Studies using mainly dietary restriction have shown that weight loss is associated with a decrease in fat-free mass (FFM) and resting energy expenditure (REE). The aim of this study was to investigate the effects of a weight-loss program relying solely on increased physical activity on FFM and REE. Forty-two overweight male military recruits (12 with initial body mass index [BMI] between 25.0 and 29.9 kg/m2, group 1; 14 with BMI between 30.0 and 34.9 kg/m2, group 2; and 16 with BMI of at least 35 kg/m2, group 3) completed a 5-month program of supervised physical activity that included both aerobic and muscle-strengthening components. All subjects lost significant amounts of weight (group 1, 8.6 kg; group 2, 15.7 kg; group 3, 22.0 kg). This weight loss was accompanied by a significant reduction in the waist to hip ratio (WHR) in all groups. FFM was maintained in all groups. REE tended to decline in all groups (group 1, from 1,595.0 +/- 46.9 to 1,511.7 +/- 53.2 kcal/d; group 2, from 1,751.4 +/- 56.0 to 1,680.0 +/- 63.1 kcal/d; group 3, from 1,901.9 +/- 93.7 to 1,740.0 +/- 67.3 kcal/d), but this decline reached statistical significance only when all 42 subjects were considered. REE normalized for FFM did not decrease except in group 1. Furthermore, differences between the slopes and intercepts of the regression lines relating REE with FFM before and at completion of the 5-month program were not statistically significant, suggesting that the relationship between REE and FFM was maintained after weight loss.(ABSTRACT TRUNCATED AT 250 WORDS)