Increasingly, medical research is showing that it isn’t. Despite concerns about an obesity epidemic, there is growing evidence that our obsession about weight as a primary measure of health may be misguided.

Last week a report in The Archives of Internal Medicine compared weight and cardiovascular risk factors among a representative sample of more than 5,400 adults. The data suggest that half of overweight people and one-third of obese people are “metabolically healthy.” That means that despite their excess pounds, many overweight and obese adults have healthy levels of “good” cholesterol, blood pressure, blood glucose and other risks for heart disease.

At the same time, about one out of four slim people — those who fall into the “healthy” weight range — actually have at least two cardiovascular risk factors typically associated with obesity, the study showed.

To be sure, being overweight or obese is linked with numerous health problems, and even in the most recent research, obese people were more likely to have two or more cardiovascular risk factors than slim people. But researchers say it is the proportion of overweight and obese people who are metabolically healthy that is so surprising.

“We use ‘overweight’ almost indiscriminately sometimes,” said MaryFran Sowers, a co-author of the study and professor of epidemiology at the University of Michigan. “But there is lots of individual variation within that, and we need to be cognizant of that as we think about what our health messages should be.”

The data follow a report last fall from researchers at the Centers for Disease Control and Prevention and the National Cancer Institute showing that overweight people appear to have longer life expectancies than so-called normal weight adults.

But many people resist the notion that people who are overweight or obese can be healthy. Several prominent health researchers have criticized the findings from the C.D.C. researchers as misleading, noting that mortality statistics don’t reflect the poor quality of life and suffering obesity can cause. And on the Internet, various blog posters, including readers of the Times’s Well blog, have argued that the data are deceptive, masking the fact that far more overweight and obese people are at higher cardiovascular risk than thin people.

Part of the problem may be our skewed perception of what it means to be overweight. Typically, a person is judged to be of normal weight based on body mass index, or B.M.I., which measures weight relative to height. A normal B.M.I. ranges from 18.5 to 25. Once B.M.I. reaches 25, a person is viewed as overweight. Thirty or higher is considered obese.

“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?” Well - Weight as Main Measure of Health May Be Misguided - NYTimes.com