Does sex impact exercise thermoregulation?

Numerous investigators have documented sex differences in exercise thermoregulation; however, until recently, it remained uncertain whether those differences were related to sex per se or simply due to secondary differences in body size and aerobic fitness between the men and women studied. Such secondary differences influence exercise thermoregulation primarily by modulating metabolic heat production and the heat loss required to prevent continued rises in core temperature. For example, due to their smaller size, heat production per unit body mass or surface area and the resultant rise in core temperature would be greater in women during weight-supported exercise at a fixed work rate (eg, cycling at 200 W). While determining the potential, magnitude of these effects during training or competition is complex, they pose no more of a concern than differences in body size or aerobic fitness among athletes of the same sex and are likely of trivial importance among athletes of the same competitive level during self-paced exercise.

Investigators in later years attempted to isolate the effects of sex per se from these secondary differences using statistical procedures or by assessing men and women with similar characteristics.2 3 Contrary to conventional wisdom, sex did not significantly modulate exercise thermoregulation.2 3 The only exception was during vigorous exercise (~60% peak oxygen consumption) in dry heat (40°C, ~12% relative humidity), where women demonstrated impairments in sweating that attenuated evaporative heat loss by ~11%.3 However, the impact of such impairments on the resultant rise in core temperature during training or competition is probably minimal in the more humid environmental conditions expected for Tokyo 2021. This is because sweat rate can approach or exceed the maximal evaporation rate possible, leading to greater non-evaporated (wasted) sweat, rather than increased heat loss.

Does menstrual cycle phase or oral contraceptive use influence exercise thermoregulation?

In eumenorrheic women, the mid-luteal phase (postovulation) is associated with increased oestrogen and progesterone and an ~0.2°C elevation in basal core temperature compared with the early follicular (menses) phase.4 Monophasic, combined oral contraceptives, which typically provides a 21-day dose of oestrogen and progestin followed by a 7-day placebo phase, also elicits an upward shift in basal core temperature (~0.2°C) during active-pill consumption relative to placebo-pill consumption (pill-free days).5 6 It appears that these hormonal fluctuations neither appreciably influence heat loss nor the rise in core temperature during exercise-heat stress.4 6 7 However, while the upward shifts in basal core temperature can result in higher exercising core temperatures that may compromise performance during prolonged, high-intensity training or events,5 this is not a universal finding.6 7 Further, any such elevations in exercising core temperature could potentially be offset with heat alleviation methods (eg, pre/percooling, acclimation).8 We are therefore doubtful that the effects of menstrual cycle phase or oral contraceptive use are of sufficient magnitude to pose a notable thermoregulatory concern for female athletes.

Do men and women adapt to heat differently?

As highlighted in an earlier editorial in this series,1 training in a hot environment (or resting heat exposure) for 60–90 min/day for ~2 weeks can induce beneficial thermoregulatory and cardiovascular adaptations (heat acclimation), which optimise performance in hot environments. To minimise disruption to training, this is best conducted ~4 weeks before competition, followed by regular training and/or resting heat exposure prior to travelling to limit adaptation decay and finishing with ~7 days reacclimation on arrival. 1However, whether sex or the underlying effects of menstrual cycling or oral contraceptives modulate the magnitude or time course of heat adaptation remains poorly understood.

Based on the limited data available, women may adapt more slowly to the same heat acclimation stimulus (ie, sessional rise in core temperature), requiring ~5 more days of acclimation to gain a similar magnitude of adaptation to men (figure 1A).9 Thus, while men could benefit from shorter term acclimation (<2 weeks), women may require more intensive or longer term acclimation (≥2 weeks) to reap similar benefits, potentially followed by a more intensive or longer reacclimation period if these sex differences in adaptation persist (figure 1B). As a tentative recommendation, female athletes may therefore consider an initial heat acclimation camp of ≥2 weeks duration ~4 weeks prior to arrival, followed by ~12 days of reacclimation after arriving at the venue. This suggestion, however, requires further scrutiny.

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Figure 1

(A) shows sex differences in the reduction in end-exercise core (rectal) temperature (T_c) during heat stress tests (ST) involving 30 min treadmill running (9 km/h, 2% grade) completed on the day before (day 0) and following 5 days heat acclimation (day 6) and a further 5 days heat acclimation (day 12). Heat acclimation consisted of 90 min intermittent cycling to maintain core temperature (T_c) at ~38.5°C. All trials were conducted in a hot environment (~40°C,~40% relative humidity). Data are group means (eight men and eight women) from Mee et al 9expressed as a change from day 0. Note the relatively rapid reduction in end-exercise core temperature in men following 5 days acclimation, whereas a similar magnitude of reduction was obtained in women only following a second 5-day acclimation period (10 days total). (B) shows a theoretical example of the same protocol completed after arriving in Tokyo following a 2-week period without heat acclimation. Note that there is a loss of acclimation (decay) of ~36% (the difference between the green dashed horizontal line and purple dotted horizontal line),1 which is reestablished (reacclimation) ~5 days earlier in men relative to women. All data are approximations assuming the same rate of adaptation in (A) from Mee et al 9 expressed as a change from day 0. Created with BioRender.com.