On the slopes of Mount Kilimanjaro, where the air thins to a whisper, Deborah Clegg, a metabolic physiologist, found herself unfazed by the altitude. Her stride remained steady, her energy unflagging. In contrast, her climbing partner, Biff Palmer—an accomplished nephrologist and mountaineer—found the climb more challenging. Despite Palmer’s vast experience, including summiting Everest and six of the world’s tallest peaks, Clegg consistently outperformed him.
Their shared experiences on mountain climbs led them to question why Clegg’s body seemed so well adapted to the low-oxygen air and prolonged exertion. Back in the lab, they sought answers. By 2014, their research revealed that estrogen played a crucial role in reducing hypoxia-inducible factor (HIF), a protein that helps the body adapt to low oxygen levels but also causes inflammation and discomfort. This discovery highlighted estrogen’s role in making altitude easier to bear and its central role in metabolic flexibility—the body’s ability to shift between fuel sources, particularly from glucose to fat.
The Science Behind Women’s Metabolic Flexibility
Deborah Clegg, now a professor of internal medicine at Texas Tech University Health Sciences Center, continues to study how estrogen and fat metabolism shape endurance. Her research is part of a growing body of evidence challenging the assumption that women’s bodies are not as strong as men’s. Science increasingly shows that flexibility—the ability to adapt, shift, and recover—is a key strength that makes female bodies resilient.
Studies have shown that male bodies tend to rely more heavily on carbohydrates for short bursts of power, giving them an advantage in explosive strength. However, female bodies excel in endurance, preferentially burning fat, which provides a steady energy source. This metabolic trait was advantageous for our female ancestors, who needed sustained energy during long gestational cycles of pregnancy and breastfeeding while still engaging in daily activities like hunting and gathering.
“Women predominantly store fat in hips and thighs, which is a really beautiful safe space to store your fat because it’s outside the abdominal cavity, where men store it inside,” explains Clegg.
This difference in fat storage is significant. Visceral fat around organs in the abdomen poses more health risks than subcutaneous fat, which is more common in female bodies. Additionally, female fat cells are more flexible, akin to spandex, allowing them to stretch and store excess fatty acids and calories healthily. This flexibility is directly related to sex hormones and provides a survival and health benefit.
The Physical Flexibility Advantage
If metabolism shows the invisible side of adaptability, movement reveals the visible one. According to Miho Tanaka, a physician treating athletes and a sports medicine surgeon at Mass General Brigham, the human body has three kinds of physical flexibility: functional flexibility, muscular flexibility, and joint flexibility. This flexibility is associated with greater muscular efficiency and strength, regardless of sex, which is why athletes incorporate it into their training.
“Flexibility and having the ability to use the full motion of your joints is important in optimizing joint biomechanics. It directly influences how an athlete generates force,” says Tanaka.
However, too much laxity can lead to joint injuries, a delicate balance that explains why women are more prone to certain injuries. Research suggests that this may not solely be due to biological factors but could also be influenced by training disparities. In sports like downhill skiing, where training is individualized and starts early, there is no sex-based difference in injury rates.
Adapting to Life’s Major Changes
Beyond metabolism and movement, the female body’s ability to undergo dramatic changes is perhaps its most astonishing flexibility. From the first period to menopause, and through pregnancy, birth, and recovery, women’s systems repeatedly reconfigure without breaking down.
These physiological changes might even offer advantages. Recent research indicates that breastfeeding may reduce breast cancer risk due to immune-cell recruitment into the breast. Additionally, some female athletes return to their sport even stronger after pregnancy and childbirth, matching or exceeding their pre-pregnancy abilities.
“Adaptation is the through-line. Across all scales—from mitochondria to muscle fiber to hormone cycle—the female body’s unsung power lies in its ability to bend without breaking,” says Clegg.
The potential for future training regimens tailored to an athlete’s specific needs could further reduce injuries, as some studies have already shown. Tanaka highlights the potential of AI and machine learning to analyze large datasets, allowing for injury prediction and customized training plans.
As research continues to uncover the unique strengths of female bodies, it becomes increasingly clear that flexibility, both metabolic and physical, is a key component of women’s resilience and strength. This understanding not only challenges outdated assumptions but also opens new avenues for optimizing health and performance across genders.
