The relative importance of exercise and weight, however, doesn't give you license to specialize in junk food. Study after study has found lower disease rates in people who eat healthfully, with an emphasis on a variety of fruits and vegetables, whole grains, lean protein, and small amounts of unsaturated fat. That diet can lead to improved health even when it doesn't lead to substantial weight loss.

British researchers, for example, recently told a group of overweight women not to diet. Instead, they emphasized good eating habits, enrolled them in exercise classes, and provided social support. Though the women lost only a small amount of weight, they were significantly fitter and reported feeling less stress and better about their bodies.

"We've been shouting from the rooftops about how bad it is to be even a little overweight, but that hasn't solved the problem," says Steven N. Blair, P.E.D., professor of exercise science at the University of South Carolina. "We've become obsessed with BMI when we should be focusing on healthful behaviors."