You still have some of the baby fat that researchers have long thought melted away after childhood.

And be glad. The baby fat, known to scientists as brown fat, burns calories and uses energy. It's called the "good fat," as compared to white fat, which just hangs around and stores energy until needed.

The finding could pave the way for new treatments both for obesity and type 2 diabetes, the study team concludes. The results are detailed in the April 9 issue of the New England Journal of Medicine.

Researchers learned in 2007 that this brown fat is good. It burns calories. The new study shows that brown fat not only exists in adult humans, but also for the first time, that the fat is metabolically active.

"The fact that there is active brown fat in adult humans means this is now a new and important target for the treatment of obesity and type 2 diabetes," said study team member Dr. C. Ronald Kahn of the Harvard Medical School.

Obesity is a major risk factor for type 2 diabetes. It's causes continue to be debated, though poor diet and lack of exercise are leading factors for most overweight Americans.

Among the findings:

  • Younger patients were more likely to have larger amounts of brown fat.
  • Brown fat was more active during colder weather, keeping with its role of burning energy to generate heat.
  • Brown fat was also more common in adults who were thin and had normal blood glucose levels.

"What is of particular interest is that individuals who were overweight or obese as measured by higher Body Mass Index (BMI) were less likely to have substantial amounts of brown fat," Kahn said. "Likewise, patients taking beta-blockers and patients who were older were also less likely to have active brown fat. For example, individuals both over age 64 and with high BMI scores were six times less likely to have substantial amounts of brown fat."

The findings, particularly those having to do with BMI, suggest a potential role for brown fat in regulating body weight metabolism, the researchers explained in a statement, suggesting that higher levels of brown fat may protect against age-related obesity.

The idea behind a new therapy would be to find a way to stimulate brown fat growth to both control weight and improve glucose metabolism.

The researchers analyzed a database of 1,972 patients who had undergone positron emission tomography/computed tomography (PET/CT) scans for a variety of reasons over a three-year period. They identified substantial brown fat deposits in 7.5 percent of the female patients and over 3 percent of males.

"These numbers clearly represent an underestimate, since PET/CT can only detect collections of brown fat cells of a certain size and activity, and could miss smaller and less active deposits," Kahn said.

In addition, the researchers identified 33 other patients whose pathology records had indicated the presence of brown fat in their necks in the same places where the PET/CT scans had identified the largest concentrations of brown fat. They tested the tissue of two of those patients and detected the presence of a special heat-generating protein called UCP-1 that is unique to brown fat.

The research was supported by the Clinical Investigator Training Program, Beth Israel Deaconess Medical Center - Harvard/MIT Health Sciences and Technology, in collaboration with Pfizer Inc. and Merck & Co.; as well as with grants from the National Institutes of Health and the Eli Lilly Foundation.