Women appear to have more 'good' brown fat
Adults who retain their 'good' baby fat may be buffered against obesity and type 2 diabetes, scientists believe.
Unlike the regular white fat, which stores energy, good brown fat, actively burns calories for heat, but has been thought only to exist in childhood.
Researchers at the Joslin Diabetes Center not only found adults still had brown fat, but that slim adults had more of it than fatter ones.
The New England Journal of Medicine findings may point to new treatments.
According to the researchers, it may one day be possible to stimulate brown fat growth to both control weight and improve glucose metabolism, thereby preventing obesity and type 2 diabetes.
In the study of nearly 2,000 patients, the scientists found signs suggestive of brown fat deposits on routine medical scans in 7.5% of the women and 3% of the men investigated.
To help corroborate this, the researchers identified 33 other patients whose pathology records had indicated the presence of brown fat in their necks - the same place where the PET/CT scans had identified the largest concentrations of brown fat.
When they tested the tissue of two of those patients they detected the presence of a special heat-generating protein called UCP-1 that is unique to brown fat.
Although the numbers with brown fat in the study were small, the scientists believe this represents an underestimate, since medical scans could easily miss smaller and less active brown fat deposits.
Found most abundantly in infancy
Function is to generate heat and keep the body warm - babies cannot shiver
Burns calories to make this heat
Indeed, they believe most adults have some - but the amount depends on certain factors, including body weight.
The study found that younger patients were more likely to have larger amounts of brown fat, and the 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.
This, say the researchers, suggests brown fat plays an important role in regulating body weight and that higher levels of brown fat may protect against obesity.
Researcher Professor Ronald Kahn said: "There has been a long debate as to whether brown fat exists in adult humans and whether it was important physiologically.
"This study demonstrates that it is both present and appears to be physiologically important in terms of body weight and glucose metabolism.
"We hope this opens up a new therapeutic area for obesity and type 2 diabetes by modifying the activity of brown fat."
Professor Kahn's team has already shown that a protein, BMP-7, known for its role in inducing bone growth, can also help promote the development of brown fat in rodents.
Professor John Wilding of Liverpool University and the Association for the Study of Obesity said: "This is interesting research but it will not create a miracle pill for obesity.
"Even if we can activate brown fat production, which is still a long way off, the effects are likely to be modest.
"But it may help in some circumstances. And part of the reason some people are slim may be down to having more brown fat tissue."