The American Medical Association (AMA) has now accepted that the use of BMI (Body Mass Index) as a metric on its own is an imperfect clinical measure, and has caused “historical harm.” In its Science and Public Health report for the year 2023, the AMA criticized the BMI classification system for being misleading about the effects of body fat on mortality rates.

The report states, “Numerous comorbidities, lifestyle issues, gender, ethnicities, medically significant familial-determined mortality effectors, duration of time one spends in certain BMI categories and the expected accumulation of fat with aging are likely to significantly affect interpretation of BMI data, particularly in regard to morbidity and mortality rates.”

The Body Mass Index is essentially a formula to measure ‘fatness’ and is calculated by dividing an adult’s weight in kilograms by their height in meters squared. The World Health Organization says that for adults, the healthy range for BMI is between 18.5 and 24.9. Overweight is defined as a BMI of 25 to 29.9, and obesity is defined as a BMI of 30 or higher. The cut-off points in adults are the same for men and women, regardless of their age or even biological differences. In the last few decades, medical practitioners have used the BMI as a referendum on individual health, yet its history proves that this index was never supposed to be used as a clinical measure, much less the optimal guide to all health concerns.

Obesity specialist Dr. Sylvia Gonsahn-Bollie said it’s like shopping for clothes and an overly keen sales associate insists you try on a shirt. “The fit is off, but the clerk insists that the shirt must fit because everyone who’s your height should be able to wear it,” Gonsahn-Bollie wrote for Medscape Medical News. Yes, it sounds ridiculous, she said. “But this is how we’ve come to use the BMI. Instead of thinking that people of the same height may be the same size, we declare that they must be the same size.”

The racial and gender based biases embedded in the BMI lead to large-scale health-based marginalization, and dissuade patients from seeking help due to the fear of being weight-shamed. The BMI was initially recognised as The Quetelet index of 1892, which itself was derived through a need to prove that the mathematical mean weight of a population was its ideal, and thus the metric to quantify l’homme moyen’s (the average man’s) weight. Quetelet obtained the formula based solely on the size and measurements of French and Scottish participants, a very limited demographic that already restricts the scope of its results. Within the next century, it gained prominence and came to be used as a measurement of optimal health, and a scientific justification for eugenics (a discipline that believed in the creation of perfect human beings via elimination so-called social ills through genetics and heredity) — and inversely participated in the systemic castration of everyone who is not a white, caucasian, able bodied man, such as disabled people, autistic people, immigrants, poor people, and people of color.

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While Quetelet’s Index had questionable grounds, it was always meant as a method of measuring populations, not individuals — having been designed for the purposes of statistics, not individual health. This remained to be the norm until the 1970s, when Ancel Keys, an obesity researcher and physiologist who studied diet, conducted a large scale study (yet only restricted to predominantly white European and American men) on fatness. The results of the study were superior to all previous attempts at quantifying height and weight tables, and thus the QI revamped itself as BMI, the metric that we know today.

This trajectory continued to seep into major healthcare definitions. In 1985, the National Institutes of Health revised their definition of “obesity” to be tied to individual patients’ BMIs, and with that— the wholly imperfect and ethically questionable BMI cemented itself in the U.S public policy.

Being a convenient measure of categorizing patients sans individualized focus on individual patient histories and variances, BMI has become a standard part of medical check-ups, despite scientific study continually reiterating that a measure built by and for white people (men) is even less accurate for people of color and women — thus leading to misdiagnosis and mistreatment. According to studies published by the Endocrine Society, the BMI overestimates fatness and health risks for Black people. A large 2003 study published in The Journal of the American Medical Association (JAMA) has shown that higher BMIs tend to be more optimal for Black people, and that Black women don’t necessarily show a significant mortality risk until they cross a BMI of 37. Meanwhile, WHO observes that the BMI underestimates health risks for Asian communities. Studies have also found significant sex-based differences in the relationship between body fat and the BMI, because the primary research conducted for these tests has a predominantly male demographic, and those assigned as female at birth prove to be at a greater health risk if diagnosed on a scale that never took them into account.

“So, when we’re talking about health in marginalized communities, we need to find out about the health issues on the ground,” says Sabrina Strings, Ph.D., an associate professor of sociology at UC Irvine, “We need to understand their context, their histories, and then we need to work with them to improve their health. There is no need for us to have a top-down approach [like BMI categories] that serves to stigmatize. Instead, we can think about ways that we can care for people without making them feel like they have to change who they fundamentally are.”

Even within the criteria the BMI is presumed to serve, it does not serve its intended purpose. A 2016 study of 40,000 Americans found that nearly half of those classed as overweight according to their BMI, 29 per cent classed as obese and even 16 per cent with severe obesity were cardio-metabolically healthy, meaning their risk of diabetes and heart disease was low based on markers like cholesterol, blood glucose and blood pressure, whereas more than 30 percent of the people classified under normal weight were metabolically unhealthy. Experts have also pointed out that BMI fails to take into account factors such as how much fat versus muscle a patient has, the distribution of fat in their body (typically, fat around the waist increases disease risk more than fat in other places), and their metabolic health.

The fact that the use of BMI in the medical profession is misleading and harmful is one that has been largely acknowledged and discussed, yet there have been very few attempts to revise this measure. Authorities still promote BMI due to its convenience, and so long as this continues, very little change can be expected. Nick Trefethen, Professor of Numerical Analysis at Oxford University’s Mathematical Institute, believes a better calculation than the present weight/height2 for BMI would be weight/height raised to 2.5.

The council report has compelled the AMA House of Delegates to adopt a policy recognizing issues with BMI, and plans to support additional research on applying extended BMI percentiles and its connection to other measures, risk factors and health outcomes and efforts to educate medical professionals on issues with BMI and other ways to diagnose obesity, reports The Hill.

In America predominantly, as well as several other nations, BMI has come to be accepted as a simple truth that determines your worth, even in medical terms. But this overreliance on the universality of BMI is actively setting us back – because BMI can never be an effective measure of fatness or thinness, much less of overall health and fitness.