Sex differences in the thermoregulatory and cardiovascular response to exercise in hot environmental conditions

Exercise during heat exposure induces skin microvascular and systemic cardiovascular changes. When standardized exercise tasks are completed, such as during military training or in workplace settings, sex differences in responses may be apparent. Nineteen males and 19 females participated in a set-pace laboratory walking test (treadmill walking 5 km/h; 2% incline) in a climate chamber (40^°C; 50% RH) for 90 min. Body composition (DXA) and aerobic capacity (V^_ O_2max) were measured in a preliminary session. Metabolic heat production, skin blood flow (SkBF; laser Doppler flowmetry), limb blood flow (Doppler ultrasound), stroke volume, cardiac output (CO), heart rate (HR), oxygen consumption (V^_ O₂), and core temperature (Tc) were measured at baseline, 30, 60, and 90 min. No sex difference in Tc at 90 min was evident (male 38.3 ± 0.5^°C vs. female 38.5 ± 0.4^°C; P = 0.403) and a similar change from baseline to 90 min (D 1.40 vs. 1.28^°C; P = 0.447) occurred, despite males producing more heat (3.4 ± 1.0 vs. 2.1 ± 0.7 W/kg; P = 0.001), exhibiting higher SkBF (192 ± 50 vs. 160 ± 21 PU; P = 0.026), and higher sweat production rate (16.5 ± 5.1 vs. 12.3 ± 3.3 mL/min; P = 0.009). Males also had higher CO (7.25 ± 1.38 vs. 6.11 ± 1.72 L/min; group P = 0.026) and femoral blood flow (1.00 ± 0.23 vs. 8.22 ± 0.19 L/min; P = 0.026) responses than females. Males compensated for more lean mass and higher metabolic heat production via a larger increase in cardiac output, with more blood flow distributed to active muscle and, as heat and exercise exposure continued, to the skin. Tc in females did not rise more than in males, possibly due to body size and/or anthropometric factors.