By custom, humanity has eaten a mixed diet; that is, a combination of animal and vegetable food substances. In America three meals a day are usual; in England, four (tea in the afternoon); and in France, two, with coffee and rolls in the morning.

Illness and infancy have needed and secured special diets everywhere civilized life has penetrated. Children are not just little adults. Their bodies are growing not simply larger but are in some respects themselves being formed. Teeth illustrate this. Other body-formation is also going on which is no less important, though not so easily seen as is the coming of teeth. Since food is for the body to use, food for children must be such as the developing body of childhood can use. (See p. 202).

In recent years, science has learned much through observation and experiment about the effect of food on health and physical development. This was not so fully known in earlier times. The kinds and quantities of food needed to sustain life, to provide energy, to promote development, to maintain health, and to regulate body-activity are now carefully studied. What is known is also being more generally taught, that through such knowledge humanity may have health for wholesome living, strength to work, length of active life.

As results of scientific study, more thorough mastication than is usual is urged for all; for all adults not at hard physical labor, less food; for all persons in health, a mixed diet. It is thus that the digestive tract is used as a whole; it is such use of it that keeps it in health.

Excess of food overworks the human system and overburdens it with waste products. Thus may be caused indolence, restlessness, illness. Lack of thorough mastication prevents full digestion of the food eaten. Health and economy are therefore both promoted by thorough mastication.