Brown fat burns calories, primarily from other fat in the body when exposed to cold, and exercise can convert white fat to brown fat, according to new research. Researchers from the University of Sherbrooke, Quebec, are trying to discover how to activate brown fat without having to expose people to continuous cold or exercise. Previously, researchers thought people outgrew the heat-generating brown fat they were born with as they developed the ability to shiver. However, scans of adults in cold rooms detected fat that absorbed glucose, leading to the discovery of brown fat in adults. Most people usually have just a few ounces of brown fat, primarily found in the upper back, sides of the neck, between the collarbone and the shoulder, and along the spine. In studies, male subjects` metabolism increased by 80 percent when kept in a chilled room, activating their brown fat, which burned 250 calories in three hours. Subjects with more brown fat kept warmer at lower temperatures before starting to shiver. Some brown fat also is interspersed with white fat. In studies with mice, exercise causes muscles to release irisin, a hormone that converts white fat cells to brown fat cells. Irisin behaves the same way in humans as in mice. Brown fat can only be seen in scans when people are cold and the fat is active, and is visible in normal and underweight people, but is not seen in obese people. Researchers do not know if this is because extra body fat keeps obese people warmer, so brown fat does not turn on, or if they do not have brown fat. Mice that cannot make brown fat also have very low bone density. More research is needed to determine whether cold exposure to induce brown fat production would be a safe and effective weight loss method.