Measurements of total energy expenditure (TEE) by the doubly labeled water method were compiled from 22 studies to identify the range of variation and significant determinants of energy requirements in healthy adults. The 126 male and 173 female subjects (aged 18-78 y) were separated into four groups: athletes, Pima Indians, people in developing countries, and others. The groups differed significantly (P < 0.001) with respect to TEE, TEE/BMR, TEE-BMR divided by weight, and TEE-BMR. Stepwise multiple regression demonstrated that fat-free mass (FFM) and age are significant variables that can explain 65% of the variation in TEE. These data demonstrate that total daily energy expenditure varies dramatically among healthy, free-living adults. The relationship between body fatness and nonbasal energy expenditure was negative at high energy outputs but considerable variation in body fatness was present among sedentary individuals, suggesting that a low rate of nonbasal energy expenditure is a permissive factor for obesity.