A protein can throw a switch on fat cells, converting them from calorie-storing white fat cells into calorie-burning brown fat cells, U.S. researchers said. Study leader Shingo Kajimura of the University of California, San Francisco, Diabetes Center and the UCSF School of Dentistry, said the discovery makes the protein PRDM16 a possible target for future obesity drugs. Compounds that promote the action of this protein might help people burn calories faster, Kajimura said. "Warm-blooded" mammals produce heat internally by a variety of means -- shivering, sweating and regulating the size of their blood vessels. Brown fat contributes by burning fatty acids, which heats the blood coursing nearby, and in turn warms the body. Scientists once thought new brown fat was only made in babies but research has shown the human body is capable of creating new brown fat cells throughout life. Doctors also have discovered the amount of brown fat in the body is inversely proportional to the likelihood of obesity -- the more brown fat people have, the less chance they are obese, Kajimura said. This essentially throws a genetic switch and converts the white fat cells to brown -- at least in mice, Kajimura said. However, the question remains whether the protein has the same impact on humans, Kajimura said. The findings were published in the journal Cell Metabolism.