Humid heat stress affects trained female athletes more than does their menstrual phase

The prevailing view is that healthy, pre-menopausal women with a regular menstrual cycle face a performance disadvantage and increased risk for heat illness when exercising in a hot environment during their luteal (post-ovulatory) phase. In our recent Journal of Physiology article [1], we challenged this view and demonstrated that for trained females their menstrual phase was of little physiological or performance consequence during self-paced exercise but that the thermal profile of the environment (dry vs. humid) was important.

The literature tells us that female body temperature is regulated ∼0.4ºC higher following ovulation (luteal phase) and that a delay (threshold shift) to the vasomotor and sudomotor thermoeffectors accompanies this [2]. Therefore, when exercising with an ambient heat load a female's endurance performance is reduced and thermal strain augmented [2]. However, this basis is questionable for endurance-trained women due to three principles, as summarized in Figure 1 (upper panels). Firstly, training status alters thermoeffector responses such that aerobically trained females demonstrate enhanced skin blood flow and sweating compared to less-trained counterparts [3]. Therefore, trained women have a greater capacity to deal with a heat load. Second and relatedly, trained women have reduced ovarian hormone concentrations and fluctuation between their menstrual phases that is associated with a smaller bi-phasic core temperature and effector difference [3]. Thus, thermoregulatory changes are minimized during their luteal phase. Finally, all previous investigations utilized a fixed-intensity exercise protocol that does not allow for our most effective and powerful form of thermoregulation: behavior. Not only is this model less face-valid, but it ignores the fundamental premise of heat balance: namely, that heat loss (predominantly evaporation) needs only equal heat production (predominantly exercise intensity) [4]. We have previously demonstrated in men that during exercise in the heat behavior results in a reduction in exercise intensity, and therefore, metabolic heat production that eases the required evaporation and decreases thermoregulatory strain [5].

Figure 1.

Upper Left: Aerobically trained females display an attenuated fluctuation in core temperature and thermoeffector responses between menstrual phases, in addition to their enhanced capacity for evaporative heat loss when compared to less trained women. Upper Right: No previous investigation has allowed female athletes behavioral thermoregulation by adopting a self-paced exercise protocol, whereby a reduced metabolic heat production minimizes phase-related differences in autonomic heat loss. Lower Center: Our findings revealed humid heat stress significantly reduces self-paced exercise performance in trained female athletes, whereas there was minimal impact according to their menstrual phase. U: Untrained females; T: Trained females; L: Luteal phase; F: Follicular phase.

We studied the responses of ten well-trained eumenorrheic women who completed four trials, with each trial consisting of consecutive resting, fixed-intensity and variable-intensity cycling allowing observation of both autonomic and behavioral thermoregulation [1]. Two trials were conducted during their early follicular and two during their mid-luteal phases, therefore maximizing the differences in estrogen and progesterone occurring naturally/endogenously. Furthermore, during each phase we recorded their responses to dry (29ºC, 81% r.h.) and humid (34ºC, 41% r.h.) ambient heat matched for wet bulb globe temperature (27ºC) to enable comparison between environments differing in their required evaporation. The novel and important results observed were that i) self-paced time-trial performance was not affected by menstrual phase but was impaired by the humid environment, and ii) the thermoeffector onset thresholds and thermosensitivities were similar between menstrual phases and environments, but the behavioral response (reducing intensity) occurred earlier and to a greater extent in the humid environment. Therefore, although it is widely known that a warm-humid environment reduces evaporative power, allowing females to behave (self-pace) during exercise nullifies this thermodynamic and autonomic difference, however at the expense of exercise performance (Fig. 1, lower panel).

From a practical perspective, these results inform female athletes and their support on conditions they could be facing at events differing in thermal profile. For example, the 2019 Doha World Athletics Championships will take place in hot-dry conditions whereas the 2020 Tokyo Olympic Games most likely in warm-humid. Equally, where athletes travel, train and compete in differing climates on a regular basis due to their schedule i.e. tropical Florida and Hawaii vs. arid Arizona and Nevada. At large, international multi-event competitions (i.e. Olympic, Commonwealth, Pan-American Games etc.) the proportion of female competitors almost numbers that of males. This cohort in large parts would be closer in fitness to the sample we investigated [1], therefore we propose that it may be of lesser consequence that females consider their menstrual cycle – although individual differences will likely still persist – but that optimal competition preparation occurs for the specific ambient heat type. However, amongst athletes, prevalence of oral contraception remains high and it is known this (chronic) administration of exogenous synthetic estrogens and progestogens alters the thermoeffector responses; therefore, our laboratory has recently completed a matched cohort of oral contraceptive users under the same conditions to determine whether their responses differ. Furthermore, a current investigation will determine whether menstrual phase alters the adaptive response to exercise heat acclimation, as the acknowledged “gold standard” in preparation for and mitigation against heat stress.