Nobody likes a frozen butt. So when François Haman attempts to recruit subjects to his studies on the health benefits of uncomfortable temperatures, he gets a lot of, well … cold shoulders. And he doesn’t blame them. “You're not going to attract too many people,” says Haman, who studies thermal physiology at the University of Ottawa, Canada.
The human body is simply lousy at facing the cold. “I've done studies where people were exposed to 7 degrees Celsius [44.6 Fahrenheit], which is not even extreme. It's not that cold. Few people could sustain it for 24 hours,” he says. (Those subjects were even fully dressed: “Mitts, a hat, boots, and socks. And they still couldn't sustain it.”)
People strive to keep cozy or cool—not shivering, and not sweaty—by flattening temperature variations in indoor spaces. It’s easy to reach for the space heater or yell “Alexa, warm my ass up!” the moment you feel a touch of discomfort. But maybe you shouldn’t tinker so much with the thermostat. Some reasons for easing up on the heat are obvious: About 47 percent of American homes burn natural gas for heat, and 36 percent use electricity, which in the US is still mostly sourced from fossil fuels. And there may be other reasons to embrace the cold—health factors that physiologists like Haman have begun to uncover.
Before industrialization, says Haman, “these extremes were actually part of life.” Bodies dealt with cold in the winter and heat in the summer. “You kept on going back and forth, and back and forth. And this probably contributed to metabolic health,” he says.
Researchers know that your body reacts when it’s cold. New fat appears, muscles change, and your level of comfort rises with prolonged exposure to cold. But what all this means for modern human health—and whether we can harness the effects of cold to improve it—are still open questions. One vein of research is trying to understand how cold-induced changes in fat or muscle can help stave off metabolic disease, such as diabetes. Another suggests it’s easier than you might think to get comfortable in the cold—without blasting the heat.
To Haman, these are useful scientific questions because freezing is one of our bodies’ oldest existential threats. "Cold, to me, is [one of] the most fascinating stimuli because cold is probably the biggest challenge that humans can have,” he says. “Even though heat is challenging, as long as I have access to water, and to shade, I will survive fairly well. The cold is completely the opposite.”
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“If you're not able to work together,” he continues, “if you don't have the right equipment, if you don't have the right knowledge–you're not going to survive. It's as simple as that." Figuring out how our bodies change in response to such a formidable and ancient opponent offers clues to how they work, and how they might work better.
Haman begins every day with a cold bath or shower. It’s a rush because the cold triggers the body to release hormones called catecholamines, which are involved in the fight or flight response. “I do have that sense of Oh my God, I'm feeling so strong, and I'm awake,” he says. “This is kind of my coffee.”
But those hormones are stress hormones, and Haman does not sugar-coat the truth: “Humans are amazingly ill-adapted to the cold.” People are fur-less and have gangly extremities. Our arms extend to distant fingers and our legs to distant toes. We have to move blood over a long distance to warm them up. And when it gets too cold, the body readily sacrifices blood flow to each, in favor of preserving the core temperature.
At rest, humans make up to 100 watts of heat. (“In French, if somebody is not very bright, you say they're not 100 watts,” Haman notes.) But if you're losing too much heat to the environment through your skin, that energy balance falls apart. The body responds by ordering more heat production. Your first urge is behavioral: You try to find warmth, whether by a furnace, under a blanket, or with the help of a cup of cocoa. The second is physiological, and it begins when your skin temperature drops by just a couple degrees: You shiver. Your teeth probably chatter first, then the rest of you. “You're contracting. And you basically have no control over your body,” Haman says.
Other animals, like mice, rats, and squirrels, aren't so poorly designed. They have plenty of “brown fat,” or adipose tissue that burns calories to create heat. Biologists refer to this trick as “nonshivering thermogenesis.”
At first, scientists thought this was unique to rodents, but in 2009, The New England Journal of Medicine published three separate discoveries proving that adult people also have brown fat—and therefore capacity for nonshivering thermogenesis. Haman has since shown that braving the cold can teach your body to stockpile more of it. In 2013, he asked his subjects to wear “cold suits” circulating water at 10 degrees Celsius (about 50 Fahrenheit) two hours a day, five days a week, for four weeks. It was cold and uncomfortable, but this “low intensity, long duration” acclimation caused people to double their amount of brown fat, which appeared around the spinal column, adrenal glands, and pelvic muscles.
