Distinct physiological responses to heat acclimation in males and females lead to similar thermal adaptations in both sexes

Although there are known differences in mechanisms of thermoregulation between males and females, it is unclear whether or how these alter physiological adaptations during heat acclimation. Our goal was to evaluate possible sex differences in responses to a 10-day controlled work rate heat acclimation (HA) protocol. We studied 27 young healthy individuals (16 females). Volunteers underwent 10 days of HA at a controlled work rate (walking at 5 km·h^-1, 2% grade, 40°C, 40% relative humidity, 1.34 m·s^-1 windspeed for 120 min). Core temperature (T_core) and heart rate (HR) were measured continuously. Whole body sweat rate (WBSR) was calculated from pre- and post-exercise nude body mass. Blood samples were taken pre-exercise on <i>days 1</i> and <i>10</i> (HAD1 and HAD10) to evaluate HSP72. Males and females successfully (and similarly) adapted to HA, as assessed by progressive decreases in peak T_core (males: HAD1 38.69 ± 0.48 vs. HAD10 38.30 ± 0.28°C, <i>P</i> < 0.001; females: HAD1 38.90 ± 0.49 vs. HAD10 38.46 ± 0.45°C, <i>P</i> < 0.001). Peak HR was higher in females throughout HA, but adapted to HA in both groups (males: HAD1 141 ± 16 vs. HAD10 127 ± 11 beats/min, <i>P</i> < 0.001; females: HAD1 170 ± 17 vs. HAD10 152 ± 17 beats/min, <i>P</i> < 0.001). WBSR increased in both groups (males: HAD1 0.73 ± 0.23 vs. HAD10 0.92 ± 0.23 L·h^-1, <i>P</i> < 0.001; females: HAD1 0.65 ± 0.15 vs. HAD10 0.72 ± 0.12 L·h^-1, <i>P</i> = 0.041) but was higher in male on HAD3-HAD5. Across HA, HSP72 increased similarly between males and females. These results suggest that males and females have a similar ability to adapt to a 10-day exercise-HA protocol but appear to do so via distinct physiological mechanisms.<b>NEW & NOTEWORTHY</b> We evaluated sex differences in the magnitude and time course of physiological adaptations to heat stress using a generalizable and practical protocol (i.e., standardized work rate in the heat). Men and women had similar adaptations in core temperature, despite greater sweating in the men. We also report for the first time that intracellular heat shock protein 72 increases similarly in males and females during heat acclimation.