Our ability to exercise under hot conditions while avoiding heatstroke
By Eva Briggs, MD
I’m writing this article at the end of summer, when the worst of the heat seems to have dissipated for this year. Which is fine with me because I’d rather not hike or bike when it’s roasting hot out.
I learned a few things about the ability to exercise under hot conditions while reading “Endure,” a book by Alex Hutchinson.
He started his chapter on heat with a look at the sad case of Max Gilpin, a Louisville, Kentucky, high school student who died of heatstroke after high school football practice in 2008. Hutchinson explored some factors that allow athletes to exercise on hot days without developing heatstroke and what perhaps goes awry for the few that do succumb to heat illness.
Approximately 58 high school football players have died from heatstroke between 1992 and 2024. The human body converts food energy into mechanical energy to power our bodies, but it’s inefficient.
For every 100 calories metabolized, 25 calories are converted to work and the remaining 75 becomes heat. That’s great when the temperature is low. In fact, most properly clothed people don’t develop hypothermia. Athletes typically develop hypothermia only if they abruptly decrease their activity level or if their clothes become wet and lose their insulating ability.
When the ambient temperature rises, the body responds in several ways to try and dissipate heat. Blood vessels on the skin dilate, allowing heat transfer from the warmed skin surface to the air. People sweat and the evaporation of sweat cools the body. In fact, sweating is the only body mechanism for cooling when the air temperature rises above body temperature. But as humidity rises, sweat drips off without evaporating, becoming ineffective for cooling.
The human body acclimatizes to hot conditions. The process takes two weeks. Sweating begins at a lower temperature. The amount of sweat increases. Skin blood vessels dilate more, increasing heat transfer away from the body. Blood volume increases, allowing the heart rate to slow.
Most people stop exercising when their core temperature reaches a critical level, generally 104 – 104.5 degrees F. The time to reach this temperature can be prolonged by pre-cooling such as soaking in cool water to lower the starting body temperature. But the temperature where exhaustion ensues is not changed.
Drinking slushies before exercise can decrease the body temperature up to one degree. Interestingly, pre-cooling via slushies also raises the critical threshold temperature by about 0.5 degree F. Why does this happen? One theory is that the process of ingesting a slushie cools the brain. And brain temperature may be as important a body temperature for signaling that it’s time to stop exercising. Another theory suggests that the stomach has temperature sensors. Some scientists theorize that this is why some cultures drink hot beverages on hot days. The temperature sensors in the gut perhaps sense the heat and trigger sweating — and therefore evaporative cooling — before the core temperature rises.
The temperature of the skin also influences the ability to exercise in the heat. This explains why cooling vests and ice packs on the neck help. They don’t lower core temperature but affect perception of heat.
Your brain’s perception actually alters your ability to endure heat while exercising. In an experiment, athletes could pedal an exercise bike significantly longer when they saw a thermometer registering 79 degrees compared to one at 89 degrees, even though the room temperature was identical. Motivational self-talk also improves heat tolerance. Athletes who were trained to suppress negative thoughts (such as “it’s too hot out”) and instead think positively (such as “you can do this”) could exercise to a higher core temperature before becoming exhausted.
Research suggests that inflammation is part of the cause of heatstroke. The process that diverts blood to the skin means the blood supply to the gut and other internal organs is decreased. Eventually toxins from these organs leak into the blood stream triggering inflammation. The inflammation interferes with the body’s normal defenses against heat, preventing the normal mechanisms causing people to quit exercising before becoming dangerously overheated.
Risk factors for heat illness include heavy poorly ventilated clothing, preexisting illness and certain drugs such as amphetamines. Many common ADHD medication such as Adderall are amphetamines.
So back to the case of 15-year-old Max Gilpin, who died of heat exhaustion after a high school football practice. All three of those factors probably contributed: heavy football outfit, he had complained of a headache and feeling poorly earlier that day and he was taking ADHD medication.
You may ask, what about dehydration? It turns out that hydration and heat illness don’t necessarily go hand-in-hand. You can have one without the other. In the case of Max Gilpin, bloodwork showed that he was not dehydrated. But a discussion of dehydration could be a topic for another day.
Eva Briggs is a retired medical doctor who practiced in Central New York for several decades. She lives in Marcellus.