That finding may come as quite a shock for Americans, who have had it drilled into them that being heavy will take years off their lives. But evidence to the contrary has been mounting, and a 2008 report on the U.S. population published in the journal Archives of Internal Medicine made the issue impossible to ignore: Half of Americans who are in the overweight BMI category and a third of those in the obese one (30 or greater BMI) are actually healthy—they have plenty of "good" cholesterol, normal blood pressure and glucose levels, and no other risk markers for heart disease.

"In other research I was doing, I was seeing a lot of people who were heavy but fine," says MaryFran Sowers, PhD, a professor of epidemiology at the University of Michigan who co-authored the study. "What was surprising about the study results was that the number of healthy overweight people was so prevalent."

Lopez-Jimenez agrees. "We've long assumed that all people who are overweight have a lot of fat, and that people with normal weight have less fat and more muscle," he says. "We now know that's not always true. There are fewer unhealthy overweight people out there, and many more at-risk normal-weight ones than we thought." In other words, BMI is an imperfect standard. A simple height-versus-weight number can't account for how a person is built, lean or stocky, big breasted or not.

So if BMI is flawed, what is the best predictor of who is healthy and who isn't? In Sowers's study, there was one reliable answer. People who qualified as heavy but still healthy tended to report being physically active. Indeed, her study isn't the first to note this. Findings published in JAMA in 2007 indicate that fitness level—regardless of weight—is the single strongest predictor of mortality risk. People with the lowest level of fitness were four times more likely to die than those with the highest. Longevity didn't require superhuman effort, either; people who could pass a minimal fitness test of walking for longer than five and a half minutes on a treadmill had half the risk of an early death as those who failed the challenge.

Taking all the new findings together, the lesson is that even at 170 pounds, Leah Dawson had it right: Eat a good diet and drop any unhealthy habits you might have. Then focus on exercise and forget about striving toward some standardized ideal of weight. "Fitness is the critical piece we're losing sight of," says Church. "People get so caught up with weight—they start to exercise, the scale doesn't move, and they get discouraged. They're not noticing that their pants fit differently, that they've actually lost dangerous visceral fat. When you become active, you're not just adding muscle, you're getting healthier." And that's what will help you live longer.

As a reminder, always consult your doctor for medical advice and treatment before starting any program.