Inter-individual variability in physiological adaptations during heat acclimation in adults: Contributions of body mass index and body size.

Heat acclimation refers to the physiological adaptations that occur during repeated heat exposures, ultimately reducing thermal and cardiovascular strain in the heat. It is unknown whether body mass index (BMI) influences an individual's ability to adapt during heat acclimation, which was tested in the present analysis. Forty-two healthy adults (16F; age: 23 ± 5 years) underwent 8 days of treadmill walking (5 km·h-1) in the heat (40°C, 40% RH). Groups were compared based on BMI (<25 and >25). We measured core temperature (TC), heart rate (HR) and whole-body sweating rate (WBSR) on days 1, 4 and 8. The BMI <25 group showed decreases in peak TC (D1: 38.62°C ± 0.58°C, D4: 38.27 ± 0.38, D8: 38.10 ± 0.32; p ≤ 0.018). The BMI >25 group showed a reduction in peak TC only on Day 8 (38.35 ± 0.45) compared to Day 1 (38.54 ± 0.53, p = 0.019). Peak TC was lower in the BMI <25 group compared to the BMI > 25 group on Day 8 only (p = 0.042). HR decreased and WBSR increased over time, with no difference between groups (p > 0.05). The BMI <25 group showed greater reductions in peak TC from D1 to D8 than the BMI >25 group (p = 0.010). These data suggest that individuals with BMI >25 may have attenuated TC adaptations to heat acclimation compared to individuals with BMI <25.