Thin adults have more brown or "good" fat to help regulate body temperature compared with heftier folks, researchers have found — a discovery that one day may help people lose weight.

Scientists knew that mammals had brown fat and that babies used it to stay warm, but it was thought to disappear with age. 

'I don't want to use the word[s] "exercise-in-a-pill," but it's doing something [that's] getting rid of calories.'— Dr. Louis Aronne

Wednesday's New England Journal of Medicine, however, includes three studies from the U.S., Finland and the Netherlands showing some adults do have brown fat, and it appears to play a role in burning off excess calories.

Excess calories are stored in regular or white fat that concentrates around the waistline. 

In contrast, brown fat burns calories, generating heat for the body. It's found around internal organs and between the shoulder blades.

Researchers from Joslin Diabetes Center at Harvard Medical Center in Boston used a type of PET/CT imaging that indicates how many calories are burned to test nearly 2,000 people diagnosed with various health problems.

The two other groups of researchers scanned small number of patients at room temperature and then again a few hours later at 15 C.

Collectively, they found:

  • Lean people had far more brown fat activity than overweight and obese people, especially among people over age 64.
  • Brown fat burns more calories and generates more body heat in cooler climes.
  • Women were more likely to have deposits of brown fat than men, and the deposits were larger and more active in women.

"This study demonstrates that it is both present and appears to be physiologically important in terms of body weight and glucose metabolism," said Dr. Ronald Kahn, head of the Joslin section on obesity and hormone action at Harvard and senior author of the U.S. study.

"We hope this opens up a new therapeutic area for obesity and Type 2 diabetes by modifying the activity of brown fat."

Theoretically, the findings could be applied as a weight loss strategy.

"I don't want to use the word[s] 'exercise-in-a-pill,' but it's doing something [that's] getting rid of calories," said Dr. Louis Aronne, former president of the Obesity Society and a weight control expert at Weill Cornell Medical Center in New York.

The next step for the researchers will be to test if they can intervene in the development of fat cells, directing them to become brown fat cells, as has been done in mice. Even if successful, a treatment is five to 10 years away.

They're also trying a simpler strategy: turning down the thermostat to see if it makes people burn more energy to lose weight. Results of a small study are expected by the end of the year.

 "Taken together, these studies point to a potential 'natural' intervention to stimulate energy expenditure: turn down the heat and burn calories (and reduce the carbon footprint in the process)," Dr. Francesco Celi of the National Institute of Diabetes and Digestive and Kidney Disease in Bethesda, Maryland wrote in a commentary.

Celi cautioned that the vision may be oversimplified, since researchers still have to test whether activating brown fat makes people lose weight, or if the body just compensates by prompting people to eat more.

But at one Canadian weight loss clinic, the discovery is already changing views. 

"It's quite clear that we're not all made equal and that not everybody has the same susceptibility to gaining weight," said Dr. Arya Sharma, director of the obesity clinic at Edmonton's Royal Alexandra Hospital.

"And I think that alone already changes how people look at obesity."

With files from Associated Press