Gagnon, D., Jay, O., Lemire, B., Dorman, L., & Kenny, G. P. (2008). Sex-related differences in dynamic heat balance: Metabolic heat production considerations. ACSM 55th Annual Meeting Indianapolis, Presentation Number 1935.

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"Sex-related differences in thermoregulatory responses during exercise in the heat have typically been attributed to anthropometric characteristics. After accounting for such characteristics, and when working at the same relative oxygen consumption (VO2), females have been shown to sweat less than males, while maintaining similar core temperatures. However, workloads based upon relative maximal VO2 do not take into consideration differences in rates of metabolic heat production). Therefore, we evaluated the hypothesis that sex-related differences in rates of evaporative heat loss during exercise at the same relative VO2 are due to differences in metabolic heat production".

This study determined if different rates of metabolic heat production elicited by workloads of the same percentage of maximal VO2 would result in proportional differences in rates of evaporative heat loss between genders. Ss (M = 8; F = 7) cycled for 60 minutes at 40% VO2 in a hot dry environment (40C, 30% RH) within the modified Snellen whole-body air calorimeter. Ss were matched for body surface area to mass ratio and for maximal VO2 relative to lean body mass. Evaporative heat loss and dry heat gain, as well as changes in body heat content were determined by direct calorimetry. Metabolic heat production was concurrently measured using indirect calorimetry. Rectal temperature was measured continuously.

Throughout exercise, metabolic heat production was higher in males compared to females. Consequently, rates of evaporative heat loss were proportionally higher. That resulted in greater changes in body heat content. This body heat storage resulted in a larger rise from baseline in rectal temperature for males despite their greater total body mass and mean specific heat of body tissues. When accounting for differences in total heat load however, there were no differences in evaporative heat loss between genders.

Implication. Despite working at the same percentage of VO2, the higher metabolic heat load in males throughout exercise resulted in a higher absolute evaporative heat loss but also greater changes in body heat content. It is important to consider metabolic heat production when comparing possible thermoregulatory sex-related differences during exercise.

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