Energy expenditure (EE) was measured at specific steady-state work rates to determine if body fat percentage or gender was associated with exercise EE, substrate oxidation, or work efficiency. Body fat percentage (leaner vs. fatter men, 9-15% vs. 20-25% fat; leaner vs. fatter women, 16-24% vs. 32-48% fat) was not related to work efficiency or submaximal EE. Fatness affected substrate oxidation in men but not in women. Compared to fatter men, leaner men had higher fat oxidation (6.7 ± 1.6 vs. 1.4 ± 2.0 mg · kg fat-free mass · ; p < .01) and lower carbohydrate oxidation (26.6 ± 4.2 vs. 39.3 ± 5.0 mg ⋅ kg ⋅ ; p< .01) at 60% . When men and women of similar fatness and relative aerobic capacity were compared, men had higher EE measured as kilojoules per minute but similar rates of EE and substrate oxidation per kilogram of FFM at 40-60% . It was concluded that body FFM, not fatness, is a determinant of exercise EE, whereas fatness is associated with differences in exercise substrate oxidation in men. Along with aerobic fitness, gender and fatness should be considered in future studies of exercise substrate oxidation.
The authors are with the USDA, ARS, Western Human Nutrition Research Center, Presidio of San Francisco, CA 94129. Direct correspondence to N.L. Keim, USDA, ARS, WHNRC, P.O. Box 29997, Presidio of San Francisco, CA 94129.