Sustenance of the body is effected through the food eaten. The repair-food keeps the body alive, the fuel-food provides it with energy and body-heat.

The tissues of the body in performing their functions break down into waste products. This process is called katabolism. This is a chemical change, that is, a change in the composition of substances. All chemical change is accompanied by production of heat. Digestion of food is also a chemical change, so produces heat. Food elements are, through digestion, built up into body tissue. This is called anabolistn. As life is lived this double process of breaking down and building up tissue goes on. Both together are called metabolism.

To build the body up as its living breaks it down, the food eaten must bring, in its heat value, the equivalent of the heat generated as the tissues break down. This is called maintaining the metabolic equilibrium. Every 5 hours 422 calories are produced by adult living and must be supplied by food. Any work done requires further heat-energy.

Well-nourished bodies produce the same quantity of heat per square unit of surface and so for the same size have the same heat-need. In the morning, fifteen hours after eating, the heat production of the body is least. A man at complete rest who weighs approximately 154 lb. (70 kg.) produces in his process of just living 70 calorics per hour or 1680 calories in 24 hours. This is called the basal heat-production. If food is eaten for simple existence, the work done in eating is about 10% of this basal heat-production, or 7 calories per hour or 168 calories per day. The existence requirement is therefore 1850 - calories per day for an average-sized man at rest.

Exercise is necessary to life. This is work for the body and requires food fuel for the heat-energy needed.