Background: In adult humans, thermoregulation can differ between sexes for physiological, physical, and psychophysical reasons. Heat tolerance refers to one’s capacity for tolerating either heat stress (e.g., work capacity in a hot environment) or heat strain (e.g., core temperature that elicits exhaustion or physiological dysfunction). Both contexts have functional relevance and provide physiological insight. Yet, human heat tolerance data are largely from men, and studies of sex effects are limited to the heat stress context and the follicular phase of the menstrual cycle, i.e., when males and females are most similar physiologically.
Aims: We sought to determine whether heat tolerance differs between men and women, when assessed in the luteal phase and in a heat strain context. We hypothesised that females would have less thermal reserve due to higher baseline core temperature but a similar ceiling.
Methods: Procedures adhered to the ethical approval granted by the Human (Health) Ethics Committee, University of Otago (H20/031). Participants were 9 males and 9 females, pooled from two studies in which equal numbers of males and females cycled at low-moderate intensity in uncompensable heat stress (skin temperature 38-39°C), to volitional exhaustion or rectal temperature of 40°C, whichever occurred first. All participants were habitually physically active, and sexes were of similar fitness (peak aerobic power 54 and 51 mL/min/kg, and 3.6 and 3.7 W/kg) and surface area-to-mass ratio (0.024 and 0.024 m2/kg). Heat stress trials occurred mid-late afternoon, with participants euhydrated and blinded to their core temperature. Previously, each participant had completed an aerobic fitness test and at least one heat familiarisation session involving a 2°C rise in rectal temperature or volitional tolerance. Statistical analyses were unpaired t tests and two-way ANOVA. Results are reported as means and 95% confidence limits (CL), for n=9 vs 9 unless stated otherwise.
Results: Core temperature was not higher at baseline for females than males (37.18 vs 37.06°C; P=0.479; CL: -0.22, 0.45). Neither was thermal reserve smaller (2.07 vs 1.75°C; P=0.271; CL: -0.28, 0.93), nor rate of heating faster (1.25 vs 1.21 °C/h; P=0.841; CL: -0.37, 0.45). Only one participant reached the 40.00°C ethical end point. End tidal CO2 pressure did not differ at baseline or decrease (5 vs 6 mm Hg) more for females than males during heat stress (Sex: P=0.081; Strain: P=0.001; Sex*Strain: P=0.634). Mean arterial blood pressure and its decrease (2 vs 6 mm Hg) were also similar between sexes (P=0.503; P=0.011; P=0.221). Perfusion of the internal carotid artery also remained similar (-1 vs -9%; P=0.879; P=0.181; P=0.294), as did estimated Intracranial Pressure (1 vs 1 mm Hg, indexed from optic nerve sheath diameter; P=0.298; P=0.073; P=0.785; n=8 for both sexes and both variables).
Conclusions: For these fit, fitness-matched females and males, heat tolerance was evidently not lower for females despite the comparison being made during their luteal phase. Cerebral haemodynamics also appeared to be minimally affected. The present results cannot be expected to apply to an unfit population because high fitness is known to lessen menstrual phase effects on sex hormones and thermoregulation.