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Why is the BMI a Steaming Pile of Sh*t

TW: The O-word will be used a few times throughout this blog post due to medical studies and reference to medical classifications. Take care of yourself!

You've probably heard of the Body Mass Index (BMI) before, but in case you haven’t - it's a formula that uses your height and weight to determine whether you're classified as “underweight”, “normal weight”, “overweight”, or “obese”. The BMI is “quick and dirty” (as one doctor put it) and isn’t great at measuring body fat in people.


To start, the BMI was developed sometime between 1830 and 1850 by Adolphe Quetelet,

a Belgium mathematician, and statistician. Very importantly, NOT a physician. Annnnnnnd, Quetelet explicitly stated that the BMI was not intended to be used as a means of medical assessment. He was using it to catalog the sizes of white men in Europe in the search for “l'homme moyen”, or the average man, which was foundational for developing the very racist science of eugenics. (Maintenance Phase Podcast about the history of the BMI). So, not only is the BMI an incredibly old practice, it is racist.

So, how is it still considered gospel in 2023 and deeply interwoven into our medical system? Insurance companies. In the 20th century, the BMI was resurrected by US insurance companies to determine what to charge policyholders. The higher your BMI the higher your premium.

The BMI is also highly inaccurate; medical science has known this since it was developed. After all, Quetelet explicitly stated it should not be used for medical purposes. The main problem with BMI is that it doesn't take into account your body composition. Weight isn’t just a measure of fat (or adiposity), it also includes water retention, bone, and muscle. The BMI has no way to tell why someone is heavy, which is why bodybuilders and athletes, often have high BMIs and older people with less bone density have low BMIs. Water weight is also the cause of daily weight fluctuations.

In the 1970s, Ancel Keys, (a prominent health researcher), studied the BMI and said, “...proves to be, if not fully satisfactory, at least as good as any other relative weight index or indicator of relative obesity.” Basically, “meh.” It’s not accurate, but it’s just as inaccurate as our other measures. Another study in 2011, found that the BMI accurately “diagnosed obesity” less than 50% of the time. Imagine using a test to diagnose cancer that was inaccurate more than 50% of the time and people considering it a “vital measurement.” Medical providers also know that the BMI doesn’t work for people of color, but insist on using it.

Next, fatness or a “high BMI” are characteristics and not indicators of health. Many perfectly healthy individuals have high BMIs and many unhealthy individuals have “normal” or “underweight BMIs. The BMI is essentially a 50/50 guess. No disease is exclusive to people with high BMIs. It can be a risk factor, but you cannot predict someone’s health based on their BMI.

The bottom line is that the BMI is an inaccurate and flawed metric that never should have been elevated to a “vital sign.” All it does well is perpetuate weight stigma, leading to delayed diagnoses of conditions in fat people. Instead, we should all focus more on health behaviors that support our lives and our health goals, like eating enough food and a variety of food (which means addressing food access), regular movement, lots of sleep, stress management, and being in the community!


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