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dissolved out of bone and meat in soup, though somewhat similar to the
albuminoids in composition, are not believed to be tissue formers; but
they are valuable in protecting the albuminoids from consumption.
That is, when the food contains gelatinoids in abundance less of albuminoids
is used.
The proteids can be so changed in the body as to yield fats and carbohydrates,
and such changes actually occur to some extent. In this
and other ways they supply the body with fuel.
PROTEIN AS FUEL FOR THE BODY.
The protein compounds are not only used for building and repairing
tissue, but are also burned directly in the body like the carbohydrates,
and thus render important service as fuel. A dog can live on lean
meat. He can convert its material into muscle and its energy into
heat and muscular power. Man can do the same; but such a onesided
diet would not be best for the dog and it would be still worse
for man. The natural food for carnivorous animals, like the dog,
supplies fats and some carbohydrates, and that for omnivorous animals,
like man, furnishes fats and carbohydrates in liberal amounts along
with protein. Herbivorous animals, like horses, cattle, and sheep,
naturally require large proportions of carbohydrates.
FATS AND CARBOHYDRATES AS FUEL.
Fats and carbohydrates are the chief fuel ingredients of food. Sugar
and the starch of bread and potatoes are burned in the body to yield
heat and power.^ The fats, such as the fat of meat and butter, serve
the same purpose, only they are a more concentrated fuel than the
carbohydrates.
The body can also transform carbohydrates of food into fat. This
fat, and with it that stored from the food, is kept in the body as
reserve fuel in the most concentrated form.
The different nutrients can to a greater or less extent do one
another's work. If the body has not enough of one kind o{ fuel it can
use another. But, while protein can be burned in the place of fats
and carbohydrates, neither of the latter can take the place of the albuminoids
in building and repairing the tissues. At the same time the
gelatinoids, fats, and carbohydrates, by being consumed themselves,
protect the albuminoids from consumption.
VALUE OF FOOD FOR SUPPLYING ENERGY.
Heat and muscular power are forms of force or energy. The energy
latent in the food is developed as the food is consumed in the body.
The process is more or less akin to that which takes place when coal is
