Laura Watts has been running for hours when a pacer suddenly appears, running behind her, doggedly spraying her with a pressurized water gun. It’s a suffocating, 124-degree day in Death Valley, and the most she can manage is running for 20-second bursts at a time. She still has more than 45 miles of the Badwater 135 ultramarathon to go.
Watts, a flight attendant from England, has already endured a slew of physical insults to reach the starting line of a race so hot that shoes are known to melt into the pavement. She spent a summer month running through her neighborhood wearing three thermal tops, lined leggings, and gloves. At home, she worked up to 70-minute sessions in a tiny, infrared sauna. She even found an environmental chamber at a local university, where she spent six 90-minute sessions running with a rectal thermometer (“that was the unpleasant bit”), noting when her core temperature spiked.
“One day they took my core temperature way up, to just below 40 degrees [104° F] for me to feel what it was like in the danger zone of overheating,” she says. “I couldn’t run. It was awful.”
Watts is part of a new wave of elite athletes using heat to sharpen their training, as extreme athletic challenges become harder, the planet gets hotter, and sport regulators are increasingly screening for and banning performance-enhancing gear and supplements. Athletes are looking for an edge, and heat can give them that, even if they don’t compete in scorching temperatures.
Heat holds promise for everyone from ICU inpatients to runners slogging through Death Valley. Like exercise, heat exposure can be a positive stressor that activates helpful heat shock proteins, lowers blood pressure, and can help prevent and even treat cardiovascular, metabolic, and autoimmune diseases like rheumatoid arthritis and fibromyalgia.
This doesn’t mean a warming planet will be favorable to human health, even though prolonged heat exposure should increase heat tolerance. Constant heat means constant stress, and that will have a detrimental impact akin to an athlete overtraining and risking injury or worse. The body needs periods of rest to recover and rebuild after stress. (Yes, this means your air conditioning could, technically, be considered a fitness device.) Plus, the constant presence of heat eventually stops triggering heat shock proteins when we adapt to it.
There are also hard limits to how much heat humans can endure. New performance equipment that allows athletes to pack ice around the head, neck, and torso during races has helped. One study even examined the most cooling drinks, awarding top status to the humble gas station Slurpee, whose icy particles cool the body from the inside out. (Ice cream works, too.)
Physiologist Tamara Hew-Butler is an expert in overhydration, a trend that has killed athletes in recent years. Drinking water, as well as heat exposure, cause a temporary increase in blood plasma volume. Blood vessels dilate, increasing the water in our circulation, and making it easier for evaporative cooling via sweat. Pressure sensors in the heart act as a control, prompting urine output when there’s too much water in circulation, or constricting vessels when there’s not enough. “If you can withstand heat, you’re able to withstand stress,” Hew-Butler says. “And so your ability to have your body withstand any type of stress will make you a little bit more resilient to things that are unexpected.”
Hew-Butler says it’s unclear which parts of that process—increasing circulation, flushing toxins through sweat, or expanding plasma volume—are beneficial. Researchers elsewhere are working to clarify that picture to explore whether activating heat shock proteins can protect muscle and liver tissue from inflammation and oxidative stress that can lead to metabolic disorders like diabetes and other chronic health conditions.
Stimulating heat shock proteins
But even if we can’t yet break apart the process of heat to determine exactly how it helps, scientists have still shown that it can have benefits. The same therapies that aid athletes like Watts, for example, can also help people who are frail or immobilized. Sitting in a sauna might not boost your fitness, but it might prevent muscle atrophy by preserving mitochondrial function if, say you break your leg and have to wear a cast for weeks. In elderly people, it could have similar benefits. In this population, sickness, hospital stays, and other periods of immobilization cause quick declines.
“A lot of times, especially in older people, those are not recoverable,” says Robert Hyldahl, a physiologist at Brigham Young University. “So you don’t get all of your function back after those periods of time.”
