TUESDAY, April 1 (HealthDay News) -- Despite appearances to the contrary, more than half of normal-weight Americans have a high percentage of body fat. And, like their overweight contemporaries, this makes them susceptible to heart disease, diabetes and other metabolic disorders, a new study says.

Men whose body fat is greater than 20 percent and women whose body fat is greater than 30 percent are suffering from "normal weight obesity," the study authors said, even though their weight may be normal for their size.

"The prevalence of people with a high amount of body fat despite a normal weight is relatively high," said lead researcher Dr. Francisco Lopez-Jimenez, a cardiologist with the Mayo Clinic in Rochester, Minn. "Many of these people have metabolic abnormalities."

These findings should alert doctors that body weight isn't the only way to protect against health problems caused by excess pounds, Lopez-Jimenez said. Even normal-weight people should be advised to exercise and eat a healthful diet to reduce their level of fat, especially belly fat, he added.

The findings were expected to be presented Tuesday at the American College of Cardiology's annual meeting, in Chicago.

For the study, Lopez-Jimenez and his colleagues collected data on 2,127 people who participated in the U.S. government's Third National Health and Nutrition Examination Survey. Specifically, the researchers looked at risk factors for heart disease, diabetes, and metabolic syndrome, a precursor of diabetes.

The researchers found that 61 percent of the participants had levels of body fat that indicated "normal weight obesity." In addition, Lopez-Jimenez's group found changes in blood chemistry that can affect heart and metabolic health, including high cholesterol; high levels of leptin, a hormone found in fat and other tissues that's involved in appetite regulation; and high rates of metabolic syndrome.

Lopez-Jimenez said the study shows that just because your weight may be normal for your size, it doesn't mean you aren't at risk for heart disease and diabetes.

"If you have a normal weight, don't feel that everything is just OK," Lopez-Jimenez said. "If you have an excess amount of fat, you might have metabolic abnormalities as well," he said.

One expert agrees that normal body weight is not synonymous with good health.

"Body weight is a very blunt instrument; it is not a reliable gauge of obesity, or health, at the individual level," said Dr. David L. Katz, director of the Prevention Research Center at Yale University School of Medicine. "For example, a muscular man may have a very high body weight, yet be perfectly fit and healthy. Many people whose body weight is in the normal range are anything but."

And some people are vulnerable to weight gain in all the wrong places, such as in and around the vital organs of the abdomen, notably the liver, Katz said.

"Even a small amount of extra fat where it matters most can wreak metabolic havoc, increasing risk for diabetes and heart disease, while leaving you with a body weight that looks perfectly innocent," Katz said. "Excess body fat in the belly is a menace, whatever your weight. This study should sensitize patients and providers alike to this concern."

More information

For more about metabolic syndrome, visit the U.S. National Library of Medicine.

SOURCES: Francisco Lopez-Jimenez, M.D., cardiologist, Mayo Clinic, Rochester, Minn.; David L. Katz, M.D., M.P.H., director, Prevention Research Center, Yale University School of Medicine, New Haven, Conn.; April 1, 2008, presentation, American College of Cardiology annual meeting, Chicago

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