Why I’m skeptical of BMI

In a comment below, Justin asked about my belief that being overweight does not necessarily mean being unhealthy. The problem I have is with using BMI to decide who is overweight. Consider that the “healthy BMI range” for adults 20 and older is supposed to be from 18.5 to 24.9. When I see a range like that, I expect that at some time in the past, prior to the onset of the obesity crisis, researchers measured the height and weight of a large, representative sample of adults, calculated their BMI, and found a quasi-normal distribution. I expect 18.5 and 24.9 to be percentiles of this distribution, perhaps the 15th and the 85th. And the 50th should be near the middle of that range, around 22.

But that can’t be right. I am 5’10” tall, which means a healthy weight for me should be between 129 and 174 pounds. I did weigh around 174 in college. I had a 30-inch waist and was as thin as a rail. At 129 pounds I would be positively skeletal.

Of course, the recommendations apply to both men and women, and 129 might be a reasonable weight for a woman of 5’10” . But if so, then there should be different BMI recommendations for men and women.

Some very young men might be also that thin, but if that’s the case then the recommendations should vary by age as well.

During the NHANES I study, the CDC actually did measure the height and weight of a large sample of Americans — 23,808 people in all. The measurments were taken from 1971 through 1975, before the onset of the obesity crisis, which is generally thought to have begun in the mid-1980s. The researchers also calculated a weighting factor for each subject based on age, sex and race, so that the data could be made representative of the US population.

I downloaded the data (available here), calculated the BMI of each subject over the age of 19 and applied the weighting factors. The result is shown in the histogram below, which should be representative of the US population in 1971-1975.

As expected, the distribution is quasi-normal, or at least “mound shaped”. But the mean is not 22 — it’s 25.4 (the median is 24.8). So here you have a reference population which nobody thought of as particularly overweight, and yet by today’s arbitrary standards, nearly half of them would be classified as overweight! Something just doesn’t add up.

BMI of US Population, 1971-1975

BMI of US Population, 1971-1975

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