A comparison of medium-term heat acclimation by post-exercise hot water immersion or exercise in the heat: adaptations, overreaching, and thyroid hormones

This research compared thermal and perceptual adaptations, endurance capacity, and overreaching markers in men after 3, 6, and 12 days of post-exercise hot water immersion (HWI) or exercise heat acclimation (EHA) with a temperate exercise control (CON), and examined thyroid hormones as a mechanism for the reduction in resting and exercising core temperature (T(re)) after HWI. HWI involved a treadmill run at 65% V̇o(2peak) at 19°C followed by a 40°C bath. EHA and CON involved a work-matched treadmill run at 65% V̇o(2peak) at 33°C or 19°C, respectively. Compared with CON, resting mean body temperature (T(b)), resting and end-exercise T(re), T(re) at sweating onset, thermal sensation, and perceived exertion were lower and whole-body sweat rate (WBSR) was higher after 12 days of HWI (all P ≤ 0.049, resting T(b): CON −0.11 ± 0.15°C, HWI −0.41 ± 0.15°C). Moreover, resting T(b) and T(re) at sweating onset were lower after HWI than EHA (P ≤ 0.015, resting T(b): EHA −0.14 ± 0.14°C). No differences were identified between EHA and CON (P ≥ 0.157) except WBSR that was greater after EHA (P = 0.013). No differences were observed between interventions for endurance capacity or overreaching markers (mood, sleep, Stroop, P ≥ 0.190). Thermal adaptations observed after HWI were not related to changes in thyroid hormone concentrations (P ≥ 0.086). In conclusion, 12 days of post-exercise hot water immersion conferred more complete heat acclimation than exercise heat acclimation without increasing overreaching risk, and changes in thyroid hormones are not related to thermal adaptations after post-exercise hot water immersion.