FOOD.
PHYSIOLOGICAL CONSIDERATIONS.—All substances are foods which, after
undergoing preparatory changes in the digestive organs (rendering them
capable of absorption into the circulation), serve to renew the organs
of the body, and maintain their functions. Foods have been classified
as _tissue producers_ or _energy producers_, the first class renewing
the composition of the organs of the body, and the second class
supplying the combustible material, the oxidation (or more correctly
the _metabolism_) of which is the source of the energy manifested in
the body. The two main manifestations of energy in the body are heat
and mechanical motion, which are to a large extent interchangeable.
All foods come under one of these heads; they are either tissue or
energy producers. They may be both, and in many cases are so. Thus,
all nitrogenous foods (as meat, legumens, etc.) not only help to form
the nitrogenous tissues of the body, but their largest share becomes
split up into fats and urea, and so forms a source of heat to the
body. Similarly fats may possibly, after assimilation, enter into
the composition of the various tissues containing fat (of which the
brain is the most important), though they usually supply an immediate
source of heat. Proteid foods are, however, the tissue producers _par
excellence_, other foods serving as the immediate sources of energy
when metabolised in the body.
Certain foods do not directly serve either as tissue or energy
producers, but are useful in aiding the assimilation of food. Such are
the various condiments which may be classed as adjuncts to food. Salt
is so necessary to the assimilation of food and to the composition
of the various tissues, that it may be ranked as an important food.
Water, again, though already oxidised, and so not an immediate source
of energy, is absolutely necessary to the assimilation of food, to
the interchange between the various tissues and the blood, and to the
elimination of effete products.
CLASSIFICATION OF FOODS.—Inasmuch as milk supplies all the food
necessary for health and growth during the first year of life, it may
reasonably be expected to afford some guidance as to the necessary
constituents of a diet for the adult; although the conditions of life
being altered in the latter, we can hardly expect the same proportions
of the different materials to hold good. In the infant rapid growth
and building up of new tissues and organs are going on, involving the
necessity for a larger proportional amount of nitrogenous food than in
the adult.
The following is the average composition of 100 parts of
┌────────────────┬─────────────┬─────────────┐
│ │ HUMAN MILK. │ COW’S MILK. │
├────────────────┼─────────────┼─────────────┤
│_Casein_ │ 2.4 │ 4.0 │
│_Albumin_ │ .6 │ .9 │
│_Fat_ │ 2.9 │ 3.5 │
│_Sugar_ │ 5.9 │ 4.0 │
│_Salts_ │ .16 │ .7 │
│ │ ──── │ ──── │
│ _Total Solids_ │ 11.96 │ 13.1 │
│ _Water_ │ 88.04 │ 86.9 │
└────────────────┴─────────────┴─────────────┘
It is evident from this analysis of milk that our food must contain (at
least) representatives of all the above divisions. We have, therefore:—
1. =Nitrogenous Foods.=
2. =Hydrocarbons or Fats.=
3. =Carbohydrates or Amyloids.=
4. =Salts.=
5. =Water.=
Condiments and stimulants (tea, coffee, alcohol) are not foods in the
strict sense of the word, and will be discussed in a later chapter.
=Nitrogenous Foods= include albumin, casein, gluten, legumen, fibrin,
and gelatin. They all agree in consisting of a complex molecule
containing many atoms of carbon, hydrogen, oxygen, and nitrogen, with
the addition of smaller quantities of sulphur, and in some cases
phosphorus. The nitrogenous substances used as food may be divided into
two groups, (_a_) those containing gelatin, and (_b_) numerous bodies
which receive the common name of proteids or albuminoids.
The percentage composition of gelatin is:—
┌───────────┬───────────┬───────────┬───────────┐
│ CARBON. │ HYDROGEN. │ NITROGEN. │ OXYGEN. │
├───────────┼───────────┼───────────┼───────────┤
│ 50.0 │ 6.6 │ 18.3 │ 25.1 │
└───────────┴───────────┴───────────┴───────────┘
The percentage composition of all proteids lies within the following
limits:-
┌───────────┬───────────┬───────────┬───────────┬───────────┐
│ CARBON. │ HYDROGEN. │ NITROGEN. │ OXYGEN. │ SULPHUR. │
├───────────┼───────────┼───────────┼───────────┼───────────┤
│ 52.7 to │ 6.9 to │ 15.4 to │ 20.9 to │ 0.8 to │
│ 54.5 │ 7.3 │ 16.5 │ 23.5 │ 1.6 │
└───────────┴───────────┴───────────┴───────────┴───────────┘
Proteids also contain a small amount of phosphorus, chiefly as
phosphate of lime, but also in minute quantity in their essential
structure. Various proteids are used in food, _e.g._ serum-albumin in
the blood and tissues of animals; egg-albumin in the white of eggs;
myosin in flesh; casein in milk; legumin, or plant-casein, in the seeds
of leguminous plants; gluten in wheat-flour, etc.
