Fat burns calories? Yep, if it's cold enough

The newest obesity theory suggests we may one day be able to fight fat with fat.

Just like good and bad cholesterol, there apparently are good and bad types of body fat. The good fat is brownish, while the more predominant bad fat is white or yellow.

For more than 30 years, scientists have been intrigued by brown fat, a cell that acts like a furnace, consuming calories and generating heat. Rodents, unable to shiver to keep warm, use brown fat instead. So do human infants, who also are unable to shiver to stay warm. But it was generally believed that humans lose brown fat after infancy, no longer needing it once the shivering response kicks in.

Now three studies -- from Boston, Finland and the Netherlands -- show that some brown fat remains in adults, affecting metabolism and potentially offering a target for weight loss.

Their papers, appearing in today's New England Journal of Medicine, indicate that nearly every adult has little blobs of brown fat that can burn huge numbers of calories when activated by the cold, like sitting in a chilly room that is 61 to 66 degrees.

Thinner people appeared to have more brown fat than heavier people, younger people more than older people; people with lower glucose levels, presumably reflecting higher metabolic rates, had more than those whose metabolisms were more sluggish, and women had more than men.

The hope is that scientists may find ways to turn people's brown fat on, allowing them to lose weight by burning more calories. But researchers caution that while mice lose weight if they activate brown fat, it's not clear that people would. The data on global patterns of obesity don't reveal whether living in a cold climate makes people thinner.

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