A food is any substance which will supply the material needs of the body. These material needs, as shown above, are in two directions: First, constructive, and second, energy-producing. The constructive need of the body is the need for material for growth or repair, while the energy need is supplied by the fuel foods, whose oxidation furnishes the heat which maintains body temperature, and the energy - motor, glandular, or nervous - peculiar to the active tissues of the body.

A food, then, is any substance which can supply any or all of these needs. A food is an article of diet; nutriment or nourishment.

Animals' foods are, as a rule, the complex substances built up by the vegetable kingdom usually for their own use. Examples of this may be cited in the cereals, which are simply seeds of the cultivated grasses; the legumes, which are seeds of cultivated members of the leguminosae; nuts, which are the seeds of certain trees and shrubs. All these seed foods were prepared for the nourishment of the young plants, as described above, but are gathered by the animals and used by them extensively the world over. Some animals - the herbivorous ones - crop or browse the green herbage of plants, thus living upon plant tissues direct, while other animals - the carnivorous ones - are cannibalistic in their tastes and kill other animals, consuming them for food.

These carnivorous animals, while subsisting directly upon animal tissues, get their nourishment indirectly from the vegetable kingdom, because they uniformly subsist upon herbivorous animals.