What BMI Doesn’t Tell You: What You Need to Know About This Outdated Health Tool
Medical professionals commonly use the Body Mass Index (BMI) as a standard measure of health.
However, it has significant limitations, particularly when it is used beyond its original purpose. The diagnostic value of BMI is often overstated, and its overreliance, especially within eating disorder assessments, can contribute to missed diagnoses and delayed care.
This blog explores what BMI does not tell us about health, distress, and care, and why continued reliance on this outdated tool has real consequences, particularly for Black people and for those struggling with food and eating.
The History of BMI
Back in the 1830s, Adolphe Quetelet, a statistician, mathematician, and astronomer, developed what we now recognise as the early version of BMI. His quest wasn’t a medical one. He was trying to quantify the characteristics of what he called the “normal man”.
Through his cross-sectional studies of human growth, Quetelet observed that, aside from the growth spurts after birth and during puberty, weight tends to increase in proportion to the square of height. He called this the Quetelet Index.
That stayed in the world of statistics until, more than a century later, it was reintroduced under a new name.
In 1972, physiologist Ancel Keys built on Quetelet’s work and officially coined the term “Body Mass Index”. His analysis drew on data from 7,426 men.
Of the 7,426 men included in the study, the vast majority were either European or American. Around 44% were of European descent, roughly 41% were from the United States, about 14% were of Asian descent, and only 1.5% were of South African descent.
Women were not included at all.
Crucially, Keys acknowledged BMI’s limitations. He wasn’t presenting it as the best measure of body fat levels. He argued it was simple and practical for large studies, even while admitting it accounted for less than half of the variation in body fat. In other words, it was never designed for individual diagnosis or clinical decision-making.
Despite that, BMI gradually migrated from population research into insurance models, public health policy, and eventually, everyday medical practice.
Why BMI Cannot Be Used as a Single Measure of Health
One of the most important points raised in the literature is that BMI cannot be used as a standalone measure of health.
Health is multi-dimensional. It is shaped by physical, psychological, social, emotional, and structural factors. No single number can capture this complexity.
Research makes clear that BMI does not measure:
Body fat percentage
Muscle mass
Fat distribution
But just as importantly, BMI also does not measure:
Eating behaviours or patterns
Nutritional adequacy
Psychological distress
Physiological stress
Relationship with food or body
I don’t believe this is a case of finding a new or supposedly “better” calculation to replace BMI. Health was never meant to be reduced to a single metric in the first place.
BMI, Black Bodies, and Misclassification
The literature shows that BMI does not perform consistently across racial groups. Which is not surprising given the limited representation of different ethnicities, races, and backgrounds used in Quetelet and Keys’ studies.
In non-Hispanic Black populations, higher BMI does not necessarily correspond to higher body fat levels. Studies have shown that BMI may overestimate body fat levels in Black individuals, particularly in girls and women, partly due to differences in muscle mass and body composition.
When BMI is treated as a single marker for health or risk, this misclassification can be very harmful.
It can lead to:
Misdiagnosis
Distorted assumptions about health
Reduced clinical investigations
Dismissal of lived experience
The harm does not come from BMI alone, but from the overreliance on it as a diagnostic tool.
BMI and Access to Care for Eating Disorders and Eating Struggles
BMI’s limitations become especially visible in the context of eating disorders and eating struggles.
In many systems, BMI is used as a threshold for:
Diagnosis
Referrals
Treatment eligibility
Level of care
But as the BMI does not reliably reflect adiposity, physiological risk, or distress, it creates a dangerous gap.
Many could be struggling with debilitating eating disorder behaviours such as restriction, chaos, fear, guilt, rigidity, loss of control, without meeting the criteria for a diagnosis.
This disproportionately affects:
People at higher weights
Black people
Those whose eating struggles do not present as dramatic weight loss
When BMI is used as a gatekeeper, access to care ends up being decided by a number, not by the person’s actual need.
And the person who is harmed most by that is the one already struggling.
If you’ve lived with an eating disorder, you’ll know there is often a voice that tells you you’re not sick enough. That you’re exaggerating. That other people have it worse. So to finally reach the point where you ask for help, only to have a single metric determine whether you “qualify”, can be devastating.
It can create:
Shame and self-doubt
Reinforced “I’m not sick enough” narratives
Pressure to perform, illness or recovery
Loss of trust in your own lived experience
For those already navigating racism, stigma, trauma, or marginalisation, this harm compounds.
Clinical guidance emphasises patient-centred care, about seeing the whole person rather than reducing them to a category. But that means very little if, in practice, a number still has the final say.
The BMI Has Too Much Say-So
Over-reliance on BMI creates problems at both ends of the spectrum.
When BMI blocks care
They aren’t a priority for treatment
There’s nothing to be concerned about
Their eating behaviours aren’t severe enough to warrant support
They just need to “keep an eye on it”
The same patterns of restriction, bingeing, or compensatory behaviours can exist across very different body sizes.
When BMI prematurely signals improvement
On the other side, weight restoration is often treated as synonymous with recovery.
But meeting a BMI target does not mean:
Fear has resolved
Food rules have gone
The food behaviours have stopped
They’re no longer experiencing triggers around food
As a result, people may be deemed “better” or discharged from care while still deeply struggling.
Moving Beyond BMI
BMI is used so routinely in medical settings that many of us rarely stop to question where it came from or what it was actually designed to do. But if we are going to keep using it in clinical practice, then we also have a responsibility to understand its origins and its limitations. The systems and guidelines we inherit are not neutral. Some were shaped by narrow samples, incomplete data, and social biases, and some may simply not offer the level of clinical accuracy we assume they do.
BMI only accounts for weight and height.
It does not account for differences related to sex, race, ethnicity, or socioeconomic context. And it certainly cannot capture the complexity of an individual person’s lived experience of health.
If we are serious about patient-centred care, we have to look beyond a single metric. Moving toward tools that consider height, sex, racial and ethnic differences would bring us closer to care that is more accurate, more inclusive, and more reflective of the people we are actually treating.
Sources
Dougherty, G. B., Golden, S. H., Gross, A. L., Colantuoni, E., & Dean, L. T. (2020). Measuring Structural Racism and Its Association With BMI. American journal of preventive medicine, 59(4), 530–537. https://doi.org/10.1016/j.amepre.2020.05.019
Heymsfield, S. B., Peterson, C. M., Thomas, D. M., Heo, M., & Schuna, J. M., Jr (2016). Why are there race/ethnic differences in adult body mass index-adiposity relationships? A quantitative critical review. Obesity reviews : an official journal of the International Association for the Study of Obesity, 17(3), 262–275. https://doi.org/10.1111/obr.12358
Humphreys S. (2010). The unethical use of BMI in contemporary general practice. The British journal of general practice : the journal of the Royal College of General Practitioners, 60(578), 696–697. https://doi.org/10.3399/bjgp10X515548
Pray, R., & Riskin, S. (2023). The History and Faults of the Body Mass Index and Where to Look Next: A Literature Review. Cureus, 15(11), e48230. https://doi.org/10.7759/cureus.48230
