Posted by: Lisa Pampuch | July 20, 2009

BMI is BS

Slate’s Jeremy Singer-Vine asks why doctors won’t stop using the discredited body-mass index or BMI as a measure of obesity. The answer seems to be that they’re lazy:

Our continuing reliance on BMI is especially grating given there’s a very reasonable alternative. It turns out that the circumference around a person’s waist provides a much more accurate reading of his or her abdominal fat and risk for disease than BMI. And wrapping a tape measure around your gut is no more expensive than hopping on a scale and standing in front of a ruler. That’s why the American Society for Nutrition, the American Diabetes Association, and other prominent medical groups have lately promoted waist circumference measurements as a supplement to, or replacement for, the body mass index.

Yet few doctors have made the switch. The waist measurements require slightly more time and training than it takes to record a BMI reading, and they don’t come with any official cutoffs that can be used to make easy assessments. The sensitivity of doctors to these slight inconveniences signals just how difficult it will be to unseat Quetelet’s equation. The body mass index is cheap and easy, and it has the incumbent advantage. In short, BMI is here to stay—despite, but also because of, its flaws.

What are those flaws? BMI cutoffs for labeling folks as underweight, overweight, and obese are utterly arbitrary. BMI doesn’t distinguish between muscle and fat. It doesn’t address placement of fat, which is a key indicator of health risks associated with fat. It doesn’t account for body type or ethnicity. It doesn’t consider age or gender. It skews epidemiological studies. And, its creator never intended for it to be used for individual diagnoses.

The next time someone tells you about their BMI, or a doctor wants to discuss yours, tell them to get a legitmate measuring stick, because BMI isn’t one.

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