By Dr Ananya Mandal, MD
Brown fat is something that occurs more in thin people than fat, younger people than older and young women than men. It is brown in color and a new study finds that one form of it, which is turned on in cold climates can burn calories from elsewhere in the body to keep the body warm. Another new study finds that a second form of brown fat can be created from ordinary white fat by exercise.
Dr. André Carpentier, an endocrinologist at the University of Sherbrooke in Quebec and lead author of one of the new papers added that more research is needed before people take to cold treatments and exercise alone to burn fat. He said it is possible, for example, that people would be hungrier and eat more to make up for the calories their brown fat burns. “We have proof that this tissue burns calories — yes, indeed it does,” Dr. Carpentier said. “But what happens over the long term is unknown.”
Earlier it was known that brown fat existed only in infants who could not shiver. The new studies showed that brown fat was visible in scans when subjects were kept in cold rooms, wearing light clothes. The scans detected the fat by showing that it absorbed glucose. Barbara Cannon, a researcher at Stockholm University, said just because the brown fat in adults takes up glucose does not necessarily mean it burns calories. “We did not know what the glucose actually did,” she said. “Glucose can be stored in our cells, but that does not mean that it can be combusted.”
A new paper in The Journal of Clinical Investigation by Dr. Carpentier and his colleagues answers that question. By doing a different type of scan, which shows the metabolism of fat, the group reports that brown fat can burn ordinary fat and that glucose is not a major source of fuel for these cells.
For the study volunteers (men) were kept chilled, but not to the point of shivering, which itself burns calories. Their metabolic rates increased by 80 percent, all from the actions of a few ounces of cells. The brown fat also kept its subjects warm. The more brown fat a man had, the colder he could get before he started to shiver.
When the investigators exposed the men to a radioactive chemical, they found the radioactivity disappeared from the brown fat in just minutes, but the radioactivity wasn't metabolized in the warm subjects. Based on the radioactivity findings, the researchers concluded all of the men showed cold-induced activation of brown fat metabolism.
Brown fat, Dr. Carpentier and Jan Nedergaard, Dr. Cannon’s husband, wrote in an accompanying editorial, “is on fire.” On average, Dr. Carpentier said, the brown fat burned about 250 calories over three hours.