Short-Term Warm-Water Immersion for Improving Whole-Body Heat Loss in Older Men.

Purpose: Exercise-induced heat acclimation can mitigate age-related reductions in heat-loss capacity, though performing repeated bouts of strenuous exercise in the heat may be untenable for many older adults. While short-term passive heat acclimation (e.g., ≤7 days of warm-water immersion) enhances whole-body heat loss in young adults, evidence of its efficacy in older adults is lacking. Thus, we examined whether 7-days warm-water immersion would improve whole-body heat loss in older adults.

Methods: Twelve habitually active older men (median [IQR] age: 68 [64-73] years; peak oxygen uptake (V̇O2peak): 34.1 [29.4-36.1] mLO2·kg-1·min-1) completed 7 consecutive days of ~90-minutes warm-water immersion (~40 °C) with core (rectal) temperature clamped at ~38.5 °C for the final 60 minutes. Before and after the warm-water immersion intervention, whole-body total (evaporative + dry) heat loss was measured via direct calorimetry during three, 30-minute bouts of cycling at increasing fixed rates of metabolic heat production (150, 200, 250 W·m-2), each separated by 15-minutes rest, in a hot-dry environment (40 °C, ~13% relative humidity). Rectal temperature and heart rate were measured continuously.

Results: Following 7 days of warm-water immersion, whole-body total heat loss was elevated by 23 [95% confidence interval: 14, 31] W·m-2 across exercise bouts (acclimation-effect: P < 0.001; interaction: P = 0.598). This was paralleled by reductions in core temperature and heart rate of 0.3 [0.2, 0.4] °C and 11 [8, 14] beats·min-1 (both, acclimation-effect: P < 0.001; interaction: P = 0.288), respectively.

Conclusions: Seven consecutive days of warm-water immersion improved whole-body heat loss and reduced core temperature and cardiovascular strain across light-to-vigorous intensity exercise in habitually active older men. Passive heat acclimation may be an efficacious alternative to exercise-heat acclimation to improve heat-loss capacity. Studies are warranted to assess effectiveness in more heat-vulnerable populations.