I was planning a post for today on why the BMI is bullshit, but Keith Devlin of NPR beat me to it.

To summarize their arguments, the BMI is a 200-year-old mathematical hack, created by a man who wasn’t even versed in the anatomical sciences of the day, let alone what we know now. It is not a meaningful measure of fitness, obesity or anything else, and is only still in use because it sounds scientific (because there’s a formula), it’s easy to calculate and it makes it easier for insurance companies to charge fit people as if they are obese.

I agree with all of these arguments except the last one. Devlin is hinting that obesity rates in the U.S. are overestimated because of BMI’s misclassification of heavy-but-fit people. This is likely true. However, body weight is not a good indicator of overall health, in the sense that, while being obese likely indicates poor health, not being obese does not indicate good health. This deserves repeating.

Having a normal body weight does not mean you are physically fit.

There are plenty of skinny people who are unhealthy because they have a high body fat percentage, no muscle tone and terrible cardio. So when BMI is used as a general indicator of health, misclassifying fit/heavy people as obese will be mirrored by misclassifying thin/unfit people as healthy. I don’t have any numbers on the extent of these two types of errors, so it’s hard to say which is more prevalent.

Regardless, the basic point holds: BMI is bullshit. Ignore any “science” based on it. Complain to any journalist who uses it.