The first national estimate of its kind bolsters the argument that you can be hefty but still healthy, or at least healthier than has been believed.
The results also show that stereotypes about body size can be misleading, and that even "less voluptuous" people can have risk factors commonly associated with obesity, said study author MaryFran Sowers, a University of Michigan obesity researcher.
"We're really talking about taking a look with a very different lens" at weight and health risks, Sowers said.
In the study, about 51 percent of overweight adults, or roughly 36 million people nationwide, had mostly normal levels of blood pressure, cholesterol, blood fats called triglycerides and blood sugar.
Almost one-third of obese adults, or nearly 20 million people, also were in this healthy range, meaning that none or only one of those measures was abnormal.
Yet about a fourth of adults in the recommended-weight range had unhealthy levels of at least two of these measures. That means some 16 million of them are at risk for heart problems.
It's no secret that thin people can develop heart-related problems and that fat people often do not. But that millions defy the stereotypes will come as a surprise to many people, Sowers said.
Even so, there's growing debate about the accuracy of the standard method of calculating whether someone is overweight. Health officials rely on the body mass index, a weight-height ratio that does not distinguish between fat and lean tissue. The limits of that method were highlighted a few years ago when it was reported that the system would put nearly half of NBA players in the overweight category.
A number of experts say waist size is a more accurate way of determining someone's health risks, and the study results support that argument.
Dr. Robert Eckel, a former American Heart Association president and professor of medicine at the University of Colorado, said the new research may help dismiss some of the generalizations that are sometimes made about weight and health.
Study co-author Judith Wylie-Rosett emphasized that the study shouldn't send the message "that we don't need to worry about weight." That's because half of overweight people do face elevated risks for heart disease, explained Wylie-Rosett, a nutrition researcher at Albert Einstein College of Medicine in New York.
But, for those without elevated risks, losing weight "might be important only from a cosmetic perspective," she said.
To arrive at the estimates, scientists analyzed nationally representative government surveys involving 5,440 people age 20 and over, and extrapolated to calculate nationwide figures.
The new study, appearing in Monday's Archives of Internal Medicine, used government surveys from 1999 to 2004 that included lab tests and height and weight measurements. Participants reported on habits including smoking and physical activity.
In all weight categories, risk factors for heart problems were generally more common in older people, smokers and inactive people. Among obese people who were 50 to 64, just 20 percent were considered healthy compared with half of younger obese people.
The results underscore how important exercise is for staying healthy, even for people of healthy weight, Wylie-Rosett said.
The authors noted that fat tissue releases hormones and other substances that affect things like blood vessels, cholesterol and blood sugar. The results suggest this interaction varies among overweight and obese people, the authors said.
The results also add to mounting evidence that thick waists are linked with heart risks.
Among people of healthy weight in the study, elevated blood pressure, cholesterol and other factors were more common for people with larger waists or potbellies. This often signals internal fat deposits surrounding abdominal organs, which previous research has shown can be especially risky.
Similarly, among overweight and obese adults, those in the "healthy" category tended to have smaller waists than those with at least two risk factors.
Dr. Lewis Landsberg, a Northwestern University obesity expert, noted that the research didn't look at heart disease, and that not everyone with high risk factors develops heart problems.
Still, he said, the study shows that waist measurements can help assess health.
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