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 (Tre) after HWI. HWI involved a treadmill run at 65% V̇O2peak in 19°C followed by a 40°C bath. EHA and CON involved a work-matched treadmill run at 65% V̇O2peak in 33°C or 19°C, respectively. Compared with CON, resting mean body temperature (Tb), resting and end-exercise Tre, Tre 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 Tb: CON −0.11 ± 0.15°C, HWI −0.41 ± 0.15°C). Moreover, resting Tb and Tre at sweating onset were lower after HWI than EHA (P ≤ 0.015, resting Tb: EHA −0.14 ± 0.14°C). No differences were identified between EHA and CON (P ≥ 0.157) except WBSR which 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.