IT'S NO COINCIDENCE HELL IS WARM:

Kajimura's lab at UCSF focuses on metabolism and energy balance, and specifically on the function of the body's fat cells. He says cold exposure during workouts may be beneficial for a number of reasons. For one thing, shivering burns a lot of calories. "Shivering is a very energy-demanding and tiring process," he says. If your goal when exercising is to lose weight, working out in the cold may help a bit.

Some of Kajimura's research has also shown that spending time in cool or cold environments converts some of the body's fat cells from unhealthy "white" fat to heat-producing, metabolically active "beige" fat. Kajimura says this conversion seems to help the body acclimatize to cold environments and increases resting energy expenditure -- the amount of calories a person burns just sitting around. Research by Kajimura and others has shown that beige fat may limit blood sugar spikes and even combat metabolic disorders like diabetes. "For now, this is just a theory," he says. "We need more investigation."

Exercise also appears to promote the development of beige fat. Kajimura says one hypothesis is that beige fat helps the body filter byproducts called metabolites that are released during exercise. Evidence suggests certain metabolites can contribute to fatigue and post-exercise soreness. So, for a lot of reasons, "Combining cold stimulus with exercise is a great idea," he says. "I highly recommend exercising outside in the early morning or swimming in cold water."

Fat and fatigue benefits aside, cool-weather training may offer other perks. Maintaining proper internal temperature is one of the body's main priorities during any activity, says Mike Jett, an instructor and lab director in exercise physiology at the University of Louisville in Kentucky. Exercise produces heat, which the body must labor to counteract if workout conditions are hot and humid. While that's not a big deal in most circumstances, it can hamper athletic performance. "If heat accumulates, exercise intensity is reduced," Jett says. Meanwhile, exercising in cool conditions (or even in cold ones, assuming you're properly bundled) seems to be optimal. The body can devote all its resources to training and performance, rather than to temperature regulation, Jett says. However, "Working out in an intentionally hot room does not make sense from a performance standpoint," he adds.

There may even be some mental health benefits associated with cold-weather exercise. A small 2004 study from Finland found that winter swimming combatted tension and fatigue and improved overall well-being among 36 middle-aged adults. Cold-water exposure seems to activate the central nervous system and stimulate the circulation of hormones that may promote improved mood and energy, write the authors.

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