It's one of life's crueler jokes: Some people must suffer with a slow metabolism, while others have one so speedy they can gobble down Krispy Kremes without putting on a single pound. But what is metabolism exactly? It's the sum of the energy-producing systems of the body, explains endocrinologist Steven Petak, M.D., of the Texas Institute for Reproductive Medicine and Endocrinology in Houston. "Every cell in the body uses fuel—glucose or something that can be turned into glucose," he explains. Your thyroid hormones determine how quickly your body burns that fuel—and…