Defining obesity: BMI isn't good enough, says new expert report

Body mass index (BMI) is the standard medical measure of obesity – and it has been for many years.

However, a landmark new report by global obesity experts argues that it's time to drop BMI in favour of a more nuanced approach.

Commissioned by The Lancet Diabetes and Endocrinology, the report contends that BMI 'provides inadequate information about health at the individual level'. Moreover, it 'undermines medically sound approaches to health care and policy'.

But if this is true, why do so many medical experts rely on BMI when defining obesity – including global institutions like the World Health Organisation?

To find out, we need to go back to basics. What is BMI – and what are its limitations?

What is BMI?

BMI is a measure of a person's body mass. You can calculate your BMI by dividing your weight (in kilogrammes) by the square of your height (in metres).

BMI = kg ÷ m²

Or, if you want to save time, you can use the handy BMI calculator on the SemaPen website.

The result will be a value in kg/m². This value can be used as a rule of thumb to determine whether you're a healthy weight for your height.

Most medical organisations consider a healthy BMI to be between 18.5 and 24.9kg/m². Anything under 18.5kg/m² is underweight. A BMI between 25 and 29.9kg/m² is overweight. And if your BMI is 30kg/m² or greater, you're considered to have obesity.

Neat? Yes. Simple? Yes. Effective? That's up for debate.

What's the problem with BMI?

BMI is a more reliable measure of obesity than body weight alone. That's because it takes your height into account, as well as your weight.

However, it's not without limitations. For instance, it can measure your body mass – but it can't tell you what that mass is made up of. It could be fat, but it could also be muscle. Bodybuilders are likely to have high BMIs, despite not having much fat.

BMI also falls short when body fat is unevenly distributed – for example, when a person has abdominal obesity (excess fat around the stomach).

However, many medical organisations still rely on BMI because it's a useful yardstick for defining obesity. As one expert told the BBC in 2018, 'It works in the vast majority of people, the vast majority of the time'.

What does the new report say?

The new report is not the first to criticise BMI as a measure of obesity. Far from it.

A 2016 study, for instance, found evidence to suggest that BMI was miscategorising people en masse.

The study looked at the overall health of 40,420 participants. It found that 29% of those with a BMI in the obese range were 'metabolically healthy', while 'over 30% of normal weight individuals were cardiometabolically unhealthy'.

But the new report goes further than this. It argues that BMI is not simply inaccurate, but it also oversimplifies individual health concerns and risks undermining medical decision-making.

'Individuals living with obesity have different health profiles and needs', the researchers wrote in an editorial accompanying the report. '[However, they] are often discussed as a single entity, defined by one single parameter (BMI), or not discussed at all.'

The researchers propose a new definition of obesity – measured not using BMI but with other indicators of body size like waist circumference and waist-to-height ratio.

This new definition aims to account for the complex causes and presentations of obesity by distinguishing between 'preclinical obesity' and 'clinical obesity'.

Under the researchers' proposed diagnostic framework, preclinical obesity is considered a health concern, while clinical obesity is 'a chronic, systemic illness'.

Both are caused by 'excess adiposity' (fat build-up). However, preclinical obesity only becomes clinical obesity when this excess fat causes 'alterations in the function of tissues, organs [or] the entire individual'.

In other words, obesity becomes clinical obesity when it transforms from a risk into a full-blown illness.

Will this new definition be accepted?

At SemaPen, we welcome this report and its recommendations for a more nuanced and patient-focused definition of obesity.

We, like many others, recognise the limitations of BMI-based diagnoses. This new diagnostic framework better aligns with our modern understanding of obesity as a complex illness with many and multifaceted causes.

However, as the researchers acknowledge, there are obstacles to its adoption – including difficulties in implementation and negative perceptions of obesity among society at large. It 'will take time and effort', they write, 'but at the heart of these proposals is the aim to improve the lives of people living with obesity'.

'We now have the opportunity to transform obesity care', they continue, 'moving away from a system in which individuals are seen under one single label toward a system that recognises the unique health and needs of each person.'

Hear hear.

As part of Phoenix Health, SemaPen has been helping people with obesity lose weight for more than 20 years. Why not learn more about our custom weight loss plans?

Sources

Rubino, R. et al. (2025) "Definition and diagnostic criteria of clinical obesity" The Lancet Diabetes & Endocrinology https://doi.org/10.1016/S2213-8587(24)00316-4

"Redefining obesity: advancing care for better lives" The Lancet Diabetes & Endocrinology (2025) https://doi.org/10.1016/S2213-8587(25)00004-X

Tomiyama, A.J. et al. (2016) "Misclassification of cardiometabolic health when using body mass index categories in NHANES 2005–2012" International Journal of Obesity, 40 https://doi.org/10.1038/ijo.2016.17