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

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 (T_C), heart rate (HR) and whole-body sweating rate (WBSR) on days 1, 4 and 8. The BMI <25 group showed decreases in peak T_C (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 T_C only on Day 8 (38.35 ± 0.45) compared to Day 1 (38.54 ± 0.53, p = 0.019). Peak T_C 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 T_C from D1 to D8 than the BMI >25 group (p = 0.010). These data suggest that individuals with BMI >25 may have attenuated T_C adaptations to heat acclimation compared to individuals with BMI <25.