Proteid foods are pre-eminently important, as they construct and keep
in repair the tissues of the body. They are not used solely for this
purpose. A large share of the energy of the body is derived from the
metabolism of proteids. The amount required for these purposes will be
discussed on page 32. Meanwhile, it may be said that it is not found
to be compatible with efficient health simply to supply an amount of
proteid food which will suffice to replace the wear and tear of the
tissues, leaving fats and carbohydrates to supply the energy of the
body. Deficiency of proteid food always leads to ill-health; and it
would appear that in all cases proteid food determines, to a large
extent, the metabolism of non-nitrogenous food, and so is favourable
to all vital action. The action of nitrogenous food in thus increasing
metabolism may make it, when in _relative excess_, a tissue waster.
Banting’s cure for corpulence is founded on this principle, lean meat
alone being taken, all starchy and saccharine foods being carefully
avoided.
By _metabolism_ is meant the changes undergone by food before it
reaches the state in which it is finally eliminated from the body.
It is commonly spoken of as _oxidation_, but this word less exactly
represents the facts. The complexity of the changes undergone by food
in the body may be better appreciated by a glance at the following
schematic statement, which only gives an approximation to the truth:-
HYPOTHETICAL TYPICAL FAT
ALBUMINOID. (STEARIN). STARCH. GRAPE-SUGAR.
C₇₂H₁₁₂N₁₈SO₂₂. C₃H₅(C₁₈H₃₅O₂)₃. _x_(C₆H₁₀O₅). (C₆H₁₂O₆).
———————————————————————————————————-┬———————————————————————————————
│
Various intermediate products, which are finally broken down
into and eliminated as
│
┌————————————————————┼————————————————————┐
│ │ │
Urea, CH₄N₂O. Carbonic acid, CO₂. Water, H₂O.
=Hydrocarbons=, or fats, consist of three elements, carbon, hydrogen,
and oxygen, the amount of oxygen present not being sufficient to
oxidise completely either the hydrogen or the carbon. Thus the molecule
of stearin, which may be taken as a typical fat, has the formula C₃H₅
(C_{18}H_{35}O₂)₈.
In respect to their comparatively unoxidised condition fats compare
favourably with starch and sugar, C₆H_{10}O₅ and C₆H_{12}O₆
respectively. It is evident that in starch the H_{10}O₅ = 5H₂O, and
that in sugar H_{12}O₆ = 6H₂O, so that in both cases only carbon
remains uncombined with oxygen. Dried fats produce by their oxidation
2¼ times as much heat as a corresponding amount of sugar or starch;
but the relative advantage of fat is not quite so great as would appear
from this comparison, inasmuch as metabolism within the body is not
identical with oxidation.
The fat obtained from food is not simply deposited in the body as such,
to form a store of combustible matter, and to fill up the interstices
between the different tissues. If this were so, the kind of fat
deposited would vary with the food, which is not the case. The fat of
the body is probably not formed directly from fatty food, but as the
result of the metabolism of nitrogenous foods when this metabolism is
incomplete. In the formation of milk this can be distinctly proved: the
fat cells are formed from the protoplasm of the cells of the mammary
gland.
Possibly carbohydrate food may be a source of fat, as well as
nitrogenous and fatty food. This appears to be the case in the
Strasburg goose, which is kept penned up in a warm room, and fed
entirely on barley-meal, in order to produce an enormous fatty liver
for the delicacy termed _pâté de foie gras_. But it may be that the
large accumulation of fat in the liver is due to the warmth and
inaction of the goose diminishing metabolism, and producing a fatty
degeneration of the nitrogenous material of the liver.
Fats and carbohydrates, unlike proteids, do not excite metabolism in
the system, and so, if in excess of the requirements of the system,
can be stored up with comparative ease. Quiet and warmth, diminishing
metabolism, conduce to the accumulation of fat in animals being fed for
the market; and the same applies to human beings.
