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.