“I can’t do this,” the woman in her 60s said as she lowered herself into the 40° Celsius water that filled a big silver tub at the end of a dimly lit room in the University of Ottawa’s Human and Environmental Physiology Research Unit. It was only after the temperature dropped a degree that she immersed herself, shoulder-deep, for the next two hours. Then she did it again — and again and again — every day for a week. “I’m going to be straight up: People hated doing this,” says Caroline Li-Maloney, a Ph.D. candidate at the university who studies how the human body adapts to extreme heat.
Li-Maloney is trying to figure out whether women in their 60s, 70s and 80s, a group particularly at risk of death during heat waves, could train themselves to cope better with hotter temperatures. If they could, it would be a convincing demonstration of heat acclimation in older individuals — and it could pave the way for protecting vulnerable groups in heat waves.
Extreme heat can kill. When a person’s core body temperature rises above 37° for prolonged periods, heat exhaustion often sets in; hotter temperatures can eventually lead to delirium, seizures, organ failure and death. But repeated exposure to high temperatures in young, healthy individuals has been shown to trigger a host of protective physical changes that help cool the body, including modifications to sweat responses and blood vessels. Now, researchers are trying to work out whether these changes hold for the rest of us — and whether we can train ourselves to better handle the heat that is becoming increasingly severe with climate change.
As heat wave–related deaths mount globally — more than 10,000 Europeans are estimated to have died during a heat wave in June alone — harnessing the body’s natural responses could be one way to survive in an increasingly hot world.
“With more frequent periods of extreme heat … can we actually acclimate older populations, especially those with chronic diseases?” asks heat physiologist Daniel Gagnon of the Montreal Heart Institute and the Université de Montréal. He’s hoping that the answer is “yes.”
How the body prepares for extreme heat
Studies, primarily in athletes, soldiers and outdoor workers, have long shown that repeated exposures to heat trigger physiological responses that help the body better cope with subsequent bouts of heat. Sweating, which uses moisture to wick away heat, starts earlier and is more intense in people who have recently been exposed to hot temperatures, compared with naïve individuals. Tiny vessels in the skin also dilate more quickly in heat-exposed individuals, allowing more blood to move from the body’s toasty interior to its surface, where heat can radiate outward. These changes initially force the heart to beat faster to keep blood pressure steady; over time, the body adapts by expanding blood volume, reducing the heart rate.
Studies in mice suggest that some of these responses are regulated by a thermostat-like brain structure called the hypothalamus. During successive heat exposures, nerve cells there become more sensitive to body temperature and are quicker to send out signals directing sweat glands to enlarge and vessels to expand, Gagnon explains.
Adaptation may also be happening at the cellular level. Integrative physiologist Jeremy McCormick, also of the University of Ottawa, and colleagues had a small group of healthy men in their 60s and 70s participate in a weeklong hot water immersion protocol similar to the one used by Li-Maloney’s team.
Analyzing the immune cells in participants’ blood at the end of the experiment showed a rise in concentrations of heat shock proteins, McCormick’s team reported in August in the Journal of Sport and Health Science. These molecules — found in most organisms, including plants and bacteria — stabilize and repair other cellular proteins damaged by heat and other physiological stressors. Alongside other molecular changes that the team observed, the increase “seemed to alleviate some of the cellular stress,” McCormick says.
Some like it hot
While researchers are still figuring out precisely how these physiological adaptations work, some scientists believe they are at least partly behind why people in climates that are hot year-round tend to suffer less from heat.
Residents of countries including Kuwait, India and Thailand, for example, show signs of being well-adapted to relatively high average daily temperatures.
Every population has a minimum mortality temperature, where the risk of death from natural causes is the lowest. Many scientists interpret this to be an optimal temperature to which a population is adapted. In cooler places such as the United Kingdom, this hovers in the high teens or low twenties. But in hot countries, “it sits far higher,” says critical care physician Abderrezak Bouchama of Saudi Arabia’s King Abdullah International Medical Research Center, who studies heat physiology.