Hyldahl is no stranger to making research subjects suffer a bit in the name of science. For an earlier project, he tested the effects of warming a single muscle using a small, thin wire catheter as a probe, inserted directly into the muscle while warming it with a special heating drum. “It’s a bit like when you measure the temperature of meat,” he says with a laugh. “But it’s less invasive.”
Improved mitochondrial function is a key benefit of exercise—and it’s also a benefit of heat exposure. Mitochondria are cellular organelles that convert the food we eat into energy, or ATP, that we use during exercise, activity, and bodily processes during cellular respiration. Participants whose muscles were heated for two hours each day for six days showed improved energy production. The next step was obvious: adapting the experiment to heating a larger section of the body.
Hyldahl got creative. He put young adult participants in a single-leg immobilization cast, measuring a 30 percent dip in mitochondrial function after 10 days along with muscle atrophy and 30 to 35 percent decline in blood vessel ability to dilate in response to stimulus, a sign of healthy vascular function. But in participants who had regular heat therapy treatments, there was no loss of vascular or mitochondrial function and only half as much muscle atrophy.
Next, Hyldahl confined healthy volunteers to motorized wheelchairs and tracked them with pedometers, limiting each participant to 1,000 steps per day, again tracking declines in fitness, function, and energy production. Some volunteers also sat in a 180-degree sauna—a typical temperature—raising their body temperature to between 102 and 103 degrees, “a normal temperature during exercise,” for about 40 minutes, Hyldahl says. Again, people who received heat therapy fared better.
Despite these benefits, exercise remains a gold standard stressor that boosts health, reduces disease risk, and is linked to longevity. While heat has many benefits, Hyldahl says they don’t compare to the perks of exercise itself. But University of Kansas Medical Center researcher Paige Geiger sees mostly upside.
“I’m an exercise physiologist at heart. I’m never going to tell people to not exercise,” she says. “But there are so many populations that can’t do enough to get the benefits that we know are possible.”
This includes people who can’t exercise due to illness, disability, or lifestyle factors, or who simply don’t want to. The CDC estimates that 53 percent of U.S. adults meet current standards for aerobic exercise. If you factor in the percentage also doing the recommended strength training, that figure falls to 23 percent.
Heat “is limited relative to the gold standard of exercise, but it’s highly applicable. It’s an easily implemented modality,” Hyldahl says. “Heating someone in a hot tub or a sauna is not difficult and it’s generally pleasurable for them,” making it a more palatable therapy.
Despite their confusing name, heat shock proteins are produced in response to a range of cellular stressors, including heat exposure and exercise. Even cold exposure stimulates heat shock proteins, but heat researchers say most study participants prefer a prescription for a hot tub or sauna to an icy plunge.
Warming up to disease prevention
Geiger has spent a decade honing preclinical models in mice and rodents, hoping to define causal links between heat therapy and disease prevention. Now, researchers are applying her work to humans.
The potential for a boon to human health using heat “is greater than the potential for adaptation to cold,” says Christopher Minson, whose research seeks to understand heat benefits in order to help athletes apply them to training. As a thermoregulation expert at the University of Oregon, Minson teams up with elite athletes who want peak hot-weather performance or who are simply looking for the bleeding-edge benefits of heat therapy. But his work has also attracted funding from groups like the American Heart Association. That’s because Minson is investigating whether the same therapies that benefit athletes could stave off chronic conditions in people with health risk factors like obesity, pre-diabetes, pre-hypertension, and high blood pressure.
“Heat is sort of like the sledgehammer. That’s why we start with it.”
In a clinical trial of people with fibromyalgia along with other chronic illnesses, Geiger and her team started them on heat therapy. Many of them were unable to exercise or, in many cases, hold a job. By the end of the four-week study, participants were sleeping better and some began taking regular walks.
“We’ve really shown that when we induce heat shock proteins via heat or by exercise, we can protect some of these tissues—like the muscle and the liver—from the inflammation and oxidative stress that lead to diabetes, for example, and we’ve also shown that we can lower blood glucose,” she says.
