised, a
certain proportion of their carbon being excreted as urea in the urine. The physiological Calorie values of food are
generally reckoned

I gram of protein
gives          4-1 Calories

   fat gives                   9-3    

      carbohydrate gives   4-1    

The Energy Output of Man

Man and animals can be put into another form of calorimeter, and the
heat given out by them under different


can be measured. In these experiments the subject is generally placed in the apparatus
for a definite and short period of time. The amount of heat given out when the
body is at absolute rest and without food, a state approximated during sleep,
is known as the basal metabolism. The extra output of heat under different
conditions as sitting, walking, riding a bicycle or at lower temperatures can
also be measured. The average man of ii stone
( = 70 kilos.) at rest gives out about 1 Calorie per hour per kilo. The figures
are generally calculated in terms of Calories per 24 hours. The average
quantities have been found to be:

Man's basal metabolism   .       
.    1700 Calories.

Man without muscular work     .   

Man with light muscular work .     3000     

Man with moderate muscular work

Man with hard muscular work .    4500 to 9000 or more.

men require a larger amount of energy, but the amount of energy per kilogram of
body-weight is less than in the case of lighter men, because the area of the
body-surface, and consequently the amount of heat lost from it, is relatively
greater in smaller men.

The energy output of the individual varies according to occupation; it
also varies according to sex, age, and the surrounding temperature. The average
woman and child has a lower energy metabolism than man. In estimating the
energy requirements of a family it is customary to use Lusk's coef&cients:
taking the man's requirement as 1; the woman requires only 0-83; boys over
thirteen, ro; boy aged ten, 0-7; girl aged ten, 07; children under six, 0-5.

The science of calorimetry offers a convenient method of comparing the
value of different diets without reference to the individual constituents. The
Food (War) Com­mittee of the Royal Society adopted the following scale of food
requirements according to occupation :





Food Requirements.


2750 Calories (approximate values)















supply the Calories for a man doing light work an average diet consisting of







grams of protein will give 60     
,,   „ fat will give 550      ,,   
„ carbohydrate

Total      3193

general, the higher Calorie values required by indivi­duals doing hard muscular
work are supplied by increasing the amount of carbohydrate and fat. During the
war, when the supply of protein foods was limited, the Royal Society Food
Committee gave as a basis :

Protein .



.    70 grams

   90   ..


Total    .

280 Calories. 810 2200


The figures very closely approximate the former figures, but this diet
differs in containing less protein and more fat.

The Protein Requirements.

The total energy value of the food is the sum of that of the three
proximal principles. Theoretically these con­stituents can be varied at will so
long as the total Calorie


reaches the amount required .for the particular occupation, but a definite
minimal quantity of protein must always be present in the diet. Various
investigators have attempted to fix this quantity. Atwater's figures are those
generally accepted as the standard. Atwater calculated that 100 grams of
protein were necessary for light work, 125 for medium, and 150 for hard work.

The protein figures refer to a mixture of animal and vegetable proteins.
Less protein can be used if it be of animal origin; vegetable proteins have a
lower value than animal proteins (see p. no). The Calories are made up to the
required amount by carbohydrate and fat in the proportion of about 10 to 1.

Salts and Water.

The salts in our food are the mineral constituents of meat, milk,
cereals, plant tissues, etc., which form the residual ash after these
food-stuffs are completely burned. The most common salts are the phosphates,
sulphates, chlorides and carbonates of
sodium, potassium, magnesium,
calcium (= lime) and iron. The mineral
salts have no fuel value, but they are essential constituents of the body and
are contained in sufficient amount in an ordinary mixed diet. The only addition
usually made is common salt (= sodium chloride) added according to individual
taste, but this addition is not a necessity unless the food consists chiefly of
cereals which are poor in sodium salts. Common salt is necessary for the forma­tion
of the hydrochloric acid in the gastric juice.

A diet composed largely of cereals is deficient
in calcium
salts, but they are most easily introduced
in milk. To ensure a proper supply of all the salts it might be better to use a
proper physiological mixture rather than a prepared table salt, which is
deprived of some of the important constituents of the natural salt.

Water is contained in most foods and is partaken of according to natural



Other Food Constituents.

Detailed analyses have shown that besides protein there are other
nitrogenous substances: creatine, purine bases, etc., in animal tissues;
amides, such as asparagine, and similar compounds of this type in vegetable
tissues. Lipoids, such as lecithin, cholesterol and lipochrome pig­ments (the
natural yellow colouring matter in butter, eggs and other fats) are contained
in animal and plant tissues. All these compounds are present in natural foods
in such small amounts that they are negligible with refer­ence to their energy
value, but they probably serve some definite physiological purpose.

Vitamins : Unidentified Constituents of the Food.

Physiologists and biochemists are ascertaining the func­tion of all the
known chemical constituents of the food. To study these questions feeding
experiments on man and animals are carried out. It is, of course, impossible to
use as protein natural products such as meat or milk which contain the other
substances. The isolated or so-called "pure" protein must be used;
also "pure" fat and " pure " carbohydrate. The preparation
of pure food­stuffs involves a great deal of laborious chemical work as a
preliminary to such feeding experiments.

The feeding of animals with pure protein, fat, carbo­hydrate, salts and
water led to the discovery of some unknown substances which must be included in
the food in order that life may be maintained. These substances are the
vitamins or accessory food-factors. Altogether three distinct vitamins have
been recognised :

Fat-soluble A-vitamin,present in
certain fats, particularly cod-liver oil, butter, egg yolk, and also in green

Water-soluble B-vitamin, present in the
seeds of plants, egg yolk, yeast and in many fruits and vegetables.

Water-soluble C-vitamin, in most juicy
fruits and vegetables.


There is a common misconception that one vitamin is the equivalent of
another, but this is not the case; there are three distinct vitamins, each one
of which ranks as an essential food-substance and cannot be replaced by another
vitamin. For practical purposes it is convenient to think of the three vitamins
as (A) cod-liver oil, (B) the germ of cereals, (C) lemon-juice. These are three
very different food-stuffs, and the vitamins which they contain are just as

The absence of these vitamins from the food leads to the so-called food
deficiency diseases. Scurvy follows on a diet deficient in C-vitamin, beri-beri
on a diet poor in B-vitamin, rickets is associated with a lack of A-vitamin.
,The curious disease, pellagra, is connected with the quality of the protein.

Although there is as yet no concrete chemical evidence of the existence
of vitamins, experimental investigations show very definitely that health and
growth depend on the presence of protein of good quality and of three accessory
factors or vitamins in the food; that these vitamins are, generally speaking,
very unstable to tem­perature and other influences; that they are very unevenly
distributed in the various food-stuffs; that a sufficient quantity must be
consumed every day; that, although they may exist in animal tissues, yet
ultimately they are derived from plant food and cannot be formed in the animal

Striking proof of the need and value of vitamins in the diet of animals
is given by the practical agricultural experiments of Dr. M. J. Rowlands,
carried out on a large scale. His animals, fed on a full vitamin contain­ing
diet, have not only grown unusually fast, but also have shown remarkable
resistance to the ordinary diseases of animals. In competition at the
Smithfield Fat Stock Show they gained the chief prizes. Further, he has shown
that the full vitamin diet is a financially sound proposition.

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