A new study confirms that calorie-burning tissue called brown fat once believed to be present only in infants is actually relatively common in adults – at least in slender ones.

Study co-author Ronald Kahn, a cellular and molecular physiologist at the Joslin Diabetes Center in Boston, Mass., says the finding adds to growing evidence that "brown fat helps people stay slim."

Unlike white fat, which stores excess calories, brown fat burns calories to generate heat – and scientists have long known that newborn humans (as well as baby and adult rats, mice and other mammals) have it to keep them warm. But researchers believed that most if not all of it was shed after infancy, because its job was done.

Kahn and his colleagues report in the New England Journal of Medicine today that they studied PET (positron emission tomography) and CT (computed tomography) full-body scans of patients and discovered that 76 (7.5 percent) of the 1,013 women and 30 (3.1 percent) of the 959 men aged 18 years and older had reserves of brown fat in their necks.

Roughly 50 percent of the people with the so-called good fat were age 50 or younger, and 40 percent had a body mass index (BMI) below 23.5 (which is considered healthy), says study co-author Aaron Cypess, an endocrinologist at Joslin.

It's not clear why some adults have brown fat and others do not. But Cypess speculates that it may be more common in women because they need it more than men for keeping warm: men have more muscle mass, and muscles produce heat when a person shivers. He also suspects that losing brown fat – which may be a part of the natural aging process like decreasing bone density -- may make people gain weight faster, because they're burning fewer calories.

Francesco Celi, a clinical endocrinologist at the National Institute of Diabetes and Digestive and Kidney Diseases in Bethesda, Md., says this study probably underestimates the number of adults who store brown fat, because the scans might not detect smaller and inactive nests of brown fat in the body.  He adds that more research should be done to identify the exact function of brown fat in adults, noting that if it turns out to be a calorie burner researchers may one day be able to use it as a drug target in the fight against obesity.

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