nder in the
general work of the body. Since these are constantly being lost in one way
or another, they also must be replaced. These different needs of the body
for new materials are supplied through
*The Foods.*--Foods are substances that, on being taken into the healthy
body, are of assistance in carrying on its work. This definition properly
includes oxygen, but the term is usually limited to substances introduced
through the digestive organs. As suggested above, foods serve at least
three purposes:
1. They, with oxygen, supply the body with energy.
2. They provide materials for rebuilding the tissues.
3. They supply materials that aid directly or indirectly in the general
work of the body.
*The Simple Foods, or Nutrients.*--From the great variety of things that
are eaten, it might appear that many different kinds of substances are
suitable for food. When our various animal and vegetable foods are
analyzed, however, they are found to be similar in composition and to
contain only some five or six kinds of materials that are essentially
different. While certain foods may contain only a single one of these,
most of the foods are mixtures of two or more. These few common materials
which, in different proportions, form the different things that are eaten,
are variously referred to as simple foods, food-stuffs, and _nutrients_,
the last name being the one generally preferred. The different classes of
nutrients are as follows:
Nutrients:
Proteids
(Albuminoids)
Carbohydrates
Fats
Mineral salts
Water
It is now necessary to become somewhat familiar with the different
nutrients and the purposes which they serve in the body.
*Proteids.*--The proteids are obtained in part from the animal and in part
from the plant kingdom, there being several varieties. A well-known
variety, called _albumin_, is found in the white of eggs and in the plasma
of the blood, while the muscles contain an abundance of another variety,
known as _myosin_. Cheese consists largely of a kind of proteid, called
_casein_, which is also present in milk, but in a more diluted form. If a
mouthful of wheat is chewed for some time, most of it is dissolved and
swallowed, but there remains in the mouth a sticky, gum-like substance.
This is _gluten_, a form of proteid which occurs in different grains.
Again, certain vegetables, as beans, peas, and peanut
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