“In this world nothing can be said to be certain,” wrote Ben Franklin in a letter to his pal Jean-Baptiste Le Roy in 1789, “except death and taxes.” This tells us two things: firstly, that Franklin knew a quotable turn of phrase when he saw one, and secondly, that he lived before the invention of the BMI.
In the last 50 years, the body mass index (BMI) has gone from a relatively obscure demographic tool to an ever-present gauge of personal worth. BMI over a certain number? Say goodbye to that life-changing surgery – in fact, good luck getting treated equally in pretty much any medical setting. Prepare to be undervalued and underpaid at work – if you can even get hired in the first place – and if you’re still at school, expect to be graded more harshly and perceived by your teachers as “more burdensome” than your thinner peers. Frankly, it’s no wonder that a higher BMI correlates with lower self-esteem, higher rates of depression and anxiety, and a higher likelihood of facing abuse from family and romantic partners.
And the real kicker in all of this? The BMI is bunk.
The BMI was not meant for you
With the level of importance we tend to impart upon the metric, you might expect the BMI to be the result of years of research by health experts. It’s not. In fact, it was never meant to be used on individuals at all.
“The BMI was introduced in the early 19th century by a Belgian named Lambert Adolphe Jacques Quetelet,” explained Stanford University mathematician and well-known science communicator Keith Devlin to NPR all the way back in 2009.
“He was a mathematician, not a physician,” Devlin pointed out. “He produced the formula to give a quick and easy way to measure the degree of obesity of the general population to assist the government in allocating resources. In other words, it is a 200-year-old hack.”
To a mathematician, or statistician, the metric kind of makes sense: over a large enough sample size, after all, the mean does indeed tend towards truth. But it was never intended to be a measure of health, or even of obesity – it was originally just an observation aimed at classifying some theoretical “average man”.
And yes, it was almost entirely men that Quetelet based his figures on – specifically, European men, mostly from France and Scotland, and if you’re starting to see a problem here then whew boy, hold on to your hats, because we’re not done yet. The concept doesn’t just assume you’re a white guy born in 19th century Ghent; it also presupposes a mostly sedentary lifestyle, a working age and able body, an average income and education level – hell, even an average complexion (three guesses as to what that was).
This is despite the fact that “our bodies, by nature, have some distinct characteristics driven by our gender, including that females generally have less muscle mass and more fat mass than males,” pointed out Nick Fuller, Charles Perkins Centre Research Program Leader at the University of Sydney, in a 2022 article for The Conversation. “We also know muscle mass decreases and shifts around the body as we age.”
Similarly, “research has also confirmed significant differences in body weight, composition and disease risk based on ethnicity,” he continued. For example, “people of Asian ethnicity should have a lower BMI, and people of Polynesian ethnicity could be healthier at higher BMIs,” he wrote.
Even in more modern variations of the index, though, the base data has been overwhelmingly white and male. In other words: not only is the BMI a population measure rather than an individual one, but you’re probably not even a member of the population it’s measuring.
The BMI does not measure health
So how does a relatively obscure population-wide statistical data point become a ubiquitous proxy for individual health? Well, it becomes a little clearer when you learn the identities of two of the main supporters of the metric’s use in healthcare: insurance companies, and drug companies.
“By 1867 the first American life insurance company created height and weight tables with the purpose of charging fat customers more,” explained author and journalist Aubrey Gordon in a 2021 episode of the podcast Maintenance Phase. “The entire history of the thing is a dude who thought he was doing a population analysis and then a series of people who grabbed onto it largely for-profit motives – first insurance companies and then drug companies.”
Working out a person’s BMI is cheap, and easy, and reduces a whole host of questions down to one relationship between two measurements – no wonder it took off. Looking at it from this perspective, its flaws as a health metric become even more glaringly obvious.
For proof of that, look no further than your favorite sportspeople. “[The BMI] makes no allowance for the relative proportions of bone, muscle and fat in the body,” noted Devlin. “But bone is denser than muscle and twice as dense as fat, so a person with strong bones, good muscle tone and low fat will have a high BMI.”
It’s why using the BMI alone will give you such unintuitive results as Tom Brady (BMI: 27.4; overweight, obese using pre-1998 definitions), Jonah Lomu (BMI: 34.5 at his prime; obese), or Shaquille O’Neal (BMI: a stonking 40.3 at his prime; severely obese) – or why Chris Hemsworth (BMI: 29.0) was playing a near-obese character in Thor: Love and Thunder even (perhaps especially) after the eponymous character’s buffing-up montage.
“Because of how Quetelet came up with it, if a person is fat or obese, he or she will have a high BMI,” Devlin explained. But "it doesn't work the other way round. A high BMI does not mean an individual is even overweight, let alone obese.”
And it’s when the BMI is compared against other ways of measuring health that its weaknesses really show through. Take, for example, the 2016 study of more than 40,000 Americans which concluded that “nearly half of overweight individuals, 29 percent of obese individuals and even 16 percent of obesity type 2/3 individuals were metabolically healthy” while “over 30 percent of normal weight individuals were cardiometabolically unhealthy.”
Similarly, the index cannot account for body fat distribution – and that matters a lot more than you might think. “If you have fat stored around your stomach, your risk of chronic disease is much higher than people who have fat stored around their hips,” explained Fuller, “because this is an indicator of how much visceral fat you have – the type of fat deep inside the belly that increases your risk of stroke, type 2 diabetes and heart disease.”
“[The BMI] lacks accuracy and clarity and, in its current form, misses measuring the many important factors that influence your risk of disease,” he wrote. “It should never be the only measurement you use.”
The BMI is statistical nonsense
But why, you might ask, is the BMI so useless? Well – why wouldn’t it be?
The legitimacy of the BMI as a meaningful metric is on shaky ground from the get-go: after all, “there is no physiological reason to square a person's height,” noted Devlin. That’s something Quetelet decided to do for more-or-less aesthetic reasons – he wanted the data collected to follow a Gaussian curve, and that was the ratio that provided one. (“If you can't fix the data, rig the formula!”, says Devlin.)
And, having calculated your BMI, things get even more esoteric. According to the CDC, a BMI of 25 or above makes a person “overweight”; 30 or more and the classification is “obese”. But nobody really knows where those cut-offs come from: “these are arbitrary numbers,” Katherine Flegal, a consulting professor in obesity epidemiology at Stanford University, told The Washington Post.
As such, it’s pretty useless for measuring health – and, frankly, it’s weird that we still use it.
“Imagine a sales rep comes to your clinic office to pitch a new gadget to gauge your patients' health,” wrote S. Bryn Austin, a professor of social and behavioral sciences at Harvard T.H. Chan School of Public Health, and Tracy K. Richmond, an assistant professor of pediatrics at Harvard Medical School, in a 2022 MedPageToday article on the BMI’s shortcomings as a diagnostic tool.
“They tell you that it's not nearly as good as the measures you already have. It performs even worse with older people and athletes. It will drive large numbers of patients away, while it worsens symptoms in others. Then the sales rep adds sheepishly, at least it's cheap and easy to use,” they suggest.
“Would you buy it? Of course not, who in their right mind would?”
The content of this article is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of qualified health providers with questions you may have regarding medical conditions.
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