Much of the work devoted to exploring potential sex-specific differences in exercise or sports performance has been derived from laboratory-based studies. While these studies are typically well-controlled and guide our understanding of physiological mechanisms, they may lack pragmatic or practical relevance to the 'real world'.
Shrinking performance gaps between the sexes in sport and extreme challenges
In 1967, Katherine V Switzer, the daughter of a US Army officer, became the first woman to complete the Boston Marathon as an officially registered competitor. She was assaulted by the race manager, Jock Semple, who attempted to remove her race bib. This shocking incident led to a ban imposed by the Amateur Athletic Union against women participating in the event until 1972. What was the reasoning behind such an act? Race officials claimed that women could not run that far, and the rules forbade it. Fortunately, the consensus evolved, and the rules changed. Katherine and Jock later became friends.
Johny Hayes, an American male set the first marathon world record during the London Olympics in 1908. A Kenyan male, Kelvin Kiptum, currently holds the marathon world record at 2h00min:35s, set during the Chicago Marathon in 2023. Ruth Chepng'etich, a Kenyan female, set the women's marathon world record in the same event at 2h:09min:56s, only 9min21s behind her male counterpart. Over the past few decades, athletes have benefited from improved nutrition and footwear, as well as the use of pacesetters to break the wind, all of which may improve performance. Athletes with anthropometric and lifestyle phenotypes well-suited for marathons also contribute to the record-breaking trend. Closer examination, however, reveals an overall shrinking gap in athletic performance between males and females.
Are women more metabolically efficient under extreme physiological circumstances?
Strength, power, speed, and endurance are typically between 10% and 30% greater in males compared to females. However, recent work suggests a shrinking reduction in the performance gap between the sexes as the distance and/or duration increases. Here we address this important issue in our recent study in Frontiers in Physiology . Using the doubly labeled water method, we reported lower total energy expenditure relative to load carriage in women compared to men during the Alaska Mountain Wilderness Ski Classic – a remote and unsupported 200km Arctic winter expedition . These findings indicate greater metabolic efficiency in women under such extreme conditions.
Sex-specific physiological differences in athletic performance are largely determined by variations in sex chromosomes and hormones. Testosterone levels increase approximately 30-fold in males during puberty and are closely linked to increased muscle mass and strength. In contrast, testosterone levels remain relatively low throughout the lifespan of females.
The menstrual cycle is marked by fluctuations in estrogen and progesterone in females, but these hormones stay relatively constant in males. Elevations in estrogen during the follicular phase have been posited to enhance fat oxidation. However, a recent meta-analysis concluded that variations in estrogen have a minimal impact on metabolism.
So what about ultramarathons and even more extreme events? Pamela Reed and Hiroko Okiyama outperformed their male counterparts in the Badwater and Deutschlandlauf ultramarathons, respectively. While these may be isolated cases, the gap in race durations between males and females has decreased by approximately 3% in events lasting six, 72, 144, and 240 hours over the past four decades. When males and females compete in similar numbers, the gap decreases even further. Fewer elite females participate in ultras compared to elite males.
In studying physiological resilience in athletes participating in the Yukon Arctic Ultra (YAU), the longest and coldest ultramarathon in the world, we observed while working alongside Dr Mathias Steinach (affiliated with the Center for Space Medicine, Berlin)that not a single participant with a body mass index (BMI) of 22 kg/m2 has ever completed the event In fact, the average BMI for this event is approximately 24 kg/m2 for both males and females, with fat mass being 30% higher in females. Despite males having greater amounts of lean tissue mass and less fat mass compared to females, the number of finishers in both sexes is essentially equivalent.
Recent studies have described an 'Arctic shift' in females, indicating the activation of cold-induced thermogenesis at a lower temperature compared to males, and potentially reducing metabolic demands under cold stress. Leveraging plasma, serum, stool, hair, muscle and adipose tissue samples from the YAU cohort, future studies will explore the mechanisms responsible for similar levels of resilience despite differences in lean tissue.
Females in combat-forward military scenarios
As a surrogate model for military operations, we measured rates of total energy expenditure (TEE) during backcountry hunting expeditions in Alaska. Hunters, who also volunteer as research participants, are dropped off via bush plane in the wilderness with a backpack, a rifle, and a satellite phone for between two and four weeks. Females typically carry more weight relative to body weight than males, but are no less capable or resilient. Although the number of participants was small, TEE/lean tissue mass was similar in both males and females, indicating no difference in energy expenditure dedicated to physical exertion. Utilizing new stable isotope methods developed with our collaborators at the University of California Berkeley, we are now studying sex-specific alterations in the structural integrity, cellular respiration and contractile function of skeletal muscle in this cohort.
Although men have historically dominated protective roles in society, emerging data from endurance events conducted in extreme environments suggest that women may be equally, if not more, metabolically resilient under physical and nutritional stress.
We think that one of the coolest (pun intended) aspects of being scientists is that we continually challenge dogma to find new answers to old problems. Perhaps the best-selling author John Gray was right with one minor correction: women may be from Mars (colder), and men may be from Venus (warmer).
