There’s at least one potential benefit to extreme winter weather: Shivering appears to torch significant calories by converting energy-storing white fat to energy-burning brown fat.
In a recent study, National Institutes of Health investigators subjected volunteers to various environments (including some really cold ones) and found that about 15 minutes of shivering increased levels of the hormones irisin, produced by muscle tissue, and FGF21, produced by brown fat, by about as much as an hour of moderate exercise.
Over the next six days, lab tests found that those hormones converted white fat cells into brown fat cells. Brown fat, discovered in 2009, is present in our bodies in very small amounts, but it acts more like muscle—sucking up white fat throughout the body and burning it for fuel. (Previous research has shown that brown fat is activated when we get cold, and that people with more brown fat tend to be thinner than those with less.)
"Shivering is the result of high-frequency contraction of muscle mass," explains co-author Dr. Francesco Celi. "Heat gets released to maintain the internal core temperature as a side product of mechanical contractions, which require burning of energy."
Before you ditch all those warm winter running layers, though, take note: Shivering while breaking a sweat won't likely have an additive effect, says Celi, since "maximum exercise" in the study did not produce any additional irisin on top of what shivering already had.
And even though your chattering teeth may be a sign that you're burning calories, you've still got to put in a bit more effort to get the other benefits of exercise, like improved cardiovascular fitness and muscle tone.
Mainly, Ceil hopes the study findings one day help doctors treat conditions like obesity and diabetes, by better understanding how to activate brown fat in the body. In the meantime, it may just help people embrace the cold a bit more, he says. "Perhaps lowering the thermostat during the winter months could help both the budget and metabolism."
Bottom line: Crawling out from under the covers and spending some time outside may help you avoid some of that dreaded winter weight gain, but it's still no substitute for your regular workout.