The effect of progressive heat acclimation on change in body heat content in young males

Abstract

Heat acclimation improves the physiological capacity to dissipate heat. As previous studies have only relied upon core temperature and local heat loss measurements, it remains unclear the extent to which whole‐body heat loss, and therefore body heat storage is altered during heat acclimation. Using direct calorimetry, we examined whole‐body heat exchange during progressive increases in the requirement for heat loss during and following a 14‐day heat acclimation protocol. Six young males (Mean±SD, age: 23±3yrs) performed intermittent exercise (3×30‐min bouts of cycling at 300, 350, and 400 W·m−2separated by 10 min rest period) in dry heat (35°C, 20% relative humidity [RH]) prior to (day 0), during (day 7), and at the end (day 14) of a 14‐day heat acclimation. On days 1–6 and 8–13, subjects cycled for 90 min (~50% VO2max) in the heat (40°C, 20% RH). Despite a similar requirement between test days, significantly lower increases in evaporative heat loss were observed for the final exercise bout in days 0 (P=0.01) and 7 (P=0.02) relative to day 14. After 7 days of acclimation, the ΔHb following the exercise did not significantly decrease from day 0 (673±165 vs. 449±109 kJ, p=0.112). However, on day 14 the ΔHb was significantly reduced compared to day 0 (363±64 vs. 673±165 kJ, p=0.04). These data suggest that 14 days is necessary to achieve sufficient heat acclimation to significantly reduce heat storage during exercise in the heat. Support: Deep Mining Research Consortium (Dr. Glen P. Kenny)