Sexual Dimorphism in the Slope of Rectal Temperature Rise Before Heat Acclimation, but Not After Heat Acclimation
Authors: Ky Alan , Szymanski Michael R. , Struder Jeb F. , Thornton Staci N. , Lee Elaine C. , Casa Douglas J. 
Published: May 2024

Purpose: Our study aimed to identify if sexual dimorphism exists surrounding the slope of rectal temperature rise during a steady state exercise in the heat before and after heat acclimation (HA). In this study, we hypothesized that women would have a greater tolerance to heat stress resulting from a lesser slope of rectal temperature rise when compared to men. Methods: Physically active men (M) (n=12, VO2max: 51.3 ± 6.1 mL·kg−1·min−1, Age: 21±2 yo) and women (W) (n=12, VO2max: 44.2±5.8±6.1 mL·kg−1·min−1, Age: 24±4 yo) completed a steady state treadmill based heat tolerance test (HTT) (3 mph 2% grade for 120 minutes (min) in 40°C, 40% relative humidity). Rectal temperature (Trec) was collected during the first 10-min (PreHAF) and the last 10-min (PreHAL) of the HTT before HA, as well as the first 10-min (PostHAF) and last 10-min (PostHAL) after HA. The slope of rectal temperature rise for the first 10min was calculated by taking the difference of Trec at 10min and 0min, with the slope for the last 10min calculated using Trec at 120min and 110min. Both Trec differences were divided by 10min to obtain the slope in °C/min. One tailed independent T-tests were used to determine sex differences and dependent T-tests to determine differences within sex, both with significance at p≤0.05. All data are presented as mean ± SD. Results: Before acclimation, W demonstrated a greater slope of Trec rise PreHAF when compared to M (M: 0.01 ± 0.02°C/min, W: 0.03 ± 0.01°C/min, p=0.006) with opposite findings PreHAL (M: 0.01 ± 0.01°C/min, W: 0.00 ± 0.00°C/min, p=0.046). Following HA, there were no differences between the slope of Trec rise in M vs. W at both PostHAF (M: 0.02 ± 0.01°C/min, W: 0.02 ± 0.01°C/min, p>0.05) and PostHAL (M: 0.00 ± 0.01°C/min, W: 0.00 ± 0.00°C/min, p>0.05). In M, the PreHAL slope of Trec rise was greater than PostHAL (PreHAL: 0.01 ± 0.01°C/min, PostHAL: 0.00 ± 0.01°C/min, p=0.019), with no differences found within the first 10min (PreHAF: 0.01 ± 0.01°C/min, PostHAF: 0.02 ± 0.01°C/min, p=0.060). No differences in the slope of Trec rise between HA statuses were found in W (PreHAF: 0.03 ± 0.01°C/min, PostHAF: 0.02 ± 0.01°C/min, p>0.05; PreHAL: 0.00 ± 0.01°C/min, PostHAL: 0.00 ± 0.01°C/min, p>0.05). Conclusion: Consistent with previous literature, men exhibit better adaptations displayed by greater reductions in rectal temperature after HA, while women demonstrated consistent responses in both the first and last 10-min. These findings contribute to our understanding of sex-specific physiological adaptations to heat acclimation and underscore the importance of considering sex-based differences in heat-related research and interventions. Funding: 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.