Fluid balance, kidney function and acute kidney injury risk in healthy men and women during simulated work-related heat stress

We tested the hypothesis that men are at greater risk of dehydration during simulated work-related heat stress (WrHS) compared to women independent of menstrual cycle phase. Twelve eumenorrheic women completed a WrHS simulation during the early follicular (EF), late follicular (LF) and mid-luteal (ML) phase of their menstrual cycle and twelve men completed one WrHS simulation. The WrHS simulation involved a four-hour exposure to 33.8±0.8°C, 54±1% relative humidity where participants walked on a treadmill for 30 min per hour at a rate of metabolic heat production of 5.2±0.7 W/kg (199±28 W/m²). Participants drank a cool non-caloric sport drink ad libitum. Percent changes in nude body weight from pre- to post- WrHS did not differ between men (-0.5±0.5%) and women (EF: -0.5±0.9%; LF: -0.3±0.9%; ML: -0.3±0.7%) during any phase of the menstrual cycle (p≥0.4694). Relative fluid intake was higher in women during LF (43±18 mL/kg LBM; p=0.0148) and ML (43±16 mL/kg LBM; p=0.0048), but not EF (36±21 mL/kg LBM; p=0.1792) compared to men (24±5 mL/kg LBM). Relative urine output was higher in women (20±12 mL/kg LBM) compared to men (8±4 mL/kg LBM) across menstrual cycle phases (p≤0.0367). Relative total sweat loss did not differ between men (700±160 mL/m² BSA) and women (675±254 mL/m² BSA) during any phase of the menstrual cycle (p≥0.8458). While the way it was achieved differed, when cool fluids are available fluid balance is similarly achieved in men and women independent of the menstrual cycle, suggesting the risk of dehydration during WrHS is not modified by biological sex.