Researchers at the Joslin Diabetes Center conducted tests on 2,000 adult patients to look for brown fat, which actively burns calories to generate heat and keep the body warm.
Experts previously believed that it only existed in childhood but the scientists found deposits in 7.5 per cent of the women and three per cent of the men in routine PET/CT scans.
They tested a further two patients whose pathology records had indicated the presence of brown fat and found that UCP-1 - a heat-generating protein - was present in all of them.
In addition, the team believes that the results, which were published in the New England Journal of Medicine, could be an underestimate as scans may have missed smaller brown fat particles.
One of the researchers, professor Ronald Khan, explained that the deposits were bigger in fatter patients, suggesting that brown fat growth could be used to control weight and improve glucose metabolism.
He said: "This study demonstrates that [brown fat] 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."
General Healthcare Group revealed recently that it saw a 30 per cent rise in demand for obesity surgery at its BMI Healthcare hospitals between October 2008 and February 2009.