The Bizarre and Racist History of the BMI

Body Mass Index has been used in recent decades as a referendum on individual health. But it was never meant to be.

A woman checking her weight on a scale.
Photo: Zave Smith/Getty Images

II walk out of the doctor’s office, swiftly folding my after-visit summary packet and tucking it under my arm. If I don’t, the strangers in the waiting room will see its bold lettering in an oversized pull-out box on the first page. BMI: 47. Super morbidly obese.

My Body Mass Index (BMI) has come to feel like a scarlet letter. It has become not only a referendum on my size, but also on my health and subsequently my character. The logic is ruthlessly consistent: anyone my size must have committed a series of unforgivable acts. I must have let myself go. I must be pathological in my need to eat, my greedy desire to stay still. This is a pathology deserving only of disdain, never empathy. Clearly, I have been derelict in my duty to keep myself thin.

Like most of us, I’ve come to accept the BMI as a simple truth. It is, I have been taught, a direct measure of my size and health. But for something as universally relied upon as the BMI, its history is much less solid — and scientific — than you might think. For many of us, especially people of color, medicine’s over-reliance on the BMI may be actively harming our health.

The invention of the BMI

The Body Mass Index was invented nearly 200 years ago. Its creator, Adolphe Quetelet, was an academic whose studies included astronomy, mathematics, statistics, and sociology. Notably, Quetelet was not a physician, nor did he study medicine. He was best known for his sociological work aimed at identifying the characteristics of l’homme moyen — the average man — whom, to Quetelet, represented a social ideal.

Quetelet was Belgian, publishing works in Western Europe during the early 19th century — a boom time for racist science. He is credited with co-founding the school of positivist criminology, “which asserted the dangerousness of the criminal to be the only measure of the extent to which he was punishable.” That positivist school laid the groundwork for criminologists like Cesare Lombroso, who believed that people of color were a separate species. Homo Criminalis, Lombroso argued, were “savages” by birth, identified by physical characteristics that he claimed linked them to primates. For Lombroso, people of color were some kind of subspecies, congenitally driven to commit crimes. In addition to paving the way for Lombroso’s work, Quetelet is also credited with founding the field of anthropometry, including the racist pseudoscience of phrenology.

For many of us, especially people of color, medicine’s over-reliance on the BMI may be actively harming our health.

Quetelet believed that the mathematical mean of a population was its ideal, and his desire to prove it resulted in the invention of the BMI, a way of quantifying l’homme moyen’s weight. Initially called Quetelet’s Index, Quetelet derived the formula based solely on the size and measurements of French and Scottish participants. That is, the Index was devised exclusively by and for white Western Europeans. By the turn of the next century, Quetelet’s l’homme moyen would be used as a measurement of fitness to parent, and as a scientific justification for eugenics — the systemic sterilization of disabled people, autistic people, immigrants, poor people, and people of color.

While Quetelet’s work was used to justify scientific racism for decades to come, he was clear about one aspect of the BMI: it was never intended as a measure of individual body fat, build, or health. For its inventor, the BMI was a way of measuring populations, not individuals — and it was designed for the purposes of statistics, not individual health.

The BMI, lost and found

Weight wasn’t considered a primary indicator of health until the early 20th century, when U.S. life insurance companies began to compile tables of height and weight for the purposes of determining what to charge prospective policyholders.

Like Quetelet’s Index, however, those actuarial tables were deeply flawed, representing only those with the resources and legal ability to purchase life insurance. Weight and height were largely self-reported, and often inaccurately. And what constituted an insurable weight varied from one company to the next, as did their methods of determining weight. Some included “frame size” — small, medium, or large. Others did not. Many didn’t factor in age. Insurers were staffed by actuaries and sales agents, not medical doctors. But despite their lack of medical expertise and insurers’ inconsistent measures, physicians began to use insurers’ rating tables as a means of evaluating their patients’ weight and health. This trend reached its peak in the 1950s and 1960s.

By the 1970s, medical science was on the hunt for a more effective measure of weight. Enter researcher Ancel Keys. Keys and a cohort of fellow researchers conducted a study of 7,500 men from five different countries, aiming to find the most effective of medicine’s existing measures of body fat, that would be both easy and cost-effective enough for regular office visits

As in Quetelet’s work, the researchers’ subjects were drawn from predominantly white nations (the United States, Finland, Italy), along with Japan and South Africa, though their study notes that findings in South Africa “could not be suggested to be a representative sample of Bantu men in Cape Province let alone Bantu men in general.” Most of their findings, the authors note, apply to “all but the Bantu men.” That is, Keys’ findings weren’t representative of, or applicable to, the very South African men included in the study. Like Quetelet’s Index, whiteness took center stage in their research.

