Science has confirmed the role that excess body fat plays in health problems like heart disease, diabetes and cancer. But conventional wisdom has always associated an increased risk for these diseases with overweight and obesity. Emerging research now brings a new classification to the forefront - "normal weight obesity" - a label that identifies people who are not overweight, but carry amounts of body fat associated with health risks.

New research, yet to be published but presented at the American College of Cardiology conference this past spring, suggests that more than half of normal weight U.S. adults carry excess body fat, typically around the waist.

After examining data collected in the National Health and Nutrition Examination Surveys (NHANES), more than half of the adults surveyed met criteria for high body fat (more than 20 percent body fat in men and more than 30 percent in women) despite a normal body mass index (BMI).

According to the researchers, these "normal weight obese" were more than twice as likely as people with a normal BMI and normal body fat percentage to have elevated blood cholesterol levels, high triglycerides, high blood sugar and elevated markers of inflammation.

Although these factors don't guarantee that someone will develop high blood pressure or diabetes, the excess fat could still be promoting inflammation throughout the body, a state that is linked with increased cardiovascular and cancer risk.

In addition, the "normal weight obese" were also more likely to exhibit a cluster of risk factors associated with cardiovascular disease and type 2 diabetes, namely abnormal blood sugar, high blood pressure, high triglycerides, low HDL cholesterol and excess waistline fat. Termed the Metabolic Syndrome, a predisposition to these risk factors is usually seen in overweight and obese individuals.

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But a new study published in August in the Archives of Internal Medicine shows that almost a quarter of adults classified at a healthy weight exhibit two or more of these risk factors. They are also more likely to have excess fat within the abdomen and liver, reduced amounts of lean tissue (including muscle) and impaired insulin function.

Of particular concern is the potential damage that excess body fat could achieve even before the classic symptoms of Metabolic Syndrome appear. Body fat - particularly excess fat around the waist - is active tissue. Fat cells produce estrogen and proteins (called cytokines) that promote inflammation and can increase blood levels of insulin throughout the body - developments that may increase cancer risk.

Some studies of women with normal BMI but high percent body fat show that even if blood pressure, blood sugar and triglycerides levels are still normal, cytokines involved in early stages of inflammation may be elevated.

But while some normal weight men and women display risk factors commonly seen in the overweight and obese, some overweight adults (almost 50 percent) show no risk factors at all. The advice is still not clear for these individuals. Perhaps they should simply focus on keeping fit and eating healthfully, or perhaps the risk of chronic low-grade inflammation from excess body fat warrants modest weight loss. More research is clearly needed.

So how does someone who currently falls in the normal weight range know when to be concerned? First consider your shape. Apple shaped adults (women with waist measurements greater than 31.5 inches and men whose midsections are over 37 inches) should take notice.

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In addition, people who have steadily been gaining weight throughout adulthood should be on guard as well. Losing even a little excess waistline fat makes a difference and exercise can help, too. Even without weight loss, 30 minutes or more of daily moderate physical activity is linked with small but significant decreases in inflammatory markers and insulin resistance.

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