Role Of Testosterone In Regulation Of Substrate Use During Exercise

Abstract

Men use proportionately more carbohydrate and less fat during submaximal exercise than women and circulating hormones appear to play a role in mediating this difference. We have shown that circulating estrogen and progesterone have potent and opposing effects on the regulation of substrate use during exercise in women, but the role of testosterone in mediating exercise substrate use in men is unknown. We tested the hypothesis that high concentrations of testosterone (T) would shift substrate use toward greater carbohydrate utilization. Nine healthy active men cycled at ∼60% of VO2peak for 90 min under 3 hormonal conditions: physiological T (no intervention), low T (pharmacological suppression of endogenous T with GnRH antagonist) and high T (supplementation with transdermal T). Total plasma testosterone was significantly different between physiological T, low T, and high T (mean±SEM, 5.5±0.4 ng/ml, 0.8±0.1, 10.9±1.0 (p < 0.0001), respectively). Despite the large change in plasma T, there were no differences in carbohydrate oxidation (27.1± 1.7 mg/kg/min, 25.5±2.0, 24.9±1.4 (p=0.68)) or lipid oxidation (5.1±0.7 mg/kg/min, 5.4±0.6, 5.9±0.4 (p=0.60)). Furthermore, blood glucose uptake (8.3±0.7 mg/kg/min, 8.2±0.7, 7.8±0.7 (p=0.88)) and estimated muscle glycogen utilization (18.8±1.6 mg/kg/min, 17.2±1.7, 17.1±1.1(p=0.66)), as measured using stable isotope dilution and indirect calorimetry, were not different between T conditions. In conclusion, circulating T does not alter carbohydrate oxidation or blood glucose uptake during submaximal exercise. These data indicate that circulating T does not play a direct role in the observed gender difference in submaximal exercise carbohydrate oxidation.