Each of these classes contains food material derived from both the
animal and vegetable kingdom, although the majority of the animal
substances belong to the nitrogenous, and the majority of the
vegetable substances to the carbonaceous group.

Therefore, for practical purposes, we will confine ourselves to the
more general terms used in Atwater's table.


Uses of Food.

First, food is used to form the materials of the body and repair its
waste; second, to yield energy in the form of (1) heat to keep the
body warm, (2) to provide muscular and other power for the work it has
to do. In forming the tissues and fluids of the body the food serves
for building and repair. In yielding energy, it serves as fuel for
heat and power. The principal tissue formers are the albuminoids;
these form the frame-work of the body. They build and repair the
nitrogenous materials, as those of muscle, tendon and bone, and supply
the albuminoids of blood, milk and other fluids. The chief fuel
ingredients of food are the carbohydrates and fats. These are either
consumed in the body or are stored as fat to be used as occasion
demands.


Water.

By referring to a preceding chapter we find that water composes
three-fifths of the entire body. The elasticity of muscles, cartilage,
tendons, and even of bones is due in great part to the water which