Sex differences in physiological responses to heat stress have been a “hot topic” over recent years, in particular with regard to risk of developing exertional heat illnesses and the important countermeasure of heat acclimation. Heat acclimation is the process through which the body garners adaptations from systematic, repeated heat exposure. These adaptations primarily include lower core body temperature (Tcore), lower heart rate (at rest and during exercise), increased sweating rate, and increased plasma volume. Mechanistically, we know there are differences between men and women in thermoregulatory responses to exercise-heat stress. Some of these differences result from physical and anthropometric differences between the sexes. For example, men are often larger and thus have lower body surface area (BSA) to mass ratio (BSA/mass^-1). This is an important biophysical factor because in individuals with lower BSA:mass^-1, heat dissipation (i.e. via sweating from the skin surface) may be limited relative to heat production (i.e. heat produced from skeletal muscle mass contraction during exercise). This physical difference may benefit women in certain environments. Additionally, female sex hormones influence thermoregulation with progesterone increasing the thermoregulatory setpoint by ~0.3-0.5°C, and estradiol increasing nitric oxide mediated vasodilation. This increase in Tcore by progesterone, during acute heat stress, does not appear to be an obstacle for women, and may prove beneficial in the acclimation process (more research is currently needed to elucidate the possible impact). The estradiol-mediated increase in vasodilation is beneficial in terms of thermoregulation, allowing for increased heat dissipation during exercise and/or heat stress. Due to the increasing utilization of hormonal contraceptives, both short and long-acting, that exogenously supplement estrogens and progestins, more work is needed to evaluate the impact of such exogenous hormonal administration on thermoregulatory processes and adaptations, including heat acclimation.

Funded by USAMRDC; Author views are not representative of the Army/DoD.