According to a report published in The New England Journal of Medicine, every adult has some amount of brown fat, cells that act like a furnace to consume calories and generate heat. The fat, which is reddish brown in color, is filled with mitochondria. These tiny energy factories of cells contain iron, thus giving brown fat its color – and its name. The amount in adult humans vary, say researchers: thin people, younger people and people with higher metabolic rates all exhibit higher levels of brown fat. Women have more active brown fat than men, and people taking beta blockers have less. Why do beta blockers affect the ability of brown fat to become activated? In addition to the cold, brown fat is also activated by hormones that are part of the “fight or flight” response called catecholamines – the reason beta blockers, which block these hormones, can suppress the activation of brown fat.
PET-CT scans were used to find the brown fat, which rapidly burns glucose to produce heat, lighting up in the scans. The scans showed that in adult humans, brown fat is located in the upper back, on the side of the neck, between the collarbone and shoulder and along the spine. Their findings contradict past beliefs that humans lose brown fat after infancy, no longer needing it once the “shivering response kicks in” to help them stay warm.
The best evidence demonstrating the effects of brown fat has been gleaned from earlier studies in mice. Leslie P. Kozak, a professor of molecular genetics at the Pennington Biomedical Research Center of Louisiana State University, conducted a study in which mice that were predisposed to obesity were placed in a 41-degree room for a week. At the same time, they were fed a high-fat diet with 2.5 times more calories than they consumed at room temperature. Despite their diet, the cool room activated their brown fat, and as a result, the mice lost 14 percent of their weight – or 47 percent of their body fat.
Jan Nedergaard of the University of Stockholm did the opposite of Dr. Kozak. He and his associate studied mice that were genetically altered so that their brown fat could not burn calories. Not surprising, the animals became fat. “Until very recently, we would have said that it is doubtful that differences in brown fat really could contribute to obesity,” says Dr. Nedergaard, who has since changed his mind, at least for now.
Scientists hope to be able to find safe ways to “turn on” humans’ ability to activate brown fat in order to allow them to lose weight by burning more calories. They express caution, however, saying that while mice lose weight if they activate brown fat, it is not clear that humans would shed pounds. Moreover, data on global patterns of obesity is not substantial enough to clearly demonstrate that living in a cold climate makes people thinner. However, according to the investigators, the studies should stimulate research on the development of techniques to activate brown fat. And notes Dr. Claude Bouchard of the Pennington Biomedical Research Center, if a drug that stimulates brown fat were developed, “It would be the first obesity drug to affect energy expenditure rather than appetite.”
News Release: Calorie-burning fat? Studies say you have it www.nytimes.com April 9, 2009
