In my previous blog post on thermal dieting, I discussed how exposure to cold temperatures can potentially ramp up fat metabolism. In figuring out what makes this possible physiologically, scientists have pinpointed brown fat as the gatekeeper. Distinct from white fat, the stuff that gym memberships are made of, brown adipose tissue is chock-full of energy-chomping mitochondria (these iron-packed structures give the tissues its brown color, in fact) that kick into gear when the body's internal temperature drops in order to generate heat. That way, when the mercury plummets, brown fat chows down on calories and fat cells. Rather cannibalistic, eh?