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BMI Is a Bad Way to Measure Health, Study Finds

Photograph by Twenty20

Millions—MILLIONS—of Americans have been told by their doctors that they're overweight or obese, and that they should lose weight for a better life. But a new study out of UCLA estimates that these 54 million Americans who have been labeled overweight or obese are, by all other measures, totally healthy.

These findings, which were published in the International Journal of Obesity, could be the end of the pernicious, contrived, ill-used body mass index (BMI), which medical professionals and insurance companies have used for decades to encourage lifestyle changes and pay more for policies, respectively.

To calculate body mass index, doctors divide a person's weight in kilograms by the square of the person's height in meters. The Centers for Disease Control and Prevention lists a healthy BMI between 18.5–24.9; the "overweight" range is 25–29.9; and the "obese" BMI is 30 or above.

UCLA psychology researchers looked at the health data, including BMI, of 40,420 participants in the 2005–2012 National Health and Nutrition Examination Survey. After analyzing reports of participants' blood pressure, triglycerides, cholesterol, glucose, insulin resistance and C-reactive protein data, all of which are linked to heart disease and inflammation, they found that 47 percent of the "overweight" people and 29 percent of the "obese" people were, metabolically, very heathy. And around 30 percent of the people whose BMI put them in the "normal" category were metabolically unhealthy.

That's not good.

BMI has long been sold to the public as a more "precise" way of measuring health than just weight alone, though many have pushed back against it. BMI was developed (by Adolphe Quetelet in the mid 1800s to study populations, not individuals) and does not account for higher than average muscle mass. Famously, Brad Pitt's BMI when he was in the movie "Fight Club" put him in the obese range. BMI health ranges also assume that there is an exact decimal place boundary between ideal weight and overweight, or overweight and obese (or underweight and ideal weight).

BMI has also been increasingly used in pediatricians' offices to determine which kids needed intervention to prevent diseases like diabetes.

"The public is used to hearing 'obesity,' and they mistakenly see it as a death sentence," lead author A. Janet Tomiyama, a psychologist at UCLA, told the Los Angeles Times. "But obesity is just a number based on BMI, and we think BMI is just a really crude and terrible indicator of someone's health."

So how should doctors make determinations about a patient's health, if not by having them hop on the scale?

"[I]f you know someone's weight and you know someone's height, then out pops this magical number," Tomiyama said. "But getting blood pressure is pretty easy too. It takes maybe 20 seconds, if you have the machine."

What about life insurance brokers, who travel with scales and a calculator and not much else? Why are they so reliant on BMI and is it a fair way to set policy rates?

"Policymakers should consider the unintended consequences of relying solely on BMI," the authors wrote in the study, "and researchers should seek to improve diagnostic tools related to weight and cardiometabolic health."

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