Hot Pockets of Brown Fat Burn Up Calories

Three studies just published in the New England Journal of Medicine have discovered that most adults have several grams of brown fat sequestered in little pockets on their necks and backs. It’s a tiny amount, but it’s big news because brown fat is not your everyday fat, the unwelcome white variety that stores calories and makes us hate mirrors. Brown fat is a busy little heat-producing fat that actually burns calories.  It’s brown because it contains special mitochondria, tiny factories within the fat cells that produce heat, lots of it, when activated by cold. 

Brown fat makes up about five percent of the body mass of babies, who are especially vulnerable to cold because they can’t shiver effectively, put on a sweater, or turn up the heat. Mice also have a lot of brown fat for the same reasons: large surface area and an inability to shiver or assemble a winter wardrobe. But until now, scientists hadn’t been able to detect brown fat in adult humans and therefore believed that adults lost all their brown fat as they matured.

PET scans, however, have changed that opinion. One of the studies just published, which examined 1,972 people, revealed brown fat in 7.5 percent of the women and 3 percent of the men. Dr. C. Ronald Kahn of the Joslin Diabetes Center in Boston, who led the study, believes that more brown fat would have shown up if the subjects had been chilled to activate their little heat factories. The fat was found by PET scans because brown fat burns glucose to produce its heat, and PET scans light up areas that are burning lots of glucose. 

The second study examined 24 healthy young men, first at a comfortable temperature and then after being chilled for two hours. They showed no brown fat while warm, but all except one man, who was obese, showed brown fat when cold.  The third study looked at five healthy adults, again while warm and while cold, and once more the brown fat was seen on PET scans when the subjects were chilly.  

If people had 50 grams of brown fat and activated it to the max by moving into an igloo, they could burn 400 or 500 calories a day.  That’s actually less brown fat than what the researchers found in some of their subjects, so speculation abounds about using brown fat to burn calories by just putting your feet in an ice bucket. But there are plenty of caveats: people who are already thin have more brown fat than those who are overweight, and it may well be that activating brown fat could just make you want to eat more.  Still, it’s nice to know that we might have a little hot pocket of friendly fat sitting somewhere on the back of our necks, ready to burn calories and warm us up to boot. 

Sources: The New York Times

Reuters Health

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