Submitted on May 13, 2005
Accepted on June 29, 2005
No Effect of Short-term Testosterone Manipulation on Exercise Substrate Metabolism in Men
Barry Braun1*, Laura S Gerson1, Todd A Hagobian1, Daniel Grow2, and Stuart R Chipkin1
1 Exercise Science, University of Massachusetts, Amherst, MA, USA
2 Obstetrics and Gynecology, Baystate Medical Center, Springfield, MA, USA; Exercise Science, University of Massachusetts, Amherst, MA, USA
* To whom correspondence should be addressed. E-mail: bbraun{at}excsci.umass.edu.
Compared with women, men use proportionately more carbohydrate and less fat during exercise at the same relative intensity. Estrogen and progesterone have potent effects on substrate use during exercise in women but the role of testosterone (T) in mediating substrate use is unknown. The purpose of this investigation was to assess how large variations in the concentration of blood T would impact substrate use during exercise in men. Nine healthy active men were studied in 3 distinct hormonal conditions: physiological T (no intervention), low T (pharmacological suppression of endogenous T with a GnRH antagonist) and high T (supplementation with transdermal T). Total carbohydrate oxidation (CHOox), blood glucose rate of disappearance (Rd) and estimated muscle glycogen use (EMGU) were assessed using stable isotope dilution and indirect calorimetry at rest and while bicycling at ~60% of VO2peak for 90 minutes. Relative to the physiological condition (T = 5.5±0.5 ng/ml), total plasma T was considerably suppressed in low T (0.8±0.1) and elevated in high T (10.9±1.1). Despite the large changes in plasma T, CHOox, glucose Rd, and EMGU were very similar across the 3 conditions. There were also no differences in plasma concentrations of glucose, insulin, lactate or free fatty acids. Plasma estradiol (E) concentrations were elevated in high T but correlations between substrate use and plasma concentrations of T, E or the T/E ratio were very weak (r2<.20). In conclusion, unlike the effect of acute elevation in estradiol to constrain carbohydrate use in women, acute changes in circulating testosterone concentrations do not appear to alter substrate use during exercise in men.