“Heat is sort of like the sledgehammer. That’s why we start with it,” Geiger says, explaining why heat itself is her preferred method of inducing heat shock proteins, despite the popularity of cold-based interventions in recent years. “It’s a very robust response.”
With a lab set up at the University of Oregon’s famed Hayward Field in the college town of Eugene, Minson is testing his theories on the ground; his university is also a hub for track stars. Hayward played host to the 2021 U.S. Olympic trials for track and field. Officials paused the meet when temperatures hit 109 degrees in Eugene, an all-time high for the city.
Hot performance gains for athletes
For athletes, Minson offers an environmental chamber, similar to what Watts used, that can simulate any environmental conditions on Earth. Like a hard workout, a heat session takes its toll, and Minson recommends that athletes schedule these on easier workout days to avoid overstressing the body. He typically starts athletes with 20- to 30-minute sessions around 95 degrees, building up to 45-minute bouts at 104 degrees, sometimes with humidity added in.
“We’ve now demonstrated in multiple different ways—and other labs have as well—that you can use our innate ability to adapt heat activation or heat stress to improve performance, even if it’s not hot out,” Minson says. He’s found that repetitive bouts of heat acclimation increase red blood cell mass, boosting oxygen-carrying capacity and thereby improving performance.
Minson tries to get athletes to an internal temperature of about 101 degrees—far from the 104 degrees Watts experienced on her toughest day in an environmental chamber. For her, researchers hoped to simulate what overheating would be like, so she and her husband, who led her support crew, could see how she reacted.
Newbie runners are actually more likely to finish than runners who’ve finished the race in the past.
Those tough training sessions paid off. After the gun went off in California’s Badwater Basin at 8pm, Watts took off through the night, clocking her first few miles of the 135-mile race in 118 degree heat. Running through the desert, “I just wasn’t hot, it was so bizarre,” Watts says. “It gave me such a confidence boost.” The feeling wouldn’t last, but Watts fared better than some by completing the race. Despite its invitation-only status and strict qualifiers, only 78 of 94 runners finished the race this year, including Watts whose final time was 40 hours, 52 minutes, and 6 seconds. According to a five-year analysis by race director Chris Kostman, newbie runners are actually more likely to finish than runners who’ve finished the race in the past. No one’s quite sure why, though finishing the race for the first time may be a powerful motivator.
Watts hopes to be one of those veterans next year, which will require a new round of heat acclimation. Researchers told her to expect a 5 percent dropoff in acclimation every few days once she stopped her sauna and environmental chamber sessions. But there have been some lingering effects.
“When I was doing the heat training, I was always cold at home,” she says. She also found she had retained some heat adaptation during another ultramarathon two months after Badwater, staying cool even on a hot day.
Crossing the blood-brain barrier
Geiger is currently investigating extracellular vesicles, bubble-like structures that are formed within cells when tissue encloses on itself. These “bubbles” are released from cells, traveling from tissue to tissue through blood circulation, with roles in tissue-to-tissue or cell-to-cell communication.
Extracellular vesicles can also cross the blood-brain barrier in mice, offering a potential muscle-to-brain communication pathway—the same may or may not be true for humans. Heat shock proteins travel in these vesicles, where Geiger hypothesizes they could impact protein folding, a key pathway to Alzheimer’s, which involves clumps of junky amyloid proteins. In a Petri dish, Geiger found that brain cells cluttered with amyloids can be “cleaned up” by heat shock proteins from extracellular vesicles dumped into the same dish. But it’s still far too early to tell whether this process can happen in actual humans, or even whether vesicles enter the brain in effective concentrations to do so. The science of heat’s benefits is still, on the whole, in its infancy.
There are layers of associations researchers still hope to untangle, observations like research subjects whose blood glucose levels plummeted during morning heating sessions, or the impacts of diet and circadian rhythms on people being treated with heat therapy.
“It’s like thinking about what we knew about exercise about fifty years ago,” Geiger says. “That’s kind of where we are now. We just don’t know all the different applications and tools that we can put into place.”
