Sex Differences in Rectal Temperature Rise: Regression Analysis of Women and Men Before and After Heat Acclimation

Purpose: Our study identified sexual dimorphism in the slope and best-fit line of rectal temperature rise during steady-state exercise in the heat, pre- and post-heat acclimation (HA). We hypothesized that women would exhibit reduced heat stress tolerance, reflected by a steeper and more linear rectal temperature (T rec ) rise compared to men. Methods: Twenty-nine men (M) (VO 2 max: 51.93±5.82 mL·kg - 1 ·min - 1 , Age: 21.86±2.83yo) and 38 women (W) (VO 2 max: 45.18±4.90 mL·kg - 1 ·min - 1 , Age: 24.03±2.61yo) completed a 120min heat tolerance test (HTT) (3 mph, 2% grade, 40°C, 40% RH) pre- (PREHA) and post-heat acclimation (POSTHA). T rec slope (°C/min) was calculated as the difference between T rec at 120min and 0min, divided by 120. Linear and quadratic fits assessed slope shape over 120min and 30min segments. Repeated measures ANOVA evaluated slope rates, and regression analysis assessed curve shape by sex and timepoints. Data are mean±SD. Results: Rate of rise (RoR) was slower in the after acclimation (POSTHA, 0.010±0.003°C/min; p=0.011) compared to before acclimation (PREHA, 0.012±0.004°C/min. M showed no changes in RoR PREHA compared to POST HA (PREHA, 0.011±0.004°C/min; POSTHA, 0.010±0.003°C/min; p=0.36). However, W exhibited a slower RoR POST HA (PREHA, 0.012±0.004°C/min; POSTHA, 0.010±0.003°C/min; p=0.01). Regression analysis indicated all T rec graphs PRE HA and POST HA in M and W were shaped quadratically (p>.05). Segmented analysis in M were linear PRE HA and POSTHA between 0-30mins, 60-90mins, and 90-120mins (PREHA: 0-30mins, r 2 =1.0; 60-90mins, r 2 =1.0; 90-120mins, r 2 =0.99; POSTHA: 0-30mins, r 2 =1.0; 60-90mins, r 2 =0.99; 90-120mins, r 2 =0.97). During 20-60mins, M exhibited a quadratic curve in the naive (PREHA: r 2 =1.0, p=0.01) and acclimated state (POSTHA: r 2 =1.0, p=0.03). W exhibited linear increases 0-30min PRE HA and POSTHA (PREHA, r 2 =1.0; POSTHA, r 2 =1.0). Between 30-60mins, W had a more quadratic shape (PREHA and POSTHA PREHA, r 2 =1.0, p=0.02; POSTHA, r 2 =0.99, p=0.01). Both 60-90mins and 90-120min, W demonstrated more quadratic curves PREHA (PREHA: 60-90mins, r 2 =1.0, p=0.02; 90-120mins, r 2 =1.0, p=0.02) and a linear shape POSTHA (POSTHA: 60-90mins, r 2 =0.99; 90-120mins, r 2 =0.97). Conclusion: Women exhibited a slower rate of rectal temperature rise post-acclimation and shifted from quadratic to more linear temperature profiles during exercise. In contrast, men showed consistent rates and curve shapes across conditions. Although more research is necessary, these findings highlight differences in sexes during heat acclimation and the need for more research. Funding Source: DoD BA200299 and DoD BA220010 This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.