Why a 200-Year-Old Metric Is Still Causing Problems
BMI — Body Mass Index — was developed by mathematician Adolphe Quetelet in the 1830s. The goal at the time wasn't to measure an individual's health. It was to study average physical characteristics across a population. That historical detail is key to understanding why BMI is still generating debate in 2026.
A study published in early 2026 reignited the conversation by showing that BMI misclassifies a significant proportion of adults — particularly people with high muscle mass and older adults whose body composition shifts without major weight change. Physically fit men with low visceral fat can show up as overweight on BMI. Meanwhile, people with a normal BMI can carry a worrying body fat percentage.
The problem is structural: BMI doesn't distinguish muscle mass from fat mass. Two people with the exact same height and weight can have radically different body compositions — and therefore very different health risks.
What the Science Recommends Instead
Research on body composition has advanced significantly since the 1990s. Several indicators have shown better predictive value than BMI, depending on the context.
The waist-to-height ratio (WHtR) is one of the strongest candidates for assessing cardiovascular risk. The simple rule: your waist circumference should be less than half your height. A meta-analysis across hundreds of thousands of participants found WHtR predicts type 2 diabetes and cardiovascular disease better than BMI alone. And all you need is a tape measure.
For a more direct measure of body composition, bioelectrical impedance analysis — the technology behind smart scales — estimates body fat percentage. Its reliability varies with hydration levels and device quality, but it's meaningfully more informative than BMI for ongoing tracking.
DEXA scan remains the gold standard. It separates lean mass, fat mass, and bone density with precision the other methods can't match. But it's expensive, involves minimal radiation, and is reserved for clinical or research contexts.
Why BMI Isn't Going Away
Despite its well-documented limits, BMI will remain a standard epidemiological tool for one simple reason: it's free, universally calculable, and allows comparison of millions of people across decades. For population-level studies, it has no practical equivalent.
But for individual use — especially in athletic contexts — its limitations are too significant to rely on alone. A professional rugby player with a BMI of 28 is not overweight. A sedentary person with a BMI of 23 can have 32% body fat. The number means nothing without context.
What This Means for Coaches and Athletes
For coaches, the conclusion is clear: BMI is not a coaching tool. Use it at most as a very rough population-level flag, never as an individual diagnosis. Someone who gains 5 kg of muscle while losing 3 kg of fat can watch their BMI stagnate or even increase — while their health and performance improve dramatically.
The relevant markers for training progress are: body fat percentage measured consistently under the same conditions, waist circumference, performance metrics like strength and VO2max, and subjective wellbeing scores. Together, these tell a story BMI alone never can.
The good news: the trend is moving in the right direction. Wearables and smart scales are making body composition data increasingly accessible. BMI is transitioning from universal tool to historical relic — useful, but only if you know exactly what it doesn't measure.