The precursor to the Body Mass Index (BMI) scale was invented more than 150 years ago by a Belgian statistician, Adolphe Quetelet. Named the Quetelet Index, it was designed as a sociological rather than medical tool, to study the average proportions of the “ideal” human body, based on a European physique. In 1972, the American physiologist Ancel Keys developed this into the BMI as a measure of obesity and its health risks.
The BMI is now used to assess whether someone has a “healthy” body weight and if they are worthy of treatment. But the scale is outdated, flawed and Eurocentric. It is a poorly evidenced metric for good health.
BMI is a simplistic measurement, calculated by dividing a person’s weight in kilograms by their height in metres squared, then categorising them into underweight, normal weight, overweight or obese. In a cash-strapped NHS, an “obese” BMI (that of 30 or above) is consistently used to inform care pathways that are more cost-effective but often discriminatory – denying people treatment or surgery.
As a sexual and reproductive health doctor, I regularly see it limit people’s access to contraception, such as the combined hormonal pill. In some cases, these people are otherwise perfectly healthy – take athletes, who have an elevated BMI due to high muscle mass. Sometimes it’s used as a cost-cutting strategy – a report from the British Pregnancy Advisory Service (BPAS) found that people are pushed towards expensive private in-vitro fertilisation (IVF), because 96 per cent of NHS trusts restrict access based on female BMI, despite guidance not mandating this.
In other cases, its use is unhelpful and counterintuitive. Those seeking breast reduction surgery often have an elevated BMI due to having more breast tissue – ironically, they end up being denied surgery that would reduce their weight.
The BMI may have more robust evidence in areas such as cardiovascular disease and stroke, as studies are much larger (albeit Eurocentric). But data from the UK Clinical Research Collaboration indicates that reproductive health receives less than 2.5 per cent of research funding in the UK, so study sizes are typically only hundreds of people.
Some doctors are researchers, but most are not – we simply work to guidelines that other people have produced. Often, these guidelines are grossly simplified and outdated. This isn’t a left-field view – the American Medical Association has acknowledged that the BMI is problematic, and that doctors need a more holistic approach.
Much better tools already exist. People can go into a gym and use a fat analysis machine, which shows where their fat is distributed. With chronic diseases such as diabetes, this is a more essential metric than overall BMI. But in pressurised NHS environments, the BMI’s redeeming features are that it’s cheap, easy and can neatly categorise people to make quick clinical decisions.
This does not justify its isolated use, and continuing to do so does patients a disservice. For certain patients, more detailed analysis of multiple factors may be required to ascertain their true risk for treatments or operations.
There is no doubt that such precision medicine will be a challenge in the NHS. But amid so much technological innovation, we need a more progressive approach that stops the systemic mistreatment of those deemed to be overweight. Using an outdated tool only exacerbates this discrimination.
This piece is part of a debate on the BMI scale. Read the other half of the debate here.
This piece first appeared in a Spotlight Healthcare print report on 13 October 2023. Read it here.