Body mass index (BMI) may not measure obesity accurately

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The scale of the obesity epidemic may be much worse than currently believed, because the usual measure, body mass index (BMI), is a very insensitive measure of excess body fat, researchers said.

In a single-center study, 66% of patients classified as obese on the basis of DEXA scanning had BMI values in the nonobese range, according to Eric Braverman, MD, of New York Presbyterian Hospital/Weill Cornell Medical Center.

Among more than 1,000 patients, 56% were obese according to the DEXA results, versus 20% using the standard BMI-based definitions.

Braverman and colleagues presented the findings during a press conference here at the American Association of Clinical Endocrinologists meeting.

Scoffing at BMI as the “baloney mass index,” Braverman said it’s “very likely that obesity is a much bigger epidemic than the 300 million people acknowledged by the World Health Organization.”

Currently 23% of Americans are labeled obese by BMI.

He explained that BMI is just a mathematical equation based solely on height and weight that is too general for diagnosing anything, especially in such an exacting field of clinicians.

“If any endocrinologist would rely on math to calculate thyroid stimulating hormone, for instance, he would be laughed at,” Braverman said.

DEXA scans, on the other hand, provide a direct measurement of body fat percentage. It can spot fat exactly, in every part of the body.

It’s particularly effective, Braverman said, for that part of the population that is known as “thin-but-unfit.” Their condition is known as normal-weight obesity, in which the BMI is low but they have a high percentage of body fat, especially compared with more favorable tissue like muscle.

These patients are at higher risk of dyslipidemia, as well as hypertension among men and cardiovascular disease among women.

The researchers conducted a retrospective analysis of 1,234 patients seen at a private medical practice in the U.S. from 2003 and 2009. All had both BMI measurements and DEXA scans available.

The BMI threshold for obesity was 30; with DEXA, a score of 25% body fat or higher in males and 30% or higher in women marked obesity.

The researchers found that DEXA identified 56% of patients as obese while BMI identified 20% as such.

Some 37% of patients were misclassified with BMI, the researchers indicated.

Among those classified as obese by DEXA, only 34% were classified as obese by BMI.

Also, 5% of patients identified as obese by BMI actually weren’t obese according to DEXA scans.

“These individuals were muscular and large, so they look like they’re high weight but they really had high muscle mass,” Braverman said.

So the researchers concluded that BMI is a highly insensitive measure of obesity prone to under-diagnosis, while direct fat measurements are superior because they show distribution of body fat.

“This means that we may have more health problems, as individuals are delaying treatment because they don’t think they’re obese,” he said. “They think they’re thin and ‘just a little flabby.’”

They called for additional studies to confirm the results and to “determine the true nature of the obesity epidemic.” Further analyses are also needed to identify which patients may be at risk of mislabeling by BMI.

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by Kristina Fiore
Kristina Fiore is a MedPage Today staff writer.

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