The human body needs, in order to grow or to be active or to work or even to live, to take in air, food, water, and to dispose of the waste products that accumulate in it.

The activity of the internal organs of the body, such as that of the heart, lungs, etc., is work that the body does. This is usually done without the person's being aware of it; some of it continues during sleep. In the waking-hours the body-activity usually appears to be work. But all its activity, whether evident or not, is work for the body and requires energy.

The body gets its energy to do this work from food. As the body is active even in living, it wears out and needs repair. It takes from food the materials that it needs for repair and to keep itself in good running order. If one is growing physically, as all do until the twenty-fifth year, the body gets the materials it needs for growth from food. How the body uses food for warmth, work, repair, and growth, physiology tells.

It has been found that some foods that will give the body energy will not provide for its repair and growth; such are fats, sugar, and many vegetables. As the body needs repair every day, it must be clearly known what kind of food or what in food will repair the body-tissues, as activity wears these out; also what kind of food or what in food promotes growth, and whether what is necessary for repair, growth, warmth, and energy is in the foods being eaten.

It has also been found that when such a combination of foods is eaten as will do all that food can for the body, each food in it is more fully used by the body than when eaten alone.

Growth, repair, health, heat, energy, for the human body must come from food. The body needs also air and water; likewise care and regulation of body-activity.