It may sound like an oxymoron, but normal weight obesity is not a contradiction. It is a real — and common — health issue.

We know from previous studies that people with a high body-mass index (BMI) who are physically fit may be at lower risk for cardiovascular and other apparently weight-related diseases than those with low BMI who are out of shape.

Now, a new study by the Mayo Clinic suggests that more than half of American adults classified as having "normal" weight have high body fat percentages, as well as heart and metabolic disturbances such as high cholesterol levels and abnormal levels of the appetite-regulating hormone leptin. This calls into question the conventional wisdom that maintaining a normal weight in itself protects us against heart disease and type 2 diabetes. High body fat is defined as greater than 20 percent for men and greater than 30 percent for women.

The researchers defined normal weight using BMI. They coined the term "normal weight obesity" for someone who is at risk for metabolism problems and heart disease, but who is classified as normal on weight charts. In other words, normal weight obesity means having a normal BMI but an abnormally high body fat percentage.

"Using the term 'normal weight obesity' is really a way of being more precise about the changing conceptualization of obesity, because the real definition of obesity is excess body fat," says Francisco Lopez-Jimenez, M.D., a cardiologist on the Mayo research team. "Our study demonstrates that even people with normal weight may have excessive body fat, and that these people are at risk for metabolic abnormalities that lead to diabetes and, eventually, to heart disease."

The Mayo team presented these results at the American College of Cardiology's Annual Scientific Session, held in Chicago March 31- April 1, 2008.