The American Medical Association (AMA) recently adopted a new policy meant to deemphasize Body Mass Index (BMI) as a measure of health.

While BMI might seem like a neutral, science-y metric, it has a troubling origin and baked-in biases that still affect healthcare today. BMI was designed based on data drawn almost exclusively from populations of white men. It was developed in 1832 by Lambert Adolphe Jacques Quetelet, a Belgian guy with expertise in astronomy, math, statistics, and sociology (you may notice a lack of medical expertise.) 

BMI is calculated by taking a person’s weight, than dividing it by the square of their height. If you’re thinking, “wait, that doesn’t really account for muscle mass,” or “doesn’t it matter where fat is stored?” you’re right!

This measurement might be useful at a population level, the AMA and other experts said, but at the individual level it fails to account for “differences across race/ethnic groups, sexes, genders, and age-span.”

Men and women are known to distribute body fat differently. Where fat is stored matters, as the American Heart Association says too much “belly fat,” or “visceral fat” stored around the waist is an indicator of “poor cardiovascular incomes,” regardless of whether the person has a “healthy” BMI or not. That means the measure might underestimate the prevalence of obesity or poor metabolic health. 

The AMA said there are “significant limitations” in using BMI in clinical settings—although it is widely used—and it can be the deciding factor in whether health insurance reimburses treatment for a patient.

A myopic focus on BMI can lead to worse patient outcomes: weight stigma can lead to poorer care, and “could ironically be driving future increases in weight,” as one doctor told The New York Times.

The inaccuracy of BMI, while possibly designating healthy people as unhealthy and vice versa, has sweeping implications. By relying on this measure, our global obesity estimates might be off by about a half billion people, give or take.

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Ashwin Rodrigues is a freelance writer. He often covers health, fitness, and things you might've heard on a podcast. He previously worked at Morning Brew as a business features reporter. Even more previous than that, he was a staff writer on the culture team at VICE. His byline can also... Read moreXRelated Stories for GQHealth