The extent to which the resting and nonresting components of 24-hour energy expenditure decrease after weight reduction has not been prospectively assessed in ambulatory, weight-stable, reduced-obese humans. Accordingly, 24-hour energy expenditure was estimated as the weight-stabilizing (+/- 50 g/d) daily caloric intake of a defined liquid diet in a cross-sectional study of ten reduced-obese subjects after a 23.2% +/- 9.4% weight loss and 18 obese subjects at baseline weight. A regression analysis demonstrated an 18% decrease in the mean daily energy requirement of the reduced-obese subjects compared with that of subjects of the same relative body weight who had never dieted. Strong linear relationships were noted between estimated 24-hour energy expenditure and fat-free mass (FFM), and between resting metabolic rate (RMR) and FFM in the subjects at baseline weight. In six reduced-obese men, the 24-hour energy expenditure was only 75.7% +/- 5.6% of the value predicted by regression analysis for the decreased FFM. In these six subjects the RMR was 97.4% +/- 7.5% of that predicted for the decreased FFM, suggesting that essentially all the energy savings relative to FFM in the reduced-obese state occurred in nonresting energy expenditure. In a subsequent group of seven subjects studied longitudinally before and after a 21.5% +/- 2.3% weight loss, the decrease in nonresting energy expenditure accounted for 582 +/- 276 kcal/d or 71% of the decrease in estimated 24-hour energy expenditure. These data suggest a decrease in the nonresting energy expenditure of ambulatory reduced-obese individuals, which is greater than previously appreciated.(ABSTRACT TRUNCATED AT 250 WORDS)