Outdated Numbers, Modern Consequences

There are few numbers in medicine with more authority than Body Mass Index (BMI).

BMI is found by dividing your weight in kilograms by your height in meters squared. This number is then used to determine whether you’re underweight (below 18.5), a healthy weight (between 18.5 and 24.9), overweight (25–29.9) or obese (30+).

There’s no differentiation between muscle, bone and fat, nor does BMI distinguish where fat is stored. Yet this number determines who qualifies for knee surgery, if you can access GLP-1 medication on the NHS, whether you’re eligible for fertility treatment or bariatric procedures. Life-altering treatment pathways hinge on weight divided by height.

Globally, around one billion people are classified as obese using BMI thresholds. Yet “there is no logic, no medical coherence to using BMI to define a disease [obesity]. It’s just not suitable,” says Professor Rubino of King’s College London. And he’s one of a growing consensus among professionals that BMI doesn’t do the job it’s being used for.

What BMI Fails to Capture

Developed in the 1830s by Belgian statistician Adolphe Quetelet, BMI (originally the “Quetelet Index”) was designed as a tool to define the “average man” for research, not to measure health. Scientist Ancel Keys renamed it as the Body Mass Index in 1972, before the World Health Organisation (WHO) adopted BMI in the 1980s to define obesity. In 1997, the WHO declared obesity a global epidemic and uptake of the index skyrocketed across the world in turn.

It spread because it was efficient. Two numbers. One equation. Simple. 

Of course, excess body fat impairs health. The issue here is whether a BMI measurement is physiologically accurate. In the 200 years or so since the Belgian sought to quantify a baseline, it’s safe to say we’ve advanced.

Over two decades of research have shown that fat distribution predicts cardiometabolic risk more reliably than total body weight. Those with the highest waist-to-hip ratios showed double the risk of developing heart disease. Further, when comparing people in their 40s, those with the highest levels of abdominal (or visceral) fat were nearly three times more likely than those with the lowest levels to develop dementia in older age.

BMI as a blunt tool undoubtedly has a place, but in modernity there are clear scientific alternatives – like waist-to-hip ratio, weight-adjusted waist and the Body Roundness Index – that often align better with health outcomes by focusing on body geometry rather than total mass alone.

Take the landmark INTERHEART study, including more than 27,000 participants across 52 countries, demonstrated more than 20 years ago that waist-to-hip ratio outperforms BMI in predicting heart attack risk. Subsequent research has shown it’s also a stronger predictor of mortality.

TL;DR: Location changes risk profile and BMI can’t detect it.

Why This Matters for Regulation

Excess visceral fat isn’t just stored energy sitting quietly under the surface. It’s metabolically active tissue that releases inflammatory chemicals, interferes with insulin signalling and shifts hormonal balance.

When low-grade inflammation becomes chronic, sympathetic drive (like increased heart rate and blood pressure) rises and cortisol rhythms can flatten or spike at the wrong times. In practical terms, that means the system becomes less efficient at handling demand and slower to return to baseline afterwards.

Essentially, your BMI can be “right” whilst your level of visceral fat is causing unnecessary strain on your wellbeing. 

Old Standards, New Demands

BMI is one example of a broader pattern in health science: metrics designed for one era becoming default tools in another.

Recommended Daily Allowances (RDAs) were established in the mid-20th century using relatively small datasets. Their purpose was specific and pragmatic, to prevent overt deficiency diseases such as rickets and scurvy. The benchmark was survival.

Preventing deficiency is a low bar, not an aim for people who expect a lot from themselves.

The demands placed on modern physiology look very different. Sustained cognitive load. Chronic psychological stress. Circadian disruption. Continuous stimulation. Nervous systems rarely receive long, uninterrupted recovery windows.

A nutrient intake sufficient to avoid deficiency does not automatically support ambitious capacity. The floor and the ceiling are not the same thing.

BMI followed a similar trajectory. It was created to describe population averages, not to determine individual health status or treatment access. Yet it became a clinical gatekeeper.

In both cases, a tool built for one purpose expanded into another. The original design was narrow. The application became broad.

There is a meaningful difference between avoiding breakdown and expanding functional capacity.

When standards are anchored to minimum viability, performance ceilings follow.

Measurement Shapes Outcome

Imprecise measurement leads to imprecise intervention.

If we rely on outdated markers designed to prevent deficiency or track averages, we aim for adequacy rather than adaptability.

Expanded capacity requires more intelligent inputs and more accurate signals.

• From total mass to fat distribution.
• From deficiency thresholds to functional sufficiency.
• From categorical labels to graded physiological assessment.

Metrics must evolve alongside our understanding of biology.

Because what we choose to measure defines the standard we accept, which determines whether we merely avoid decline or build systems capable of sustained performance.