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

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.