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 (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.