Can you be normal weight and fat at the same time?

That's the implication of a provocative recent report from the Mayo Clinic, which suggests that fat in your body can get you and your heart into trouble even if you don't look fat and if the scale tells you you're healthy.

The Mayo researchers, led by cardiologist Francisco Lopez-Jimenez, have coined a term for the phenomenon: normal weight obesity. In a study that looked at data from 6,171 Americans with normal body size, as measured by body mass index, those with a high percentage of body fat were at significantly greater risk of future heart problems than those with low amounts of fat. Their bodies "behave like they are obese, but they are not," Dr. Lopez-Jimenez says.

People don't have to be overweight to have excess body fat. Instead, these people have a higher ratio of fat to muscle tissue than do people with low body fat. Indeed, even people of the same weight, or those with comparable body mass index, which factors together weight and height, can have different body-fat percentages.

Based on results of the nine-year study, as well as U.S. Census and obesity data, Dr. Lopez-Jimenez and his colleagues estimate that as many as 30 million Americans may fall into the normal-weight-obesity category, many of them unaware they may be at increased heart risk.

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