Over the course of a year, a typical American consumes nearly a million calories and yet weight generally fluctuates very little. That's because the body has mechanisms for keeping track of calories and carefully balancing food intake and energy output. And that's what makes dieting so difficult. As we cut calories and lose weight, our metabolism slows, making it more difficult to take off the pounds. Obesity, then, is not the result of a complete lack of discipline but is largely a function of biology.
Studies of twins and adopted children show that obesity is heritable and that genes play an important role in determining body size. In the past decade, researchers have discovered some of these genes and are learning how they influence eating and body weight. Studying mice that are massively obese, Dr. Friedman and his colleagues identified the gene for leptin, a hormone produced by fat cells. Leptin—named after the Greek word for "thin"—feeds into the circuit of neurons in the brain that controls eating and energy expenditure. When we lose weight, leptin concentrations fall. This dip in leptin levels instructs the body to find more food. For this reason, most diets eventually fail.