A high-dietary fat intake may be an important environmental factor leading to obesity in some people. The mechanism could be either a decrease in energy expenditure and/or an increase in caloric intake. To determine the relative importance of these mechanisms we measured 24-h energy expenditure in a whole body calorimeter in 14 nondiabetic subjects and in six subjects with non-insulin-dependent diabetes mellitus, eating isocaloric, weight-maintenance, high-fat, and high-carbohydrate diets. All subjects were Pima Indians. In nondiabetics, the mean total 24-h energy expenditure was similar (2,436 +/- 103 vs. 2,359 +/- 82 kcal/day) on high-fat and high-carbohydrate diets, respectively. The means for sleeping and resting metabolic rates, thermic effect of food, and spontaneous physical activity were unchanged. Similar results were obtained in the diabetic subjects. In summary, using a whole body calorimeter, we found no evidence of a decrease in 24-h energy expenditure on a high-fat diet compared with a high-carbohydrate diet.