What the body can do itself in utilizing its food, it needs to do in health. But digesting food is not the only activity of the body. Important as digestion is, it needs to be so accomplished that the body is prepared through it for other work and not simply absorbed in its own living. Though all unnecessary digestion impairs body usefulness, if not health itself, wholesome digestive activity is essential to healthful digestion.

Fat in moderation aids the general working of the body; without it disorders and difficulties ensue. For the same weight fat furnishes over twice the quantity of heat-energy which sugar or starch can produce. Active children and physically laboring adults can use more fat than others use fully or digest freely. Fat passes as heat-energy and is probably not stored as body-fat. Sugar and starch eaten beyond the immediate need of the body become body-fat. Body-fat protects other body-tissue from use for energy by itself furnishing heat and energy first. Fat-reserve serves thus in illness and food-deprivation of any type. Excess fat in the body or in food usually interferes with health.

Tissue-sparing is a function of some protein foods. Gelatin (p. 94) is a form of protein which will not build tissue, but by being present in the diet can prevent body-tissue from being worn out by work. This has a use even in health, as the unnecessary breaking down and renewal of tissue consumes energy. All needless body-functioning destroys instead of preserves wholesome body-activity and the body itself in a state of healthful repair. In illness tissue-sparers are often necessary. They save a body weakened by disease from the drain upon it that would otherwise be required to sustain the work of repair beyond the repair-need absolutely imperative to preserve life.