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

Abstract

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.

Look to the stars—Is there anything that public health and rehabilitation can learn from elite sports?

“…Altitude/hypoxic training ( 69 ) and heat acclimatization ( 70 ) are very important components of preparation in elite sport. Recent innovative methods have been developed [e.g., to cite only few: repeated sprint training in hypoxia—RSH—for team sport athletes ( 71 ); warm bath ( 72 ) for heat acclimatization); full body cryotherapy for improving recovery or rehabilitation ( 73 )…]. Combination of several environmental stressors is also investigated ( 74 ).…”

Molecular biomarkers for assessing the heat-adapted phenotype: a narrative scoping review

“…More recently, in an unpublished study, McIntyre [ 128 ] demonstrated that the change in resting plasma total T4 after a 6-day HA was related to the magnitude of the accumulated endogenous thermal ‘dose’ (total area under the curve for rectal temperature > 38.5 °C, r = − 0.32, P = 0.021), with a larger endogenous thermal dose resulting in a greater reduction in plasma total T4. In a subsequent study examining two different 12-day HA regimens, the same group demonstrated a significant reduction ( P = 0.006) in resting plasma free T3 following a 12-day hot water immersion HA regimen [ 129 ], but no significant change in plasma T4 levels. Interestingly, in unpublished data from this group [ 128 ], it appears that the plasma free T3 level was related to the reduction in resting rectal temperature ( r = 0.47, P = 0.044), as well as the endogenous thermal dose accumulated during a 12-day HA regimen ( r = − 0.57, P = 0.017).…”