When one is growing, food that will build physically is especially needed and should be eaten. All body-activity, even simply living, wears the body out so that it needs repair; that is, rebuilding of its tissues.

The body contains more water than any other constituent. Water keeps the liquids of the body, as blood, in a fluid state. The next largest quantity of a body-constituent is mineral salts; the body-skeleton is mainly mineral matter. Protein, the tissue-substance, is next in quantity; it forms tissue, as body-muscle. Body-fat is next. Carbohydrate is least and in very small quantity in the body.

It is strength of body which tissue-building food (protein) promotes. It forms the body during growth; it repairs for tissue-activity as one lives and works. Beef and mutton build, repair, spare tissue, and stimulate. Chicken and oysters do not stimulate. Eggs, milk, cheese, build, repair, and give energy, as do cereals, breads, graham crackers, macaroni. Beans, peas, lentils, build, repair, and give energy. Most foods do this somewhat. Mineral matter builds bone and aids growth and body-activity too.

Science finds the overeating of meat one of the mistakes of human diet. Red meats through their extractives may so stimulate as to leave a body feeling alive but neither strong for work nor able to sustain activity. A body so fed may tire quickly after eating ; it may feel hungry soon. Constant need of food may keep a body so occupied digesting food that it is able to do little else. The digestive tract may be worn out by such overuse.

Even the tissue-formers without extractives (eggs, milk, white meats, grains), if overeaten, require the body to dispose of body-waste and food-waste that need not have troubled it, do exhaust it, and may poison it, instead of repairing it for wholesome working.

For the quantity of tissue-forming food needed, see p. 222.