Sex differences in substrate oxidation during aerobic exercise in obese men and postmenopausal obese women

The aim of this study was to compare substrate oxidation during aerobic exercise in obese men and postmenopausal obese women. Ten obese men (mean age, 55.4 ± 2.2 years; body mass index, 27.5 ± 0.4 kg/m2; peak oxygen uptake [Vo2peak], 44.4 ± 1.9 mL/kg fat-free mass/min; mean ± SE] and 10 postmenopausal obese women (mean age, 57.2 ± 1.2 years; body mass index, 27.9 ± 0.5 kg/m2; VO2peak, 39.9 ± 1.3 mL/kg fat-free mass/min) performed a 40-minute bout of cycling exercise at 50% VO2peak. Blood samples were collected for assessment of metabolic variables and 17β-estradiol concentration at baseline and during aerobic exercise. Breath samples were collected to estimate carbohydrate and fat oxidation using a digital computer-based breath-by-breath exercise analysis system during aerobic exercise. Serum 17β-estradiol concentration was not significantly different between the men and women subjects at baseline (P > .05). Serum free fatty acid concentration tended to be higher in the men than in the women (P = .07) during the exercise, but the respiratory exchange ratio during exercise was lower in women than in men (P < .05). Fat oxidation adjusted for fat-free mass was higher (P < .05) in women than in men. These results suggest that fat utilization was higher during aerobic exercise in postmenopausal obese women than in obese men and did not depend on resting serum 17β-estradiol concentration.