by Courtney Rubin
First let's address that 15 percent! it can add up to a bonus 77 calories on a 5-mile run. Pretty major. And here's how it works: When you feel cool, your body actives brown fat. This so-called good fat uses sugar and fat as fuel to generate body heat -- a seemingly simple process, but it requires a ridiculous amount of energy (aka caloric burn -- or 15 percent) to pull off, The Journal of Clinical Investigation found. But there's a Goldiocks clause, researchers say: To turn on your brown fat, you can't begin a workout so bundled up that you feel toasty or so freezing that you're shivering. Create a just-right temp and burn max calories with our tips.
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* COVER LESS: Hats can trap too much warmth. The result? You overheat and brown fat hibernates. Our trick: Insulate just your ears with a headband.
* LAYER WISELY: Take a three-piece approach to dressing. Wear a sweat-wicking base in a technical fabric and a body-insulating fleece, plus a weatherproof.
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* FIGHT A CHILL: Just inhaling arctic air can cause shivers, says John Castellani, Ph.D., exercise physiologist at the Army Research Institute of Environmental Medicine. Cover your mouth with a neck gatier, which acts like a humidifier.
* TRY A MIND TRICK: When clothes fail, think about a great kiss. Sweet memories can make you feel warmer, a study in Emotion suggests.