=Carbohydrates or amyloids= include the various starchy and saccharine
foods. They are inferior to fats in nutritive power, but, being very
digestible, are in much greater favour. In the process of digestion,
starch is converted into grape sugar, and starch and sugar are
practically equal in nutritive power.
Even when carbohydrates are entirely absent from the food, they may be
produced in the organism by the breaking up of nitrogenous matter. This
certainly happens in diabetes, in which the nitrogenous food rapidly
becomes converted into sugar and urea.
The deprivation of carbohydrate food is much better borne than that of
fats, because in the latter the hydrogen is not completely oxidized,
and because fats aid the assimilation of other food.
=Salts=, and especially common salt (chloride of sodium), are essential
to health. An average adult human body contains about seven pounds of
mineral matter, of which about five-sixths is in the bones. On analysis
the whole body yields about five per cent. of ash.
Chloride of sodium is necessary for the production of the acid
(hydrochloric) of gastric juice, and of the salts of bile; half the
weight of the ash of blood consists of it. An adult requires 150 to 200
grains of salt per day; a large part of this is taken in meat, bread,
etc.; and but little need be taken as a condiment. Potassium salts form
an important part of milk, muscle juice, and the blood corpuscles.
They are obtained from bread and fresh vegetables and fruits. It has
been maintained that deficiency of potassium salts causes scurvy (see
page 28); but this is now discredited, and probably potash is chiefly
useful because of the vegetable acids with which it is associated
in fruits and vegetables, which when oxidised, help to maintain the
alkalinity of the blood, _e.g._, tartrates, citrates, and malates,
which become carbonates in the circulation. Calcium phosphate (bone
earth) is essential for the growth of bones, and is very important
for the young. The best source for it is milk. There is more lime in a
pint of milk than in a pint of lime water. Next to milk, come eggs, and
then cereals, especially rice as a source of calcium. Lime salts and
phosphates as drugs do not benefit like the same substances taken in
natural food, and rickets is not curable by taking such drugs.
Oxide of iron is always present in the ash of blood and muscles, and
in smaller quantities in milk. Fish and veal are usually deficient in
it, while beef and yolk of egg are foods richest in iron. The amount of
iron required in food is minute, and it is amply supplied by ordinary
diet.
Phosphorus is an essential building material for the body. It is
contained in foods chiefly in organic combination. The foods richest in
it are yolk of egg, sweetbread (thymus), fish-roe, calves’ brains, and
the germ of wheat. Milk and cheese are very rich in phosphates.
=Water= forms an important article of diet. This is evident from the
fact that 80 per cent. of the blood consists of it, and 75 per cent. of
the solid tissues; and from the fact that the daily loss of water from
the system averages 50 ounces (2½ pints) by the kidneys, and about
40 ounces by the skin and lungs. Water is not simply received into
the system as a liquid. It forms a large proportion of the solid food
taken. Thus, 87 per cent. of milk, 78 per cent. of fish, 72 per cent.
of lean meat, 38 per cent. of bread, 13 per cent. of peas, and 92 per
cent. of cabbage, consist of water.
Solid food is dissolved in the alimentary canal by the watery
secretions derived from the blood. Water swallowed as food, begins to
pass on into the intestine at once. The statement that free consumption
of water at meals delays digestion by diluting the gastric juice is
therefore not well grounded. In the blood, water serves to carry
nutrient materials to all the tissues; and, at the same time being
circulated all over the system, equalises the temperature, favours
chemical changes, and washes all the tissues. By water again, the
effete matters which have been separated by the kidneys are washed out
of its tubes.
The =Oxygen= of the air, in a broad sense, forms one of the foods of
the system. This will be considered later.
Besides the above classification, foods have also been classified as
follows:—
1. _Inorganic food_—Oxygen, salts.
{Animal {Nitrogenous.
{ {Non-nitrogenous.
2. _Organic foods_ {
{Vegetable {Nitrogenous.
{ {Non-nitrogenous.
Or, as—
{Animal {Nitrogenous.
{ {Non-nitrogenous.
1. _Solid foods_ {
{Vegetable {Nitrogenous.
{ {Non-nitrogenous.
{Water.
2. _Liquid foods_ {Milk and its products.
{Tea and similar beverages.
{Alcoholic beverages.
3. _Gaseous foods_—Air.