What if the body mass index is wrong?
A brief romp through the history of the BMI - and why more people than ever think it's time to retire it as a metric for health
I was 15-years-old the first time I remember having my BMI calculated. A GP was quizzing me on weight gain (“it’s the pill,” I told him, “it makes me hungry”).
He pointed to a chart that was pinned behind him: “you’re getting into the unhealthy ranges,” he said. “Remember: the pill isn’t putting food in your mouth.”
I don’t think I was too bothered by this conversion at the time. In fact, I thought it was kind of brilliant that a simple equation could tell the doctor so much about my health. Over the years, I clung to the range as a marker of optimal wellbeing. I rarely questioned the efficacy of BMI given that everything pointed to it being
Recently though, I’ve been wondering.
That equation now underpins entire healthcare systems; it is used as a gatekeeping tool for everything from fertility treatment to surgery; it is used to sell drugs and therapies, to shame, to pathologise.
And yet, the science supporting it is looking increasingly shaky.
Meet Adolphe Quetelet
The Body Mass Index (BMI), has its roots in the 19th-century work of Belgian polymath Adolphe Quetelet. Born in 1796 in Ghent, Quetelet was a mathematician, astronomer, statistician, and sociologist. His interdisciplinary pursuits led him to seek patterns in human characteristics with the aim of defining the “average man” through statistical methods.
Between 1830 and 1850, Quetelet developed what he called the Quetelet Index, a simple formula: weight in kilograms divided by height in meters squared (kg/m²). His objective was not to assess individual health but to understand population averages, particularly among white European men.
He argued that by identifying the average of various human traits, a standard or norm could be established. Quetelet believed that deviations from this norm represented abnormalities or pathologies. As he explained at the time: “if the average man were perfectly determined, we might consider him as the type of perfection; and everything differing from his proportions or condition would constitute deformity or disease.”
For over a century, the Quetelet Index remained a tool primarily for academic studies, without widespread clinical application, though that began to shift in the mid-20th century, largely because insurance companies began to recognise the correlation between body weight and mortality rates.
In the 1940s, Louis Israel Dublin, a New York-based statistician and the vice president at Metropolitan Life Insurance Company, developed height-weight tables based on data from policyholders, aiming to identify “ideal” weight ranges associated with lower mortality. These tables were among the first tools insurers used to assess individual health risks related to body weight.
By the 1950s, Metropolitan Life Insurance Company had officially adopted these tables, applying them to their underwriting processes. The company observed that higher body weights were linked to increased claims, prompting the use of weight as a factor in determining policy premiums and eligibility.
It was the American physiologist Ancel Keys, however, who settled on the Quetelet Index as the most effective way to assess obesity in epidemiological studies. This was in 1972 and it was during this period that he coined the term Body Mass Index (BMI), rebranding Quetelet’s formula for broader use.
Keys emphasised that while BMI was suitable for population studies, it was not appropriate for individual diagnoses. Despite this caution, BMI gained rapid traction in both research and clinical settings due to its simplicity and ease of use.
By the 1980s and 1990s, BMI had become the standard for assessing obesity, replacing earlier, more varied definitions of excess body weight.
If this romp through the history of BMI shows us anything, it’s how arbitrary a BMI actually is.
Quetelet’s initial ideas were shaped by 19th-century beliefs about morality, appearance, and superiority. The implication that deviations from average were “deformities” laid the groundwork for systems of thought that were racist, ableist, and deeply exclusionary.
It’s not that BMI is completely useless
BMI can show broad trends in population health - but when it’s applied to individuals, it quickly falls apart.
It doesn’t tell you anything about where your body stores fat. It can’t distinguish between fat and muscle. It doesn’t consider age, sex, ethnicity, or bone density. You could be a sprinter with a high BMI, and still have extremely low body fat. Equally, your BMI could be in the “normal” range but you might have poor metabolic health.
And while BMI is often used as a gatekeeping tool it doesn’t even predict risk equally across groups. Asian populations, for instance, tend to experience obesity-related illnesses at lower BMIs, while Black populations may be healthy at higher BMIs. A one-size-fits-all number doesn’t reflect the full picture.
The future is nuanced
Earlier this year, 58 researchers in the US proposed a new framework that looks at body composition, fat function, and organ health – not just a single ratio.
The European Association for the Study of Obesity also launched a model that distinguishes between pre-clinical obesity (where fat isn’t affecting your health) and clinical obesity (where it is). This is key: not everyone with a high BMI is sick. Not everyone with a “healthy” BMI is well.
Some doctors now use waist-to-height ratio, DEXA scans, blood work, and even fitness tests to get a more accurate picture. It takes longer but it’s better.
At the population level, BMI is still helpful. It can be used to track trends, enabling governments to make best guess analyses about where and how to send resources.
But for most of us - particularly if we’re interested in understanding our health - it’s probably time to move on.
For too long, BMI has been used as a definitive statement on our health. Health is complex. People are complex. And one number has never told the whole story.
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BMI is still shaping how our bodies are judged and treated. Let’s start asking better questions – about what health looks like, and who gets to define it.