C. Ronald Kahn: Is Brown Fat the Good Fat That Can Erase Bad Fat?

Fat you want to pack on, not take off, because it burns up calories instead of storing them as unwanted blubber? C. Ronald Kahn, head of the Joslin Diabetes Center's section on obesity and hormone action, has tracked down this miraculous stuff. Now he's trying to figure out how to direct the body to produce more and make it work harder.

C. Ronald Kahn, Director, Joslin Diabetes Center, Boston

Kahn and his research team did not discover brown fat, as it is called because of its color. It was well known that infants are born with deposits between the shoulders that generate heat until the baby's body can regulate its thermostat on its own. Then the fat usually disappears. But Kahn suspected that the fat hangs around in many adults. Radiologists had complained for years that in some patients, stray globs of dark-toned fat got in the way during scans for head and neck cancers. "We wondered if maybe they were right," says Kahn.

A large-scale study published by Kahn's team in the spring in the New England Journal of Medicine confirmed that they were. Scans of nearly 2,000 people showed brown fat around the neck and behind the collarbone of about 1 in 13 women and 1 in 30 men. The fat was more likely to be switched from passive into calorie-burning mode in people who were younger or thinner than in those who were older or heavier.

In one of those research coincidences, two other groups had also found brown fat in adults. Their much smaller studies were published along with Kahn's, with an editorial calling the findings of the three studies "a powerful proof of concept" that brown fat might be harnessed as an anti-obesity weapon. It might be possible to figure out how to tell the body to make more or how to rev it up.

How either would be accomplished remains to be seen, but the goal is irresistible. "You don't have to have very much active brown fat to burn up a lot of calories," Kahn says. Less than 2 ounces of active brown fat can gobble up as much as 20 percent of calories taken in, he says. For a man who consumes a typical 2,500 calories a day, that's 500 calories—as much energy as expended by jogging about 4 miles.