When you're struggling to button your pants around your ever expanding waistline, it probably doesn't occur to you to wonder whether your body fat is brown or white. But perhaps you should. Researchers have long known that brown fat, so called because it is packed with dark-hued mitochondria (the engines that feed cells with energy), actively breaks down sugar into heat and consumes a lot more energy than white fat does. In other words, brown fat burns energy instead of storing it. However, researchers also known that while brown fat is abundant in rodents and newborns, who need it to keep warm right out of the womb, those brown-fat stores shrink and white fat emerges as people age. But now it seems that adults retain more brown fat than previously thought, in deposits in the front and back of the neck, according to a study by Swedish researchers, published in the New England Journal of Medicine in April. Two other studies published in the same journal found that lean people tend to have more of these deposits than obese folks and that brown-fat cells are more active in the cold. Could a fat-based fat fighter be far behind?