Sex-related differences in evaporative heat loss: the importance of metabolic heat production

Abstract

We evaluated the hypothesis that different rates of metabolic heat production between sexes, during exercise at the same percentage of maximum oxygen consumption $\left( {\dot{V}{\text{O}}_{2\max } } \right),$ give proportional differences in evaporative heat loss. Seven males and seven females, exercised at 41.3 ± 2.7% $\dot{V}{\text{O}}_{2\max }$ for 60-min at 40°C and 30% relative humidity. Whole-body direct air calorimetry measured rate of whole-body evaporative heat loss $\left( {\dot{H}_{\text{E}} } \right),$ while metabolic heat production $\left( {\dot{M} - \dot{W}} \right)$ was measured by indirect calorimetry. $\dot{M} - \dot{W}$ was greater in males (243 ± 18 W m⁻²) relative to females (201 ± 4 W m⁻²) (P ≤ 0.05) throughout exercise. This was paralleled by a greater $\dot{H}_{\text{E}}$ at end-exercise in males (207 ± 51 W m⁻²) relative to females (180 ± 3 W m⁻²) (P ≤ 0.05). Differences in metabolic heat production between sexes during exercise at a fixed percentage of $\dot{V}{\text{O}}_{2\max }$ give differences in evaporative heat loss. To compare thermoregulatory function between sexes, differences in metabolic heat production must therefore be accounted for.