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

Abstract

Purpose: Exercise-induced heat acclimation can mitigate age-related reductions in heat loss capacity, although performing repeated bouts of strenuous exercise in the heat may be untenable for many older adults. Although 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 of warm water immersion would improve whole-body heat loss in older adults.

Methods: Twelve habitually active older men (median [interquartile-range] age, 68 [64-73] years; peak oxygen uptake (V̇O 2peak ), 34.1 [29.4-36.1] mL O 2 ·kg -1 ·min -1 ) completed 7 consecutive days of ~90-min warm water immersion (~40°C) with core (rectal) temperature clamped at ~38.5°C for the final 60 min. Before and after the warm water immersion intervention, whole-body total (evaporative + dry) heat loss was measured via direct calorimetry during three, 30-min bouts of cycling at increasing fixed rates of metabolic heat production (150, 200, and 250 W·m -2 ), each separated by 15-min 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.

Trial registration: ClinicalTrials.gov NCT05838612.

Keywords: AGING; CLIMATE CHANGE; HEAT STRESS; HEAT WAVE; PASSIVE HEAT ACCLIMATION; THERMOREGULATION.