Physiological Adaptations to Controlled Work Rate Heat Acclimation Do Not Differ Between Highly Trained Male and Female Endurance Runners

The purpose of this study was to determine whether the trajectory and magnitude of physiological adaptation to controlled work rate heat acclimation (HEAT) differ between male and female highly trained endurance runners, while comparing adaptations with work rate matched temperate exercise training (TEMP). Eighteen highly trained endurance runners (8 males, 10 females) completed two seven-session interventions consisting of HEAT (40°C, 35% relative humidity [RH]) and TEMP (22°C, 35% RH) in a randomized, counterbalanced crossover design. Each session consisted of 60 min of running at 50% of velocity at maximal oxygen uptake. Physiological adaptation was quantified as the change (Δ) relative to session 1 for core temperature (T_c), skin temperature, heart rate (HR), whole-body sweat rate (WBSR), and plasma volume (PV). No intervention × sex × session interactions were observed for T_c, HR, or WBSR (p > 0.05), indicating that the trajectory of adaptation did not differ between sexes or interventions. Likewise, no sex differences in the overall magnitude of adaptation were observed for any HEAT outcome. Significant main effects of intervention demonstrated greater reductions in mean and peak T_c; baseline, mean, and peak HR; as well as greater increases in WBSR during HEAT compared with TEMP (p < 0.05). PV did not change following either intervention. Male and female highly trained endurance runners exhibited comparable thermoregulatory and cardiovascular adaptation to controlled work rate heat acclimation. Although work rate matched temperate exercise induced partial physiological adaptation, environmental heat stress enhanced the overall adaptation magnitude without altering the adaptation trajectory between sexes.