| Abstract | Recent reports show reductions in evaporative heat loss during exercise in hot dry conditions in adults as young as 40, with differences being more pronounced with advanced age. It remains unclear how changes in ambient humidity, which can alter the evaporative efficiency of sweating, may influence the extent to which impairments in heat dissipation occur.PURPOSE: To compare evaporative heat loss in 60 healthy, unacclimated males, 20 to 70 years old, during intermittent exercise in a hot-dry (35°C, 20% relative humidity [RH]) and hot-humid (35°C, 60% RH) environment.METHODS: Participants performed four 15-min bouts of cycling at a rate of heat production equal to 400 W, interspersed with 15-min recovery periods in both environments. Direct calorimetry measured evaporative heat loss (HE) and dry heat exchange while indirect calorimetry measured metabolic heat production. The cumulative change in body heat content was measured as the temporal summation of heat production and total heat loss.RESULTS: HE was reduced by 17, 18, 21, 25 and 20% in males 20-30 (n=12), 40-44 (n=12), 45-49 (n=12), 50-54 (n=12) and 55-70 (n=12) years old (all P<0.001), respectively, during exercise in the hot-humid versus hot-dry environment. HE was also reduced between the hot-humid and hot-dry conditions in males 50-54 (-14%, P<0.001) and 55-70 years old (-11%, P=0.016) during recovery. During exercise in the humid heat stress condition, HE was greater in males 20-30 years old compared to males 45-49 (P=0.012), 50-54 (P=0.002) and 55-70 (P=0.002) years old. HE was also greater in males 40-44 years old compared to males 50-54 (P=0.029) and 55-70 years old (P=0.024) in the hot-humid condition. Body heat storage in the hot-humid condition was greater than in the hot-dry condition across age groups (P<0.001). Young males stored less heat in the hot-dry environment than males 40-70 years old (all P<0.005), and in the hot-humid condition compared to males 45-70 years old (all P<0.010).CONCLUSION: We show that middle-aged and older males have reduced evaporative cooling capacity, and therefore store more heat, than young males during exercise in both hot-dry and hot-humid environmental conditions. This Project was funded by research grants provided by the Workplace Safety and Insurance Board (Ontario) and the Canadian Institutes of Health Research. |
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