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They are also used as fuel-that is, they are burned in the body to
yield energy-and they are to some extent transformed into fat and
stored in the body, but these are their less important uses. The protein
compounds are most abundant in some of the animal foods, as
lean meat, though the cereals contain them in considerable, and peas
and beans in large proportions. The gelatinoids are less valuable than
the albuminoids for nutriment.
The third class, the so-called extractives, are included with the protein
compounds because they contain nitrogen, but they differ greatly
from the albuminoids and gelatinoids. They are the principal ingredients
of meat extracts, beef tea, etc. They are believed to neither
build tissue nor furnish energy, but to act as stimulants and appetizers.
The craving which some persons have for meat is perhaps due in part
to a desire for these extractives. The nitrogenous compounds of
potatoes and other vegetable foods contain more or less of so-called
amids, like asparagin, which are analogous to the extractives of meat,
and like them can not build tissue, and hence have an inferior nutritive
value.
FATS.
Fats occur chiefly in animal foods, as meats, fish, butter, etc. They
are also abundant in some vegetable products, such as olives and cotton
seed, from which they are expressed as oil, and occur in considerable
quantities in some cereals, notably oatmeal and maize (whole kernel),
and in various nuts. In our bodies and those of animals fats occur in
masses under the skin and in other localities, and in minute particles
scattered through the various tissues. The amount of fat in the body
varies greatly with food, exercise, age, and other conditions. When
more food is taken than is necessary for immediate use part of the
surplus may be stored in the body. The protein and fat of food may
thus become body protein and body fat; sugar and starch of food are
changed to fat in the body and stored as such. When the food supply
is short this reserve material is drawn upon for supplementary fuel.
Fat forms about 15 per cent, by weight, of the body of an average
man. Well-fed or over-fed people with little muscular exercise often
grow fat, but the tendency to fatness or leanness is more or less a question
of personal idiosyncrasy or some other little understood factor,
and not decided by food and exercise alone.
CARBOHYDRATES.
These include such compounds as starches, different kinds of sugar,
and the fiber of plants or cellulose. They are found chiefly in the
vegetable foods, like cereal grains and potatoes; milk, however, contains
considerable amounts of milk sugar, which is a carbohydrate.
