Fitness-related differences in the rate of whole-body total heat loss in exercising young healthy women are heat-load dependent

Abstract

Aerobic fitness has recently been shown to alter heat loss capacity in a heat-load dependent manner in young men. However, given that sex-related differences in heat loss capacity exist, it is unclear if this response is consistent in women. We therefore assessed whole-body total heat loss in young (21 ± 3 years) healthy women matched for physical characteristics, but with low (Low-fit) (35.8 ± 4.5 mLO₂·kg⁻¹·min⁻¹) and high (High-fit) (53.1 ± 5.1 mLO₂·kg⁻¹·min⁻¹) (both n = 8) aerobic fitness (indexed by peak oxygen consumption (V̇O_2peak)), during three 30-min bouts of cycling performed at increasing rates of metabolic heat production of 250 W (Ex1), 325 W (Ex2), and 400 W (Ex3), each separated by a 15-min recovery, in hot, dry conditions (40°C, 11% relative humidity). Whole-body total heat loss (evaporative ± dry heat exchange) and metabolic heat production were measured using direct and indirect calorimetry, respectively. Body heat content was measured as the temporal summation of heat production and loss. Total heat loss did not differ during Ex1 (Low-fit: 215 ± 16 W; High-fit: 231 ± 20 W, P > 0.05) and Ex2 (Low-fit: 278 ± 15 W; High-fit: 301 ± 20 W, P > 0.05), but was lower in the Low-fit (316 ± 21 W) compared to the High-fit (359 ± 32 W) during Ex3 (P < 0.01). Consequently, the Low-fit stored 1.3-fold more heat (429 ± 61 kJ) throughout the three exercise bouts relative to the High-fit women (330 ± 113 kJ, P < 0.05). We show that aerobic fitness independently modulates heat loss capacity during exercise in hot, dry conditions in women separated by a V̇O_2peak of ∼17 mLO₂·kg⁻¹·min⁻¹ starting at a metabolic heat load of 400 W.

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