Sex Differences In Energy Expenditure, Metabolic Heat, And Substrate Utilization During Exercise-Heat Stress
Authors: Brewer Gabrielle J. , Szymanski Michael R. , Struder Jeb F. , Thornton Staci N. , Morrissey-Basler Margaret , Filep Erica M. , Lee Elaine C. 
Published: May 2024

Purpose: To understand the influence of repeated heat exposures and biological sex on metabolic outcomes surrounding heat acclimation (HA). Based on previous data, we hypothesized that repeated heat exposures would result in higher resting caloric expenditure and exercising metabolic heat production in men (vs. women). Methods: Twenty participants (65% men, 22±3 yrs, 70.1±13.7 kg, VO2max 49.9±6.8 mL·kg −1·min −1) had 8 daily exercise-heat sessions (40°C, 40%rh, at least 60min) including heat tolerance tests (HTT; 120 min, 3.1 mph, 2% grade) on days 1 and 8. Resting energy expenditure (REE) and respiratory exchange ratio (RER) were assessed pre- and post-HTT on days 1 and 8. Metabolic heat production (MHP) and RER were collected during the HTT. Data are presented as mean difference (MD) (95% confidence intervals). For REE and RER, a 2x2 ANOVA compared metrics before HTT vs. after HTT session, pre-HA vs. post HA with sex as a between-subjects factor. For MHP and RER during exercise, a dependent samples t-test compared pre-HA vs. post-HA and independent samples t-test compared sexes pre-HA vs. post-HA. Results: Pre-HA: RER was higher before vs. after the HTT (MD:0.07 (0.05,0.09) A.U., p<0.001) overall with no difference between sexes. REE was not different before vs. after the HTT (p>0.05) overall, but males had higher REE before HTT (MD:564 (291,836) kcal·day −1, p<0.001) and after HTT (MD:666 (395,938) kcal·day −1, p<0.001) than women. Post-HA: RER was higher before vs. after the HTT (MD:0.08 (0.06,0.10) A.U., p<0.001) overall with no difference between sexes. REE was not different before vs. after the HTT (p>0.05), but males had higher REE before (MD:581 (328,834) kcal·day −1, p<0.001) and after HTT (MD:617 (411,823) kcal·day −1, p<0.001) compared to women. Comparing HA Status: RER was not different between HA status before or after the HTT (p>0.05). REE was not different between HA status before the HTT (p>0.05). After HTT, REE was greater pre-HA compared to post-HA (MD:142 (26,259) kcal·day −1, p=0.019). During activity, MHP was greater pre-HA compared to post-HA (MD:14 (-2,31) W/m2, p=0.042); RER was not different (p>0.05). Pre-HA there were no differences in MHP or RER between sexes (p>0.05); post-HA, men had greater MHP than women (MD:36 (3,67) W/m2, p=0.016); there were no sex differences in RER. Conclusion: Men exhibited greater REE than women at each time point under both conditions and men produced more metabolic heat following HA. In response to completing 8 heat exposures as part of a HA protocol, men and women differed in caloric expenditure and MHP, but not in substrate utilization. Thus, our findings provide supporting evidence for sex-specific nutritional recommendations surrounding exercise-heat stress. Funding Source: DoD BA200299. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.