The measurement of energy expenditure.

This is a brief account of the development of energy expenditure measurements, from speculations by early philosophers on the nature of the "innate fire," through the beginnings of quantitative animal calorimetry and to the combined material and energy balances of Rubner and of Atwater and Benedict, which established the science of nutritional energy. The equivalence of oxidation rate and heat loss led to the simplification of indirect calorimetry, followed by the era of studies of basal metabolic rate. Current practices are reviewed for measuring energy expenditure by indirect calorimetry (respiration chambers, ventilated hoods, doubly labeled water) and direct calorimetry (rooms, suits). Because problems remain in the exact account of energy balance during weight change, growth, pregnancy and exercise, perhaps it may be time to combine once again carbon balance with energy balance, using modern methods.