Normally, the basal metabolic rate or heat production is approximately 1 kcal per kg of body weight per hour, or 37 to 40 kcal per square mater of body surface per hour, or 1500 to 1800 kcal per day. An individual restricted to bed rest will have a daily metabolic exchange of about 20 per cent above the basal level. When ambulatory but not engaged in actual manual work, approximately twice the basal energy requirement is expended. During heavy labor, 5000 to 7000 kcal per day are required. The basal metabolism is higher in men (39.5 kcal per square mater of body surface) as compared to women (37 kcal). It declines gradually from about 50 kcal per square mater of body surface at age ten to 36.5 (in men) or 34 in women) at age sixty.

The basal metabolic rate, reflecting as it does the energy exchange of the organism, is proportional to the surface area of the body.

The last – named may be computed by determining the rate of excretion of nitrogen in the urine. Since this is derived from protein only, it serves as a measure of the amount of this nutrient which is being utilized. Subtracting the oxygen used in the metabolism of the protein (0.95 liter per gm) from the total oxygen consumed leaves that derived from carbohydrate and fat. Since these require different amounts of O₂ for their oxidation and yield different amounts of carbon dioxide, the ratio of CO₂ eliminated to O₂ utilized (respiratory quotient or R.Q.) simultaneously determined, gives the data necessary for determining the relative amounts of carbohydrate and fat utilized. One can accordingly calculate, from the observed respiratory exchange and urinary nitrogen excretion, the amounts of protein, fat, and carbohydrate which are being utilized and can compute the total caloric exchange under the conditions of the experiment.