For more than 30 years, scientists have been intrigued by brown fat, a cell that acts like a furnace, consuming calories and generating heat. Rodents, unable to shiver effectively to keep warm, use brown fat instead. So do human infants, who do not shiver very well. But it was generally believed that humans lose brown fat after infancy, no longer needing it once the shivering response kicks in. That belief, three groups of researchers report, is wrong. Their papers, appearing Thursday in The New England Journal of Medicine, indicate that nearly every adult has little blobs of brown fat that...