Indirect calorimetry is the only accurate and clinically feasible method of measuring energy expenditure. It is called “indirect” because the caloric burn rate is calculated from a measurement of oxygen uptake. Direct calorimetry implies a measurement of heat released by the body, which is technically difficult and clinically impractical.

Indirect calorimetry relies on the fact that burning 1 calorie (Kilocalorie) requires 208.06 milliliters of oxygen. Because of this very direct relationship between caloric burn and oxygen consumed, measurements of oxygen uptake (VO2) and caloric burn rate are virtually interchangeable. Oxygen uptake requires a precise measurement of the volume of expired air and of the concentration of oxygen in the expired air.