The BMI myth? You might be fatter than you think

The Globe and Mail

(Stock photo | Thinkstock/Stock photo | Thinkstock)

Patients leaving the doctor’s office with a body mass index under 25 – the cutoff before they’re considered overweight – might be tempted to reward themselves with a doughnut. But not so fast.

As a tool for measuring obesity, BMI falls short, researchers have found.

In a study published in the journal PLoS, lead author Eric Braverman, a clinical assistant professor at Weill Cornell Medical College, estimated that 39 per cent of participants who were classified as overweight by their BMI were actually obese, Time reports.

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Dr. Braverman and colleagues reached the conclusion after comparing the BMI of 1,400 men and women to their percentage of body fat, as measured by X-ray imaging.

BMI is a simple ratio of a person’s height and weight. But “it’s the percent of body fat, not BMI, that makes you obese,” Dr. Braverman told HealthDay News.

Using BMI as a measure of obesity is even more inaccurate for women, Dr. Braverman said, since women are more likely to lose muscle and bone and replace them with fat, which is lighter.

Taking body fat composition into account, Dr. Braverman said, the percentage of Americans who are obese may be close to 60 per cent.

While BMI is the standard measuring tool, there’s evidence that waistlines may be a better measure of body fat composition and obesity risk.

Canadian researchers have found that using a tape measure may help single out which teens have unhealthy levels of body fat, the CBC reports.

Their study, published in the Archives of Pediatrics & Adolescent Medicine, involved 4,104 Grade 9 students in the Niagara Region.

Michael Khoury of the University of Toronto and the Hospital for Sick Children and his co-authors measured their heights, weights, waistlines, blood pressure and cholesterol levels.

The researchers found that children with normal waist measurements generally had healthy blood pressure and cholesterol levels, even if they were classified as overweight according to their BMI.

The study's authors noted that BMI doesn’t distinguish between muscle and fat or different body types, particularly among growing teens.

But unlike a slightly undesirable BMI, no one can dismiss an expanding waistline as a matter of being “big boned.”

Do you calculate your BMI as a measurement of your health?