In the processes of nature there goes on all the time a breaking down of complex substances into simpler and a building up of simple substances into more complex. Bacteria break down complex substances; plants and animals utilize these. Human bodies take for food the more complex substances prepared in plants and animals. Waste products of body-activity are complex; bacteria break these down and return simpler forms to the soil and atmosphere for nature's further use.

Though human foods have concentrated in them many chemically complex substances, study of food-composition has shown that all foods are not equally complex or condensed. Some are principally water; the solid nutrients in these may be relatively small. Other foods show condensed solid nutrient substances, as do grains, but without all of these always being fully available as digestible human food. Still others contain very little that is not nutritive, and in a form to be fully assimilated by the body. (See eggs, p. 108.) Such are nature's concentrated foods.

Such foods are of great value, but they cannot be used exclusively. As the body is and now works it needs some bulk to its food for its digestive tract to function. In the variety in which nature makes food available much that is found in food that is not itself nourishing may aid the body in utilizing food, as does water. It is, however, important in what quantity even natural constituents in food, as cellulose, be eaten, if not themselves nutrients, that is, nourishing substances. Though such non-nutrients may aid digestion somewhat, they do not supply the material that makes either the body or its energy.

Concentrated foods of nature, though they contain in themselves all food-constituents that nourish and are free from the dangers of condensed foods of commerce, are not all-sufficient as human diet, exceedingly important factors as they are in it.