At a 100-mile trail race in 90-degree heat this June, some runners finished strong while others were doubled over with cramps and nausea. As temperatures rise and more people — especially women and older athletes — take up outdoor endurance sports, understanding individual heat risk matters more every year.
Bodies regulate temperature well under normal conditions, but exercise in heat taxes the heart, nervous system, and muscles faster than it can recover. Not everyone is taxed equally, and the “why” is still genuinely debated: most foundational exercise physiology research was done exclusively on men, so general coaching advice may not even apply to women. Compared to men, women sweat less, start sweating at a higher temperature, and are affected by where they are in their menstrual cycle — heat dissipation is measurably worse in the high-hormone phase and after menopause.
Age matters less than you’d think, on its own — the apparent decline in heat tolerance with age is mostly explained by fitness loss, not age itself. A fit older athlete tolerates heat much better than a sedentary younger one.
Practical takeaways: humidity and dehydration make heat stress worse; pre-cooling, shade, and hydration help in the moment; and watch for cramps, dizziness, nausea, or skin that’s either cold and clammy or hot and dry — signs to stop, cool down, and get help before it becomes heat stroke. Sports science needs more research across sex, age, and ethnicity before we have real answers — but until then, know your own risk.