BMI Underestimates the Prevalence of Obesity

May 2, 2010— -- The scale of the obesity epidemic may be much worse than currently believed since the usual measure, body mass index (BMI), is a very insensitive measure of excess body fat, researchers said at a meeting earlier this week.

In a single-center study, 66 percent of patients classified as obese on the basis of dual-energy X-ray absorptiometry (DEXA) scanning had BMI values in the non-obese range, according to Dr. Eric Braverman of New York Presbyterian Hospital/Weill Cornell Medical Center.

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

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

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 percent 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 cholesterol problems, 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 United States 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 percent body fat or higher in males and 30 percent or higher in women marked obesity.

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

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

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

Also, 5 percent 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.'"

The researchers 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.