U.S. obesity rates may be underestimated, study finds: Blame BMI test?

In a recent study, researchers found that women with mild obesity (a body mass index of 30) had a 35% greater risk of headaches than those with a lower BMI. Severe obesity (BMI of 40) upped the chances to 80%.More from Health.com: The 5 kinds of headaches istockphoto

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(CBS News) America may have a worse weight problem than anyone thought. Current estimates from the Centers for Disease Control and Prevention show roughly one-third of Americans are obese. But the authors behind a new study that questions the test commonly used to measure obesity think that rate might be underestimated.

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"Roughly 30 percent of Americans are obese," based on their body mass index (BMI), study author Dr. Eric Braverman, president of the nonprofit research group, the Path Foundation in New York City, told HealthDay. "But when you use other methods, closer to 60 percent are obese. We call BMI the 'baloney mass index.'"

A BMI is a fat measurement that takes a person's weight and divides it by the square of his or her height. According to the National Institutes of Health, a BMI of 25-29.9 is considered overweight and a BMI of 30 or greater is considered obese.

But the authors of the study, published in the April 2 issue of PLoS One, say that measurement leads to many false diagnoses.

For their study, the researchers reviewed more than 9,000 charts of patients who visited a medical private practice in New York City. Patients were an average of 51 years old, and 63 percent were women. Researchers wanted to compare how BMI tests measured against a different test to measure obesity, called a duel-energy x-ray absorptiometry (DXA). The DXA is a machine that uses x-rays to measure muscle, bone mass and body fat.

What did they find?

BMI tests found 26 percent of patients were obese, while DXA indicated 64 percent were obese. What's more, 39 percent of patients found to be not obese by BMI standards were classified as obese by the DXA test. BMI tests were twice as likely to misclassify women, the researchers found.

"A 55-year-old woman who looks great in a dress could have very little muscle and mostly body fat, and a whole lot of health risks because of that - but still have a normal BMI," Braverman told CNN. "People aren't being diagnosed [as obese], so they're not being told about their risk of disease or being given instruction on how to improve their health."

So why doesn't every doctor's office use a DXA machine? They're very expensive, so it's not a practical routine test. But Braverman told HealthDay that a simple blood test that measures levels of the appetite-regulating hormone leptin along with a BMI test can provide a more accurate obesity measurement. The researchers found high leptin levels correlated with body fat, especially for women, and those with the lowest leptin levels were skinniest. Leptin blood levels range from 0 to around 200, and Braverman told WebMD optimal levels are under 5.

The researchers also suggest changing the BMI cut-offs for obesity to 24 for women and 28 for men to maximize accuracy and get more doctors warning their patients of obesity-related diseases.

Obesity is associated with an increased risk of diabetes, hypertension, heart disease, stroke, and cancer, among other diseases.

WedMD has more on obesity.

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