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Hot Workouts Vs. Cold Workouts: What's Scientifically Proven to Work Better

Sure, they'll make you mentally tough. But when it comes to burning calories and performing faster, which direction should you spin the thermostat?

Your training program is tough enough. Whether you're grinding through windsprints on a football field or a grueling 10-mile run through the backwoods, you're at your limit even on the best days. But factor in some bad weather—icy winds, say, or a swampy summer day—and suddenly your workout got that much tougher.

But there is good news: Research has shown that you can harness hot and cold conditions for your personal training gain. Which is better? In short, it depends what you're trying to accomplish.

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Harder, Faster, Colder

Ever since Rocky traded heavy-bag training for tenderizing beef in a meat freezer, cold-weather training has been a popular way to toughen up and train at the same time. And if you're planning on competing in the cold—training for the New York City marathon in November, maybe, or simply preparing for ice hockey season—it helps to train in the cold. But while you might burn slightly more fat during prolonged endurance exercise in cold conditions, it's not much more than a trivial amount.

Instead, "people need to think about total calories they’re burning,” says Michael Joyner, M.D., an endurance athlete and expert in human performance at the Mayo Clinic. “In the cold, your body can regulate its temperature a little better, meaning you can often exercise farther or longer, so you can burn more calories.” In other words, use the frigid conditions to your advantage—but don't expect your body to do the work for you.

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It’s also true that shivering burns a significant amount of calories, Joyner notes. But if you’re shivering during a workout, you’re either not working hard enough or you’re under-dressed.

Snow White, Brown Fat

Exercise aside, there are some clinical signs that simply contending with cold weather encourages the body to transform everyday white fat—specifically belly fat and thigh fat—into calorie-burning “beige fat.” People have more genetic markers for brown fat in the winter than they do during the warmer months, according to a 2014 University of Kentucky study published in the Journal of Clinical Endocrinology and Metabolism, and that could signal slightly more calorie burn in the winter as the body insulates itself.

“Browning fat tissue would be an excellent defense against obesity,” said Philip A. Kern, M.D., one of the study authors, in a release. “It would result in the body burning extra calories rather than converting them into additional fat tissue.”

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Heat Acclimation: The Battle for Blood

Working out in the cold might help you train longer and harder, but sustained endurance training in the heat has one unusual benefit: It can help you perform better in cool weather.

Yes, you read that correctly. In a 2010 study, researchers at the University of Oregon put 20 trained cyclists from the university’s cycling team and nearby cycling clubs through 10 days of moderate workouts (45 minutes of cycling, a 10-minute break, and another 45 minutes of cycling). The only difference: One group trained in brutal 104-degree heat, while the control group trained at a crisp 55 degrees.

After 10 days of the same training protocol, all the riders were grouped back together and tested at 55 degrees. The differences between the groups, lead study author Santiago Lorenzo, M.D., told the New York Times, were “dramatic”: The heat-trained riders not only rode a whopping 6% faster in cool temperatures, but also produced 5% more aerobic power (measured as V̇o2max), 5% more power output at lactate threshold, and about 9.1% more cardiac output.

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So what was behind the changes? The study authors pointed out that the heat-trained cyclists had more plasma in their blood. Cardio exercise in the heat, Joyner says, ultimately comes down using blood for thermoregulation. “When you’re exercising in the heat, you need to send blood to your muscles so you can keep exercising, and you need to send blood to your skin so you can cool,” Joyner says. “There may competition between the muscles and the skin for blood flow.”

When your body senses that it doesn’t have enough blood to go around, it can create more plasma—and that can improve your performance when you’re in less scorching conditions. In turn, Joyner says, “That increase in blood volume might stimulate the heart and increase the oxygen capacity of the blood.”

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While the findings are indeed striking, the researchers cautioned that there are limitations to the study. It doesn’t reveal much for people who are focused on strength training, for example, or how heat-trained athletes perform in extremely cold conditions. But it does help explain the cross-country runner’s old adage that championships are won in the summer.

And if you’re not exactly eager to haul your stationary bikes into the sauna, know that hot workouts can have a positive impact on your diet, too. When your body has an elevated core temperature—far more likely after a hot workout than a cold one—it suppresses your appetite, Joyner says. “If you’ve been exercising in the cold, you can come back and feel hungry right away. On the other hand, when you’re working out in the heat, you’re probably not very hungry.”

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Mental Toughness: Beat Heat, Conquer Cold

It’s difficult to prove objectively, but workouts in extreme conditions can give you an elusive mental edge—even if you’re only imagining it.

“Any time people can do things to harden themselves mentally, or give themselves confidence, that can definitely be a competitive edge,” Joyner says.

Remember, though, that training in Death Valley or the Arctic Circle doesn’t necessarily guarantee a performance boost—and at the very least, you need to be extra in-tune to your body’s warning signs.

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“If you’re not feeling well, it’s okay to back off,” Joyner says. “When it’s warm and humid, then you can sweat a lot, and it leads to something called ‘wasted sweating’: The sweat doesn’t evaporate, so you lose fluid without getting cooled.”

So no, you shouldn’t just wear a t-shirt for your light jog through a blizzard. Nor should you wear a plastic bag on the treadmill. But if you’re really hunting for that competitive edge, then don't let chilly or steamy outdoor temps have you running for the gym. Embrace them.

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