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.
They also found the other type of brown fat that is harder to study because it often is interspersed in the white fat and does not occur in large masses. Now, in a recent article, Bruce Spiegelman, professor of cell biology and medicine at the Dana-Farber Cancer Institute, and his colleagues report that, in mice at least, exercise can make it appear, by turning ordinary white fat brown. When mice exercise, their muscle cells release a newly discovered hormone that the researchers named irisin. Irisin, in turn, converts white fat cells into brown ones. Those brown fat cells burn extra calories.
Dr. Spiegelman suspects that humans, like mice, make brown fat from white fat when they exercise, because humans also have irisin in their blood. And human irisin is identical to mouse irisin. “What I would guess is that this is likely to be the explanation for some of the effects of exercise,” Dr. Spiegelman says. The calories burned during exercise exceed the number actually used to do the work of exercising. That may be an effect of some white fat cells turning brown.
“We still don't know if activating it (brown fat) is a good idea or not and whether it will work to treat obese people or people with type 2 diabetes…So it is still premature to use that as a therapeutic target for (obesity),” Dr. Carpentier said. He also doesn't recommend people deliberately chill themselves in a bid to lose some weight. “It is still too early to cool yourself in a suit and in the hope that you will lose weight because we don't know how the body adapts over the long run to this type of stimulation, whether these stimulations can increase appetite or change the metabolism of the body over time,” he said.
“However, it remains to be demonstrated whether chronic and frequent bouts of cold exposure may contribute to increase [brown fat metabolism] and/or activity and may be a viable adjunct therapeutic strategy to other lifestyle interventions to prevent or treat obesity and its metabolic complications,” the authors of the latest study concluded.
The study was funded by the Canadian Diabetes Association.