“People get confused by the words and the mental image they get,” said Katherine Flegal, senior research scientist at the C.D.C.’s National Center for Health Statistics. “People may think, ‘How could it be that a person who is so huge wouldn’t have health problems?’ But people with B.M.I.’s of 25 are pretty unremarkable.”

Several studies from researchers at the Cooper Institute in Dallas have shown that fitness  determined by how a person performs on a treadmill  is a far better indicator of health than body mass index. In several studies, the researchers have shown that people who are fat but can still keep up on treadmill tests have much lower heart risk than people who are slim and unfit.

In December, a study in The Journal of the American Medical Association looked at death rates among 2,600 adults 60 and older over 12 years. Notably, death rates among the overweight, those with a B.M.I. of 25 to 30, were slightly lower than in normal weight adults. Death rates were highest among those with a B.M.I. of 35 or more.

But the most striking finding was that fitness level, regardless of body mass index, was the strongest predictor of mortality risk. Those with the lowest level of fitness, as measured on treadmill tests, were four times as likely to die during the 12-year study than those with the highest level of fitness. Even those who had just a minimal level of fitness had half the risk of dying compared with those who were least fit.

During the test, the treadmill moved at a brisk walking pace as the grade increased each minute. In the study, it didn’t take much to qualify as fit. For men, it meant staying on the treadmill at least 8 minutes; for women, 5.5 minutes. The people who fell below those levels, whether fat or thin, were at highest risk.

The results were adjusted to control for age, smoking and underlying heart problems and still showed that fitness, not weight, was most important in predicting mortality risk.

Stephen Blair, a co-author of the study and a professor at the Arnold School of Public Health at the University of South Carolina, said the lesson he took from the study was that instead of focusing only on weight loss, doctors should be talking to all patients about the value of physical activity, regardless of body size.

“Why is it such a stretch of the imagination,” he said, “to consider that someone overweight or obese might actually be healthy and fit?”