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.