Six issues with BMI and the UK’s current obesity definition

Emma Clarke
·Contributor
·4-min read
Color image depicting a man in his 30s jogging on a tree tunnel in a sunlit forest. The man is wearing sports training clothing - red t shirt, black shorts and trainers.
Under the current system, those with a BMI of over 30 are considered "obese". (Getty Images)

According to the current measurement, those with a body mass index (BMI) of over 30 are classed as "obese" – in the UK and many other countries.

But now, global experts are calling for a redefinition, to not only take into account the many nuances surrounding excess weight, but to also help differentiate those at risk of disease in the future because of their weight, and those who have a disease but remain healthy.

Professor Francesco Rubino, from King's College London, who chaired the group of more than 50 medical experts, says that obesity is a spectrum.

"We are calling for a change – a radical change – because obviously, in the context of one billion people being classified as having obesity in the world today, and with a number that is projected to increase, no country is rich enough to be able to afford inaccuracy in the diagnosis of obesity."

He added: "The message remains that obesity can be a health risk. What we're proposing is to say that 'it's not only a health risk, for some it is actually an illness and that needs to be diagnosed'."

Here, we take a look at the issues with the current BMI system – and the potential implications of this definition of obesity.

Female feet standing on electronic scales for weight control on wooden background. The concept of slimming and weight loss
Experts are calling for a redefinition of obesity. (Getty Images)

As of February 2024, over one in four (26%) adults and 23.4% of children aged 10-11 years in England are living with obesity.

According to the report, published in the Lancet Diabetes & Endocrinology journal, over one billion (of the total eight billion) people in the world are now estimated to be living with obesity, thanks to the current measurements.

While the experts state that timely medical treatment should be offered to those who are found to be clinically obese, they outline that those who are pre-clinically obese – people who have excess body fat but their organs and tissues work normally – should instead receive health counselling and evidence-based interventions to reduce their individual health risk.

Professor Rubino says that overdiagnosis can in turn lead to the “unwarranted use of medications and surgical procedures, with potential harm to the individual and staggering costs for society.”

The report also outlines that, owing to the current BMI threshold for obesity, the current approach to the obesity crisis is somewhat flawed and ineffective.

"BMI is not a direct measure of fat, does not reflect its distribution around the body and does not provide information about health and illness at the individual level," it reads.

Professor Robert Eckel of the University of Colorado Anschutz Medical Campus, adds: "People with excess body fat do not always have a BMI that indicates they are living with obesity, meaning their health problems can go unnoticed. Additionally, some people have a high BMI and high body fat but maintain normal organ and body functions, with no signs or symptoms of ongoing illness."

Female surgeon typing on digital tablet in hospital or surgery. She is wearing a dark blue nurse’s top and has her stethoscope around her neck. She is looking at her patients records on her digital tablet and sending emails.
The obesity crisis is putting a huge strain on the NHS – and costing billions each year. (Getty Images)

Studies have shown that those with obesity can suffer with their mental health – including low mood, self-esteem, quality of life and overall body image.

While these things can motivate individuals to get help and seek treatment, they can also have an adverse effect on their progress, setting them back or preventing them from achieving their health goals.

As outlined by Professor Rubino, overdiagnosis means unwarranted treatments – and, therefore, extra burden on health services like the NHS. In the UK alone, obesity costs the NHS around £6.5 billion a year.

Last year, health secretary Wes Streeting announced plans to roll out weight-loss jab Mounjaro on the NHS, to help combat the obesity crisis in Britain.

Due to the sheer scale of those who meet the criteria – 3.4 million are eligible for the treatment plan as it stands – the government will only be able to offer 220,000 (or 10%) of those eligible the treatment in the first instance.

Health experts have warned that given how quickly patients can be processed, the plan will take years to fully implement.

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