During the past decade, female involvement in physically demanding activities in hot conditions has continued to rise1,2 in both occupational3 and sports settings1. This brings a strong need to understand the differences between the sexes that might affect physical performance, safety, or health in the heat.
Thermal Balance
Thermal balance is the result of total heat gain and heat loss, and it is affected by heat exchange with the environment (evaporation, convection, radiation, and conduction) as well as metabolic heat production within the body itself.10 When total heat gain from metabolism and the environment is larger than total heat loss, body temperature rises which can lead to a decrease in physical performance or cause heat-related illnesses – a potential scenario when exercising in the heat.
Heat exchange with the environment is significantly influenced by anthropometric measures especially body surface area, as with a higher body surface area more of skin’s surface is exposed to the environment increasing heat exchange. Smaller individuals, typically females, have a smaller body surface area resulting in smaller total heat loss capacity.5 When females and males are matched for surface area and body mass, there are no significant differences in heat exchange during low and moderate exercise. However, when exercise reaches a moderate to high level of intensity, females may present lower maximal sweat rates, attenuating their capacity for evaporative heat loss5.
Even in relatively cool conditions, where heat loss through methods of heat exchange is efficient, metabolic heat production can surpass the capacity of heat loss as large amounts of heat are produced by the working muscles (e.g.: ˃1000 W). Metabolic heat production during exercise is mainly driven by the absolute exercise workload, and it’s important to note that if work is done relative to maximum work capacity (e.g., % of Pmax) or maximal oxygen consumption (% of V̇O2max) less heat is produced by individuals with a lower maximum work capacity. Females typically have a lower work capacity and often perform at a lower absolute workload, which decreases the amount of metabolic heat gain.2 During heat exposure females should be aware that they may be at a slight disadvantage when exercise intensity and environmental temperature are high and sweat rates are maximal, as they generally seem to have a smaller capacity for heat dissipation through sweating5, though this may be at least partially counter-balanced by the lesser metabolic heat production during high intensity exercise when absolute work rate is not fixed. Despite these differences both males and females can work, train, and compete safely and effectively in the heat. Cooling methods, timed breaks, and adjusting exercise intensity can help support performance and health in the heat.
Thermal Load
Thermal load describes the physiological strain accumulated from both environmental and metabolic heat gain and it is influenced by the factors of thermal balance and heat storage capacity. As with thermal balance, anthropometric measures influence the body’s heat storage capacity. A lower total body mass and lean body mass decrease the capacity to store heat leading to quicker and greater changes in core body temperature during exposure2. Females are often smaller in stature and have less lean body mass and thus may have a reduced heat storage capacity compared to males which may lead to a faster rise in core temperature during exercise in the heat when working at the same absolute workload. Heat storage capacity is especially important during transient bouts (hours) of heat exposure and the importance of heat loss capacity increases as exposure time increases10.
Sex-Hormones and Core Temperature
During the menstrual cycle, females experience a changing concentration of sex hormones that influences core body temperature. On average, core temperature is 0.3-0.7°C higher during the luteal phase when the concentration of the sex-hormone progesterone is higher versus the pre-ovulatory follicular phase6. This rise in temperature occurs in both eumenorrheic females and females taking oral contraceptives2,5. The change in core temperature is coupled with a higher temperature threshold for the onset of heat loss effector functions such as skin vasodilation and sweating.6 With regards to exercise in the heat, the increase in core temperature may reduce the rate of heat storage and total thermal load that occurs during heat exposure2 and reduce time to exhaustion in hot environments6. However, females who are well-trained often display smaller fluctuations in core temperature and tend to perform at a fairly consistent level irrelevant of the phase of the menstrual cycle8. As a result, from a practical perspective, when working with healthy and trained individuals these small variations in core temperature are unlikely to result in substantial differences in physical performance9.
