Jacob Bowie was testing heat tolerance in the lab when he noticed something the textbooks had missed: the women were passing the standard test at higher core temperatures than the test itself considered acceptable. That simple observation has just overturned decades of heat acclimation science and may reshape training protocols for athletes and military personnel worldwide.
The issue cuts to the heart of how science gets built on incomplete foundations. Heat tolerance tests—which measure how well someone's body adapts to exercise in extreme heat—have been the gold standard for determining when athletes can return to play or when military personnel are ready for duty after heat-related illness. But these tests were developed using only male bodies. No one had seriously asked whether they worked the same way for women until now.
At the University of Connecticut's College of Agriculture, Health and Natural Resources, Bowie, a postdoctoral fellow working under professor Elaine Choung-Hee Lee and in collaboration with Douglas Casa, Board of Trustees Distinguished Professor of Kinesiology, ran a study using the widely implemented heat tolerance test: participants walk on a treadmill at a slight incline for two hours in 40 degrees Celsius (104 degrees Fahrenheit) with 40 percent humidity. The standard threshold for passing is a core temperature below 38.5 degrees Celsius and a heart rate below 150 beats per minute.
Here's where it diverged from what the textbooks predicted. Female participants started the test at higher baseline temperatures and heart rates—and they stabilized at higher levels as they acclimated. In other words, women's bodies adapted differently to heat stress, but the test's rigid thresholds didn't account for that physiological reality. "When these tests were developed only using male data, we could expect the results to be biased towards men in terms of their interpretation, and that's what we found," Bowie explains in the research published in Physiological Reports.
The real-world stakes are significant. An overly stringent standard unknowingly penalizes female athletes. Olympic-bound professionals on impossibly tight schedules could be forced into unnecessary extra heat acclimation sessions. More critically, military women might be held back from missions or training when their bodies are actually ready. "When we don't account for those sex-based differences, we could be limiting participation in military training or missions, or we could be tagging athletes as not ready to return to play but they actually are," Bowie notes.
Lee frames the tension honestly: "In one way it's great because we're protecting their safety to prevent exertional heat illness. But they're very limited in their training schedule leading up to competition." The challenge isn't choosing between safety and fairness—it's building better science that doesn't sacrifice either.
This work is part of a larger U.S. Department of Defense-funded initiative to rethink how heat acclimation is measured. Bowie is now developing new protocols using a treadmill test that pushes participants until they reach 39.5 degrees Celsius—closer to the critical threshold where serious complications emerge. These tests, he argues, better reflect what athletes actually experience: sustained high-intensity exertion, not a leisurely treadmill walk. The goal is clear: standards that protect everyone equally, based on how everyone's body actually works.