Heat Acclimation By Warm-water Immersion Enhances Whole-body Heat Loss In Older Adults

Whole-body heat loss declines with increasing age (~3-4% per decade). Consequently, relative to their younger counterparts, older adults experience greater increases in core temperature and cardiovascular strain during heat stress. A recent report showed that heat acclimation induced by 90-min of continuous moderate-intensity exercise performed over 7-days in the heat increased whole-body heat loss in physically active older adults. However, such a protocol may not be tenable for many older adults who are sedentary or not engaged in regular physical activity. Heat acclimation via warm-water immersion has been shown to enhance heat loss and reduce core temperature in young adults during heat stress. Therefore, such an intervention may be an effective alternate approach to exercise to improve heat dissipation in older adults. PURPOSE: To determine if heat acclimation by warm-water immersion over 7-days would induce an increase in whole-body heat loss in older adults. METHODS: Twelve habitually active older males (mean (SD): 69 (6) years) completed 7-days of warm-water immersion (~40 °C) for 60-minutes at a fixed core temperature of 38.5 °C. Before (Day 0) and after (Day 8) the heat acclimation protocol, whole-body dry and evaporative heat loss was measured by direct calorimetry during three bouts of incremental exercise, consisting of 30-minutes of cycling at increasing fixed rates of metabolic heat production of 150, 200, and 250 W/m2, each separated by 15-minutes of recovery in the heat (40 °C, 15% relative humidity). Core (rectal) temperature and heart rate were measured continuously. Data in the last five minutes of the final exercise bout (250 W/m2) were analyzed via a paired t-test. RESULTS: Relative to Day 0, whole-body heat loss was elevated by 29 W/m2 [95% Confidence Interval: 19, 39] on Day 8 (P < 0.001). This was paralleled by a decrease in body heat storage (-82 [-116, -48] kJ) and rectal temperature (-0.3 [-0.5, -0.2] °C; both P < 0.01), as well as a reduction in cardiovascular strain evidenced by a decrease in heart rate (-12 [-7, -17] beats/min) on Day 8 (P < 0.0001). CONCLUSION: We showed that a brief 7-day warm-water immersion heat acclimation protocol improved heat loss capacity in older males leading to a reduction in thermal and cardiovascular strain during exercise heat stress. Natural Sciences and Engineering Research Council of Canada and Mitacs.