The Interactive Effects of Age and Sex on Whole-Body Heat Exchange During Exercise in the Heat

Abstract

It is well established that older adults display marked impairments in the heat loss responses of sweating and skin blood flow relative to young adults which can exacerbate body heat storage by compromising whole-body heat loss (evaporative + dry heat exchange). Similarly, young women display reductions in whole-body heat loss relative to young men during exercise in dry heat. As such, it is possible that the age-related decline in whole-body heat loss will be greater among women relative to men. To examine whether the age-related decline in whole-body heat loss would be greater in women relative to men during exercise in dry heat, and whether this response would be more pronounced with a greater elevation in the level of heat stress, whole-body heat loss (evaporative ± dry heat exchange) was evaluated in 80 individuals (46 men, 34 women) aged between 18-70 years. Participants completed an incremental exercise model involving three, 30-min bouts of semi-recumbent cycling at fixed rates of metabolic heat production (150, 200, 250 W/m2), each separated by a 15-min recovery period in hot-dry conditions (40˚C, ~15% relative humidity). Whole-body heat loss was measured using direct calorimetry whereas metabolic heat production was measured using indirect calorimetry. Whole-body heat loss declined with age (across men and women) during moderate- and vigorous-intensity exercise by 4.2 and 6.6 W/m2 (both P < 0.050), respectively, however, these relationships were not modified by sex (both P > 0.050). Nonetheless, whole-body heat loss was lower in women relative to men during moderate- and vigorous-intensity exercise by 8.4 and 12.1 W/m2 (both P < 0.05), respectively. Therefore, the results of this thesis demonstrate that the age-related decline in whole-body heat loss is not dependent on sex.