Quick — what proportion of American adults are obese? About one-third, as you probably already know. How many of those folks are otherwise metabolically healthy? Twenty-nine percent. And how many American adults who are at a healthy weight are metabolically unhealthy? Thirty percent.

The latter two numbers come from new research published in the International Journal of Obesity. The study's results illustrate the pitfalls of relying too much on body mass index (BMI) as a measure of overall health. To be sure, doctors use a number of measures to evaluate their patients' health, apart from BMI. (Though research shows health-care professionals tend to have a negative attitude toward their overweight patients, which suggests they're overly reliant on BMI in their own way.)

But there is one place in health care where BMI reigns unfairly, the study scientists point out: The Equal Employment Opportunity Commission is considering rules that would allow companies to penalize employees with higher health insurance premiums for falling outside of "healthy" ranges for certain measures, including BMI. The penalty for not meeting a BMI standard can be as much as 30 percent of the usual cost of the insurance. The new study suggests such policies are way off-target, penalizing large numbers of healthy folks and letting off unhealthy ones.

Relying on BMI alone would wrongly classify 75 million American adults' metabolic health, write the study scientists, a team of psychologists and a statistician from the University of California–Los Angeles and University of California–Santa Barbara.

For the study, the team looked at health measures from a nationally representative sample of more than 40,000 American adults. In addition to BMI, the researchers analyzed other important measures of risk for diabetes and heart disease, including blood pressure, triglyceride levels, cholesterol levels, and insulin resistance. By those measures, about half of people classified as overweight are healthy, as well as 29 percent of obese people and 16 percent of very obese people. Thirty percent of people with BMIs within the healthy range are metabolically unhealthy.

In addition to showing that BMI-based measures shouldn't get encoded into health-care law, the study adds some important science to a major cultural conversation happening now. As many activists and social scientists have pointed out, heavier people in the United States often face discrimination based on the stereotype that they are unhealthy and lack willpower. Even leaving aside the fact that discriminating against people for being unhealthy is a jerk move, research like this shows those stereotypes aren't true. There's a good chance a person who has a high BMI and looks heavy is, in fact, healthy, and just about as good of a chance that a thin, lower-BMI person is unhealthy.

You can't eyeball a person's habits, character, or health by looking at his waistline.

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