Once it appears, brown fat doesn’t just sit around: Its activity replaces shivering as the body’s go-to heat factory. “Everything is being compensated by nonshivering thermogenesis,” says Haman. For the participants in the study, wearing the cold suit also tripled how active that fat was, or how much it burned. Shivering decreased about 10 to 20 percent after acclimating, according to his study. In other words, he concluded that the subjects acclimated to the cold by producing more brown fat, which in turn made them more comfortable at lower temperatures, without needing to shiver.
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Then, in 2019, Haman aimed higher. Or perhaps lower. He recruited seven men to undergo seven days of intense cold acclimation. Each day, they sat in 58-degree-Fahrenheit water, submerged up to their clavicles, for up to one hour, until their core temperatures dropped to 95 degrees. They were then dried and slowly warmed back up. “It's basically an hour of, uh … not having fun,” Haman says. “But after seven days, you're basically a totally different person.” Participants could go an hour longer before shivering than they could before the trials. And they would shiver 36 percent less intensely, on average.
Other labs around the world have tried to figure out if brown fat matters in other ways. In rodent studies, activating brown fat with cold temperatures has been found to regulate fatty acid and glucose levels. That led some researchers to suspect that the tissue can help protect against dysfunctional glucose processing in diabetes and fatty acid processing in obesity. So far, some studies in adult people have linked brown fat’s presence to leanness and normal blood sugar. (In 2013, WIRED covered an independent researcher’s quest to harness brown adipose for weight loss.)
But it’s not as simple a proposition as braving a little cold, tacking on some brown fat, and then losing weight. The story is a bit more complicated.
After the brown fat discoveries in 2009, Joris Hoeks, a diabetes researcher at Maastricht University in the Netherlands, was curious about its role in controlling blood sugar. His team recruited people with type 2 diabetes for a cold acclimation study. An important hallmark of type 2 diabetes is insulin resistance, in which organs take up less sugar from the blood. Participants endured six hours of cold, right on the edge of shivering, for 10 days. Their sensitivity to insulin, a key hormone in controlling blood glucose, improved by 43 percent on average—a boost comparable to the effect of a 12-week workout program.
“We thought, ‘OK, that's a great result,’” Hoeks recalls. The cold seemed to have caused the change in insulin response. But there wasn’t a clear connection to brown fat activity. “It was stimulated by the cold, but not much,” he says.
So Hoeks’ team doubled down. In a study published in March 2021, they repeated the test but took precautions to avoid all shivering by raising the temperature and giving the subjects extra clothing if needed. In these conditions, mild cold acclimation caused no improvements in glucose regulation or fat metabolism.
Instead, the results from this pair of studies point to changes in muscle as more important for diabetes than brown fat. Muscle cells change in the cold. Proteins responsible for transporting glucose fuel into muscle cells appear to migrate toward the outside of the cell. Hoeks thinks that change may help the body process more glucose, either because of mild or unnoticeable shivering contractions, or some other muscle process altogether. “We don't know what it is,” he says.
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“Cold works, it really works. But it’s not going through brown fat” to make diabetics more sensitive to insulin, Hoeks says. Other studies have shown that muscle is in fact responsible for metabolizing about 50 times more glucose than brown fat because muscle is so much more prevalent in the body. And Haman agrees that muscle cells are likely very important in regulating blood sugar. “If I'm doing this, all day,” Haman says, flexing his bicep with a couple of quick curls, “I'm likely using way more glucose and fatty acids than what brown fat would be.”
So far, the evidence seems to support Haman and Hoeks’ hunches that cold acclimation is good for people—but there’s still much more to learn. For Haman, the next step is to try to factor in dietary restrictions. In the future, he’d like to figure out how cold exposure and calorie restriction affect weight loss. One group will restrict their diet, another will do that in the cold, and another will just be cold. The study will track how much weight they lose. But, of course, Haman says, recruiting volunteers will be a slog: “How easy do you think it's going to be to recruit the people that are just going to do cold exposure for nine weeks?”
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