More than 1 in 3 Americans who are classified as slightly overweight based on their body mass index (BMI) scores may actually be obese, a new study suggests.

Researchers say adding a simple test that measures blood levels of the hormone leptin to BMI could better identify obese people who are at risk for diabetes, heart disease, and other conditions, but an expert who spoke to WebMD is skeptical.

The study was conducted by Eric R. Braverman, MD, of Weill-Cornell Medical College in New York and Nirav R. Shah, MD, MPH, who is now the New York State Commissioner of Health.

When the two doctors reviewed X-ray scans that directly measured body fat, they concluded that 39% of people classified as overweight based on their BMI scores alone were actually obese.

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Is BMI an Inaccurate Obesity Measure?

Body mass index is an indirect measure of body fat based on a person’s height and weight. A score of 25 to 29.9 is considered overweight, while a BMI of 30 or above is considered obese.

Braverman and Shah say the test tends to under-diagnose obesity and the risk of obesity-related diseases.

The researchers analyzed dual-energy X-ray absorptiometry, or DXA, scans for more than 9,000 patients seen at a private practice in New York between 1998 and 2009.

The DXA scan measures body fat as well as muscle mass and bone density.

Based on BMI alone, 26% of the participants in the study were classified as obese, while 64% were considered obese based on DXA results.

The study appears online in the journal PLoS ONE.

“BMI works about half the time -- that’s why I call it the ‘baloney mass index,’” Braverman tells WebMD, adding that the test is particularly inaccurate in older women.

The researchers suggest that a more appropriate cut-off point for obesity might be a BMI of 24 for women and 28 for men.

While DXA testing would give a more accurate measure of body fat, Braverman concedes that it would be too costly to use on everyone to determine obesity.
BMI + Leptin: A Better Obesity Test?

Combining BMI with a blood test to measure leptin would give a more accurate picture of a patient’s body fat with very little added cost, he says.

Leptin circulates in the bloodstream and is strongly correlated with body fat.

Leptin blood levels range from 0 to around 200, and Braverman says optimal levels are under 5.

He says like cholesterol and blood pressure, leptin blood testing should be routine.

But weight loss surgeon Mitchell S. Roslin, MD, of Lenox Hill Hospital in New York City, says is it not clear that a leptin blood test will help clinicians better assess risk for so-called 'metabolic' conditions like diabetes and heart disease.

While he acknowledges that BMI alone can be an inaccurate measure of such risk, he says blood leptin levels may also be.

“Blood leptin level may be a more accurate measure of body fat, but when you factor in what you know about a patient -- such as if they are sedentary or active -- I’m not sure it tells you much,” he tells WebMD. “There is a relationship between body fat and metabolic disease, but it may not be a linear relationship.”