incipally as stimulants and
appetizers. It has been believed that they serve neither to build tissue
nor to yield energy, but recent investigations[2] indicate that creatin
may be metabolized in the body.

The _fats_ of food include both the animal fats and the vegetable oils.
The _carbohydrates_ include such compounds as starches, sugars and the
fibre of plants or cellulose, though the latter has but little value as
food for man. The more important function of both these classes of
nutrients is to supply energy to the body to meet its requirements above
that which it may obtain from the proteids. It is not improbable that
the atoms of their molecules as well as those from the proteids are
built up into the protoplasmic substance of the tissues. In this sense,
these nutrients may be considered as being utilized also for the
formation of tissue; but they are rather the accessory ingredients,
whereas the proteids are the essential ingredients for this purpose. The
fats in the food in excess of the body requirements may be stored as
body fat, and the surplus carbohydrates may also be converted into fat
and stored.

To a certain extent, then, the nutrients of the food may substitute each
other. All may be incorporated into the protoplasmic structure of body
tissue, though only the proteids can supply the essential nitrogenous
ingredients; and apart from the portion of the proteid material that is
indispensable for this purpose, all the nutrients are used as a source
of energy. If the supply of energy in the food is not sufficient, the
body will use its own proteid and fat for this purpose. The gelatinoids,
fats and carbohydrates in being utilized for energy protect the body
proteids from consumption. The fat stored in the body from the excess of
food is a reserve of energy material, on which the body may draw when
the quantity of energy in the food is insufficient for its immediate
needs.

What compounds are especially concerned in intellectual activity is not
known. The belief that fish is especially rich in phosphorus and
valuable as a brain food has no foundation in observed fact.

2. _Metabolism of Matter and Energy._--The processes of nutrition thus
consist largely of the transformation of food into body material and the
conversion of the potential energy of both food and body material into
the kinetic energy of heat and muscular work and other forms of energy.
These various processes are generally designated by th