But unlike Quetelet, Keys and his colleagues set out to test which diagnostic tool was the best existing measure of body fat. In Keys’ landmark study, he and his fellow researchers hedged their findings significantly:

Again the body mass index […] proves to be, if not fully satisfactory, at least as good as any other relative weight index as an indicator of relative obesity. Still, if density is truly and closely (inversely) proportional to body fatness, not more than half of the total variance of body fatness is accounted for by the regression of fatness on the body mass index.

That is, the BMI was the strongest of three weak and imperfect measures (alongside water displacement and the use of skin calipers). Its claim to fame? Accurately diagnosing “obesity” about 50% of the time. As recently as 2011, that number held fast, as the Journal of Obstetrics and Gynecology found that the BMI detected less than 50% of “obesity” cases in Black, white, and Hispanic women.

In Keys’ same landmark study, he renamed Quetelet’s Index the “Body Mass Index.” And with that, a statistician’s largely forgotten index entered the world of individual health care — directly counter to its inventor’s wishes.

Moving the goalposts

By 1985, the National Institutes of Health had revised their definition of “obesity” to be tied to individual patients’ BMIs. And with that, this perennially imperfect measurement was enshrined in U.S. public policy.

In 1998, the National Institutes of Health once again changed their definitions of “overweight” and “obese,” substantially lowering the threshold to be medically considered fat. CNN wrote that “Millions of Americans became ‘fat’ Wednesday — even if they didn’t gain a pound” — as the federal government adopted a controversial method for determining who is considered overweight.”

That second change paved the way for a new public health panic: the “Obesity Epidemic.” By the turn of the millennium, the BMI’s simple arithmetic had become a de rigeur part of doctor visits. Charts depicting startling spikes in Americans’ overall fatness took us by storm, all the while failing to acknowledge the changes in definition that, in large part, contributed to those spikes. At best, this failure in reporting is misleading. At worst, it stokes resentment against bodies that have already borne the blame for so much, and fuels medical mistreatment of fat patients.

The increasingly complex science of fatness

Since then, the cultural conversations about fatness, health, and respect reflect that significant failure in reporting. Views haven’t progressed, although the science has started to. In 2015, researchers at Harvard University and the University of Sheffield released a study identifying six different types of obesity, each of which had their own etiology and called for different types of treatment. By the next year, researchers at Massachusetts General Hospital had observed 59 different types. With so many types of fatness — and more being identified every year — what could the brutally oversimplified arithmetic of the BMI meaningfully contribute to our understanding and treatment of fat patients? As clinical psychiatry professor Sylvia R. Karasu, M.D., put it, “Despite all the progress we have made in science since Quetelet’s 19th century index, we are still far from being able to measure our body’s fat conveniently and accurately in a physician’s office.”

But more than that, science has repeatedly demonstrated that a measure built by and for white people is even less accurate for people of color — and may even lead to misdiagnosis and mistreatment. According to studies published by the Endocrine Society, the BMI overestimates fatness and health risks for Black people. Meanwhile, according to the World Health Organization, the BMI underestimates health risks for Asian communities, which may contribute to underdiagnosis of certain conditions. And, despite the purported universality of the BMI, it papers over significant sex-based differences in the relationship between body fat and the BMI. That is, because so much of the research behind the BMI was conducted on those assigned male at birth, those assigned female may be at greater health risk if their diagnosis hinges on a measurement that was never designed for them.

Despite all its proven inaccuracies for society as a whole, the BMI soldiers on. Employers host “Biggest Loser” weight loss competitions and offer bonuses to workers who lower their BMI. Doctors frequently mandate a lower BMI from trans patients before providing lifesaving, gender-affirming health care.

The science has disproved many common myths about size, health, and weight loss for years. Yet instead of recognizing the evolving and increasingly complex science around fatness, people stick stubbornly to the truisms that allow them to freely marginalize fat people.

Like phrenology and positivist criminology before it, the Body Mass Index is a product of its social context. And, even according to its biggest champions, it’s not an effective measure of fatness, much less overall health.

Your Fat Friend writes about the social realities of living as a very fat person.

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