29-07-2025
Why BMI Still Won't Die
July 29, 2025 — The body mass index was born in judgment.
Its creator, the 19th-century Belgian astronomer and statistician Adolphe Quetelet, believed that greatness arose from averageness. The closer an individual was to the average size and shape of their time and place, the closer they were to perfection.
Any sports fan instantly sees the flaw in this logic: How boring would basketball be if the average NBA player was 5-foot-9 instead of 6-foot-7?
But it gets worse: Quetelet asserted that the further someone deviated from the population average, the more flawed they were.
First, however, he had to figure out what 'average' was.
Starting with a database of measurements from Scottish soldiers, Quetelet developed a formula of weight (in kilograms) divided by height (in meters) squared.
More than a century later, in 1972, legendary nutrition scientist Ancel Keys coined a new name for Quetelet's formula: body mass index, or BMI.
What was conceived in judgment remains quite judge-y. BMI continues to serve as a demarcation between a 'normal' or 'healthy' body weight (a BMI between 18.5 and 24.9) and the deviance of being 'overweight' (a BMI of 25 to 29.9) or 'obese' (a BMI of 30 or more).
Today, there's nothing 'normal' about a sub-25 BMI. Not when the average American adult has a BMI of 30 — just a couple of sandwiches past 'overweight' —and the CDC estimates that 42% of U.S. adults have obesity.
That's why, over the past decade, a growing number of doctors and scientists have argued that BMI as a health metric is past its sell-by date.
But before we talk about what's wrong with BMI and what health professionals can use instead, we need to look at how it became so ubiquitous and what it tells us — and doesn't.
What BMI Can and Can't Tell Us
'The advantage of BMI,' said obesity specialist Yoni Freedhoff, MD, is that 'it's easily calculable.' Just run your height and weight through a BMI calculator.
Freedhoff, an associate professor of family medicine at the University of Ottawa and medical director of the Bariatric Medical Institute, also acknowledges that 'BMI has a basis in statistical risk.'
We've known for a long time that a person with obesity has a higher risk of developing heart disease, diabetes, and some cancers. During the COVID-19 pandemic, we learned that someone with a BMI over 30 was statistically more likely to develop a severe or even fatal illness.
But when we look at overall risk of dying early from any cause, the link to excess body weight doesn't line up with expectations.
A 2023 study found that, among U.S. adults, the likelihood of early death was 5%-7% lower among people with a BMI in the 'overweight' range, compared to those with a BMI between 22.5 and 24.9.
The results varied significantly for older vs. younger groups.
For those 65 and older, the chance of early death was about the same across BMIs from 22.5 to 34.9 — from the high end of 'healthy' to the low end of 'obese.'
But for participants younger than 65, the lowest death rates were more constrained: from 22.5 to 27.4.
'BMI alone does not capture metabolic risk well,' said study author Aayush Visaria, MD, an instructor of medicine and clinical researcher at Rutgers University. That's because it can't distinguish between fat mass and lean tissue (muscle, bone, water), much less account for how a person's fat is distributed.
That's important, Visaria said, because health professionals may overlook potential health risks in a patient who has a 'normal' BMI but poor body composition — the ratio of fat to muscle.
The combination isn't as rare as it sounds.
Research shows that many people with a 'healthy' BMI have excess body fat, defined as 25% or more in men and 35% or more in women.
So what are the alternatives to using BMI to assess a patient's health risks?
A New Paradigm for Diagnosing Obesity
'BMI by itself doesn't do anything for me,' said Fatima Cody Stanford, MD, MPH, an obesity medicine specialist at Massachusetts General Hospital and an associate professor of medicine and pediatrics at Harvard Medical School.
'I call it street-corner medicine. You're looking at the person like you're sitting on the street corner and you're like, 'That person has this issue.''
That's the message of a recent report that Stanford developed along with dozens of obesity experts from across the globe.
The report puts obesity on a continuum. Where a person lands depends on how much body fat they have and how it affects their health and abilities.
Toward the healthier end, you'd have someone whose BMI puts them in the overweight or obesity range but who has no weight-related health problems.
They also wouldn't have excess fat mass, which you can indirectly measure with a tape measure. If their waist circumference, measured at the belly button, is less than 35 inches (for a woman) or 40 inches (for a man), you can assume they have a healthy body composition.
A lot of athletes and other highly active people would fit into this category.
At the other end of the continuum is clinical obesity: a chronic illness caused by excess body fat.
Clinical obesity affects the person's health and/or quality of life at a functional level. They might have sleep apnea or joint pain; high blood pressure or heart problems; or high blood sugar or low HDL cholesterol. Or it might be some combination.
Whatever the symptoms are, clinical obesity has a significant effect on the patient's present and future health status.
Somewhere in between is preclinical obesity.
In this category, a person has objectively high body fat (whether measured directly with DEXA or indirectly via waist circumference) but doesn't yet have obesity-related complications.
Those complications are by no means exclusive to people with obesity.
In a recent study, Stanford and her co-authors found that 61% of participants with a BMI of 30 or higher had at least one obesity-related complication — typically muscle or joint pain, high cholesterol, and high blood pressure.
But so did 50% of participants with a 'normal' BMI.
'I don't know anything about [a patient] until I do a full assessment,' Stanford said. In fact, she won't see a new patient until she has access to a full metabolic workup, including fasting blood lipids and glucose, as well as their height and weight measurements. 'I don't even go over BMI with patients until it gets very severe, which is that 40-plus group.'
What No Measurement Can Tell Us
Someone with such a high BMI is unlikely to be surprised by hearing it.
'People who have excess weight know they have excess weight,' Freedhoff said. 'The doctors know. Everybody knows.'
That's why Freedhoff doesn't think it matters if we replace or combine BMI with any other metrics.
'None of those numbers tell you if the individual in front of you has health consequences of their excess adiposity,' he said.
Even more important, he added, is whether they themselves have any concerns about their weight.
'And if the answer to all those questions is no' — they have no medical conditions that require treatment, and they don't think their weight affects their quality of life — 'they're good to go. Just monitor, like we would with any other medical condition.'
Why BMI Won't Go Away
So if BMI doesn't offer uniquely valuable information, why is it still so ubiquitous?
Why is it still used to assess who is or isn't at risk for diabetes or heart disease? Why is BMI the basis for prescribing in-demand weight loss medicines or for approving a range of procedures from joint replacements to organ transplants?
'It all comes down to what's the easiest, best number to use?' Freedhoff said. 'I'm not saying it's BMI, but I'm not saying it isn't.'
If excess body fat is what medical providers should be monitoring, he added, 'BMI is pretty darned good' at detecting it.
That's supported by a new study in the Journal of the American Medical Association. It showed that an overwhelming majority of participants with a BMI above the obesity threshold do, in fact, have excess body fat, as measured by DEXA.
Still, Freedhoff said, no number has perfect prognostic value. That applies to any tool doctors use for any chronic condition.
The difference with BMI is that it comes with the onus of personal responsibility. Whereas a doctor would never suggest that a patient's cardiac arrhythmia is a choice, that implication is almost always part of the conversation when it comes to obesity.
'I marvel at how challenging it seems to be for society as a whole, including health care, to consider obesity to be just another chronic medical condition that the person did not choose, that does not always guarantee problems, that does respond to treatment, and that should be free from blame,' Freedhoff said.