To compare the heat tolerance of people from different places, Bouchama and colleagues calculated the minimum mortality temperature for two groups: the residents of Mecca, Saudi Arabia, and the hundreds of thousands of international visitors who traveled to the city for the annual Islamic pilgrimage known as the Hajj from 2006 to 2014. Looking at patterns of natural deaths for both populations during the five-day period revealed a minimum mortality temperature of 26° for Meccans and 23.5° for pilgrims, the scientists reported in 2023 in Environmental Health Perspectives. But even the residents of Mecca have physiological limits, the team found in another study: Their mortality numbers started skyrocketing above 38°.
In general, such differences at the population level could be due to many factors, Bouchama says, including access to air conditioning, outdoor conditions and possibly even genetic factors. But physiological acclimation probably also plays a role, he says.
In regions with distinct warm and cold seasons, there’s some evidence that young adults can acclimate over summer, exhibiting slightly higher sweat rates and lower skin temperatures at the end of the season versus the beginning.
But the reverse can also happen, molecular biologist Elaine Lee of the University of Connecticut in Storrs and colleagues have found. The team’s unpublished study of several dozen athletes suggests that it takes them longer to acclimate in the late winter compared with early winter, hinting that they lose their summer adaptations over the cold season.
Some scientists have posited that acclimation is why early summer heat waves are often deadlier than ones later in the summer. Because people undergoing heat acclimation protocols typically lose their adaptations within weeks, Gagnon suspects they would similarly lose any acclimation benefits in the cooler periods between heat waves.
“We know for sure that the body can acclimate, but we have to be exposed to heat on a regular basis,” Gagnon says. “It has to be a pretty considerable dose.”
Finding the right dose
The science of heat adaptation in young, healthy people is well-established. But what about older individuals or those with chronic health conditions? Scientists have long known, for example, that the ability to effectively sweat and vasodilate declines with age.
New research suggests that intentional heat acclimation can boost these responses just as effectively in older adults as in younger people. Gagnon and colleagues asked 15 adults in their 20s and 30s and 16 adults in their 60s and 70s to immerse themselves in 40° water for an hour and then spend the next hour alternating between shoulder-high and waist-high water levels to ensure their body temperature stayed at or above 38.5°.
After repeating this process for seven days, both groups started to sweat and vasodilate earlier upon heat exposure. They also experienced a 0.4° lower body temperature, on average, than they did at the beginning of the experiment. The scientists also observed a five-beats-per-minute lower heart rate, on average, in both groups by the end of the protocol, they reported in May in the Journal of Physiology.
The upshot of the study, Gagnon says, “is that even if we’re older, our body still has the ability to adapt to heat.”
That idea led some of the women in Li-Maloney’s study to take part, hoping the experience would brace them for summer heat waves. “They love the idea that a study like this might make them at least feel better when summer starts,” she says.
To find out, Li-Maloney’s team had the women endure an artificial heat wave before and after the experiment. Volunteers spent an hour sitting in a room set to 36° and 45 percent humidity. “It sucks, I’m not going to lie,” Li-Maloney says.
The team saw a significant difference in participants’ physiology during the second artificial heat wave. After the acclimation period, the women maintained a 0.2° lower core temperature and a several beats-per-minute lower heart rate than before the acclimation period, the researchers reported in April at the American Physiology Summit in Minneapolis. The team was particularly struck by how long the women avoided hitting the peak body temperatures they had reached during the first heat wave.
What’s still unclear is whether this translates to meaningful reductions in heat-related illness. Scientists will have to deeply consider the safety of such protocols, especially for people with heart disease, mobility issues or cognitive impairment. “If they were ever left to do it by themselves outside of a laboratory environment, are they potentially at risk?” asks physiologist Jessica Mee of the University of Worcester in England.
But with further study, hot water immersion could one day be a viable strategy to prepare vulnerable populations for the extreme heat of summer months — and a safer alternative to other proven acclimation methods like hot yoga or walking or running at hot temperatures, Li-Maloney says. “By the time the first heat wave hits, you should be more protected.” Some participants in the study are already convinced. One woman traveled to Thailand, exploring its sweltering capital in heat often in excess of 30°. “‘Everyone [else in] my tour group was suffering,’” Li-Maloney recalls the woman telling her, “‘and I felt great.’”
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