By Markham Heid
November 15, 2017
TIME Health
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A 50-degree fall day sends you running for a down jacket, while a 50-degree day in spring feels almost balmy. What’s up with that? Your body has an innate ability to “acclimatize” to colder temps—a skill scientists have recognized since a groundbreaking 1961 study.

For that month-long study, a U.S. Army researcher exposed 10 nude men to temperatures in the low 50s for eight hours a day. Not surprisingly, the poor study subjects did a lot of shivering—which is the body’s quick-fix way to generate heat. But by day 14, the men had mostly stopped shivering, and their bodies seemed to be making heat some other way.

Today, experts understand that special heat-producing fat cells—known as “brown fat”—deserve the credit for our ability to acclimatize. Whereas normal “white fat” cells store energy derived from the food we eat, brown fat cells burn energy to produce heat, says Barbara Cannon, a professor of biomedical sciences at Stockholm University in Sweden.

“Cold exposure increases the amount of brown fat that is present in the body,” says Cannon, who has published research on brown fat and its health benefits.

So while the body’s first response to cold is to shiver, it eventually makes and activates enough brown fat to take over those heat-producing responsibilities, she explains. In either case, your body is burning extra calories in response to cold. That can even translate to some body-weight benefits. As long as you’re not overeating to make up for the extra energy your cold-exposed body is using up, you can expect to lose some weight in response to cold, Cannon says. (How much depends on the person.)

There may be some additional metabolic benefits. Among people with higher levels of brown fat, “we see better insulin sensitivity, lower levels of circulating fatty acids and also lower levels of triglycerides,” says Sven Enerbäck, a professor of cell biology at Sweden’s University of Gothenburg whose research suggests that people’s stores of brown fat change (and mostly decline) as they age.

Enerbäck says the healthy metabolic shifts associated with brown fat are in some ways just the opposite of what happens in people with type 2 diabetes. There’s some excitement among scientists that cold exposure and brown-fat genesis could be used to counteract or prevent diabetes, obesity and other metabolic disorders.

MORE: TIME’s Guide to Weight Loss

But there are reasons to be wary—especially for those who suffer from heart disease, or are at risk for a heart event or stroke.

“When exposed to cold, the body tries to prevent heat loss by shrinking blood vessels, so you get increased blood pressure and heart rate,” says Shingo Kajimura, an associate professor of cell and tissue biology at the University of California, San Francisco.

These cold-induced blood-pressure swings could trigger a heart attack or stroke in people who are at risk. “This is why there are so many 911 calls at three A.M. in the middle of winter,” Kajimura says. “Older people get up to go to the bathroom, and when they step out on the cold floor, that stimulates blood vessel constriction and stroke.” (Research has shown that a roughly 5-degree drop in ambient temperature increases a person’s risk for stroke by 11%.)

Kajimura recently published a study on “beige fat”: basically white fat that has been partially converted to brown fat in response to cold exposure.

How much cold do you have to put up with in order to increase your body’s levels of beneficial brown and beige fat? That depends on a lot of factors. For one thing, your body’s existing deposits of insulating white fat will determine how much cold exposure you can take before you feel chilly. But Kajimura says existing studies suggest two hours a day spent in a 65-degree room—dressed so that you’re cool and shivering, but not freezing—should be enough to increase your stores of brown and beige fat.

Even if you can stand the cold and increase your body’s healthy fat stores, you’ll have to keep exposing yourself to cool temps in order to derive any fat-burning or metabolic benefits. “If you develop a lot of brown fat by being in the cold, this will not help you to stay slim when you are in a warm environment,” Cannon says.

It’s also not clear whether the weight-loss and metabolic changes associated with brown fat will offer you long-term health benefits. Many diet studies have shown that almost any intervention can help you drop weight in the short term. But the body likes to return to homeostasis. “This is why it’s so hard to maintain weight loss,” Kajimura says.

“I think chronic cold exposure will turn out to be good for metabolic health and for type-2 diabetes,” he adds. “But we need more investigation in human clinical trials.”

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