BMI Is Flabby Measure of Obesity, Study Shows

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If you think you’re overweight, it’s possible you’re being too soft on yourself: You may actually be obese.

As many as 40 percent of people classified by BMI as overweight actually had body fat levels suggesting they're obese, according to a new study co-authored by Dr. Nirav Shah,  New York’s top medical doctor before he became the state’s Health Commissioner.

The report challenges the Body Mass Index, or BMI, which is the standard yardstick for measuring who’s too skinny and who’s too fat.

It was published this week in PLoS-One, a leading scientific journal. Shah declined to comment, because the research predates his current position.

“BMI is about 75-percent accurate,” said Dr. Eric Braverman, the other co-author of the study. “No one in science is happy with something that’s wrong a quarter of the time – it just doesn’t add great value.”

BMI is calculated by multiplying height with weight. For years, it’s been acknowledged as a crude instrument. People who are short and very muscular can weigh a lot and have a high BMI without being obese.  

Still, it’s considered a useful snapshot – a first pass that can help get people to go see a doctor or dietician and a way of assessing the nation’s waistline as a whole.

But people grouped by BMI in “Normal” and “Overweight” categories can also have high amounts of body fat – and in the latter category, especially, that can be dangerous.

“What we’re calling overweight – a lot of that’s obesity,” Braverman said.

The dictionary definition of obesity  -- “a condition characterized by the excessive accumulation and storage of fat in the body,” according to Merriam-Webster – isn’t much different from the clinical definition. The question for doctors and epidemiologists is how to measure it, and where to draw the line.

The numerical cut-off is based on the amount of fat associated with medical problems such as high cholesterol, stroke or heart disease.

“Is it a perfect cut point? There’s no such thing,” Braverman said, “because there are so many genetic and other medical factors that any given individual can survive with more fat and not be so sick, or another person could have less fat and be sick.”

The study examined 1,393 patients between 1998 and 2009, and compared their BMI with other tests, including a relatively cheap test for a hormone called leptin.

Braverman said his research added to earlier studies showing leptin levels more accurately predicted obesity-related problems, and that as many as 40 percent of people with BMI’s in the overweight category had leptin levels classifying them as obese.
“The leptin tests cost $1 or less, and the return on investment is huge,” Braverman said. “The return is earlier treatment of obesity, which lengthens life.”

Dr. Abel Romero-Corral, a cardiology fellow at Albert Einstein Medical Center in Philadelphia, has done similar research – and examined figures for a population 10 times larger and spread across the country.  He isn’t convinced using leptin to measure body fat is practical.

“Why not just measure the body fat directly, instead of using a hormone marker for it?” Romero-Corral said, suggesting a cheap and widely available technology called bio-impedance, which sends an electrical charge through the body and measures resistance. “They have scales now that you can get at Target and Wal-Mart that measure your body fat pretty well.”

The two physicians agree that getting a handle on body fat, and doing so early and often is the key.

“If you're not measuring, you don't know what the goal should be,” Romero-Corral said. “You need to know what you're aiming for.”