The Impact Of Sex And Heat Exposure Schedule On Heat Acclimation

Females are underrepresented in thermoregulatory research and thus there is a need to examine how sex-related differences impact heat acclimation. Moreover, current global heatwave incidences warrant investigation into optimizing strategies for human tolerance to heat. PURPOSE: Determine how sex and temporally partitioning identical heat exposure time impacts heat acclimation. METHODS: Over a week, 30 females and 30 males were heat acclimated daily (38 °C, 60% RH) on a treadmill (1.6 m·s-1, 5% grade) in one of two 90-minute exposure groups. Sustained groups completed 90-minute sessions continuously and periodic groups completed three separate 30-minute sessions 3 hours apart. Heat exposure time was thus identical across four groups of 15 participants: sustained male, sustained female, periodic male, and periodic female. On Days 1, 4, and 7 body temperatures, sweat metrics, and skin blood flow (SkBF) were measured during the first 30 minutes of heat exposure. RESULTS: Sex differences were present throughout heat acclimation. Core (p < 0.001) and mean surface (p = 0.036) temperatures were lower in males (37.2 ± 0.3, 36.9 ± 0.4 °C) than females (37.5 ± 0.3, 37.1 ± 0.4 °C). Core temperature at sweat onset was lower in males (37.0 ± 0.3 °C) than females (37.3 ± 0.4 °C, p < 0.001). SkBF was higher in males (101 ± 113 A.U.) than females (61 ± 55 A.U., p = 0.007) following 30 minutes of heat exposure/exercise. Heat exposure schedule differentially influenced heat acclimation. Heat acclimation induced core and mean surface temperature decreases in the sustained (37.3 ± 0.3, 36.9 ± 0.4 °C, p < 0.001) but not periodic group (37.4 ± 0.3, 37.0 ± 0.4 °C, p > 0.05). Regardless of group, core temperature at onset of sweat decreased throughout heat acclimation (37.3 ± 0.4, 37.2 ± 0.4, 37.1 ± 0.5 °C, p < 0.05). SkBF was higher in the sustained (61 ± 80 A.U.) than periodic group (37 ± 81 A.U., p = 0.005). CONCLUSION: Thermoregulatory sex-differences are present prior to and continue throughout 7-days of heat acclimation. Only the sustained group experienced physiological changes indicative of heat acclimation whereas the periodic group did not. Thus, temporally partitioning identical heat exposure time insufficiently induces heat acclimation. Supported by: Supported by the United States Department of Defense, Department of the Air Force (FA8650-19-C6124).