The energy expended by the body at any given time is the summation of (1) the mechanical work performed, including that expended by the activity of the respiratory muscles, heart, and other viscera which are in constant motion; (2) the energy expended by the cells in the maintenance of their integrity and activity’ and (3) the potential energy stored by the accretion of tissue formed in the process of growth. Over a relatively short period of time, the last – named is negligible. The energy exchange at rest in the post absorptive state is designated as the basal metabolic rate. It equals the energy liberated from the body in the form of heat which is derived ultimately from the oxidation of the protein, fat, and carbohydrate stores of the body. The sum total of the chemical reactions occurring in this process may be expressed by the equation

Metabolites + C O2 = O 2 + capabilities + ?H

Where ?H represent the heat liberated in the reaction. At rest, under basal conditions, fixed proportions of protein, fat, and carbohydrate are consumed, and hence one can measure the rate of energy exchange in the body by determining either the heat liberated or the oxygen consumed during unit time. For technical reasons, it is more convenient to determine the latter (indirect calorimetric).

Relation of Oxygen Consumption to Total Metabolism

Since the caloric equivalent of protein differs from that of fat and carbohydrate, it is possible to utilize the oxygen consumption as a measure of the caloric exchange only when the nature of the foodstuffs which are being consumed is known.

These are energetic reactions which are accomplished in the organism by coupling reactions involving an increase in free energy content with energetic reactions capable of yielding this necessary energy.