Sex-Differences in Heat Acclimation
Acclimation by repeated heat exposure produces physiological adaptations that decrease core and skin temperature, increase sweating rate, lower sweating threshold, decrease sweat electrolyte concentration, increase plasma volume, and decrease heart rate resulting in improved physical performance, increased thermal comfort, and a decreased risk of heat-related illnesses.10 The magnitude of adaptions seems similar between males and females, but females tend to require a larger number of exposures for adaptations to emerge. It is suggested that during acute heat stress, core temperature rises more rapidly in women, reaching temperature thresholds more quickly, consequently leading to shorter exposure times. Reasons for this discrepancy includes larger muscle mass (improved heat storage) and physical fitness (improved heat dissipation) in men, as well as menstrual cycle, which may increase resting core temperature before exercise is initiated leading to core temperature limits more rapidly. As females may be exposed to a lower thermal load and a weaker stimulus for heat adaptations they may require a longer acclimation period to establish adaptations that significantly reduce their cardiovascular and thermal strain2. Thus, females in particular should ensure that their acclimation period is sufficiently long (10 to 14 days) to get the performance benefits of heat acclimation. Further studies are necessary to define whether sex-specific heat acclimation protocols could shorten the time course of acclimation for females.
Most differences between males and females in the heat emerge from alterations in anthropometrics and work capacity, affecting the capacity for heat exchange, heat production, and heat storage. These differences can affect thermal load during exposure and thus the time course of heat acclimation, with females generally requiring a greater number of heat exposures than males. Eventually both males and females can acquire a similar magnitude of heat adaptation. With proper preparation both males and females can perform in hot environments effectively and safely. EP2 FINLAND brings the aid and assistance of field experts to your disposal. Utilizing these resources to guide you through thermally challenging environments can increase your chance of success.
References:
1. Yanovich, R., Ketko, I., & Charkoudian, N. (2020). Sex Differences in Human Thermoregulation: Relevance for 2020 and Beyond. Physiology, 35(3), 177–184.
2. Wickham, K. A., Wallace, P. J., & Cheung, S. S. (2020). Sex differences in the physiological adaptations to heat acclimation: a state-of-the-art review. European Journal of Applied Physiology, 121(2), 353–367.
3. Corbett, J., Wright, J., & Tipton, M. J. (2020). Sex differences in response to exercise heat stress in the context of the military environment. BMJ Military Health, jramc–2019–001253.
4. Cramer, M. N., & Jay, O. (2016). Biophysical aspects of human thermoregulation during heat stress. Autonomic Neuroscience, 196, 3–13.
5. Gagnon, D., & Kenny, G. P. (2012). Does sex have an independent effect on thermoeffector responses during exercise in the heat? The Journal of Physiology, 590(23), 5963–5973.
6. Baker, F. C., Siboza, F., & Fuller, A. (2020). Temperature regulation in women: Effects of the menstrual cycle. Temperature, 1–37.
7. Giersch, G. E. W., Morrissey, M. C., Katch, R. K., Colburn, A. T., Sims, S. T., Stachenfeld, N. S., & Casa, D. J. (2020). Menstrual cycle and thermoregulation during exercise in the heat: A systematic review and meta-analysis. Journal of Science and Medicine in Sport, 23(12), 1134–1140.
8. Lei, T. H., Stannard, S. R., Perry, B. G., Schlader, Z. J., Cotter, J. D., & Mündel, T. (2017). Influence of menstrual phase and arid vs. humid heat stress on autonomic and behavioural thermoregulation during exercise in trained but unacclimated women. The Journal of physiology, 595(9), 2823–2837.
9. Charkoudian, N., & Stachenfeld, N. S. (2014). Reproductive Hormone Influences on Thermoregulation in Women. Comprehensive Physiology, 793–804.
10. Périard, J.D., Eijsvogels, T.M.H. & Daanen, H.A.M. 2021. Exercise under heat stress: thermoregulation, hydration, performance implications, and mitigation strategies. Physiological Reviews, 101(4), 1873-1979.
11. Havenith, G. (2001). Human surface to mass ratio and body core temperature in exercise heat stress—a concept revisited. Journal of Thermal Biology, 26(4-5), 387–393.
Authors:
Jere Borgenström, MSc;
Christina Kuorelahti, PhD;
Juha Peltonen, PhD, Adjunct Professor;
Dominique Gagnon, PhD, Adjunct Professor