A new definition of obesity could help treat millions of people

Rethinking the way we define obesity could help millions of people worldwide, argue a team of researchers who want to introduce a new category of “preclinical” obesity.

The current definition of obesity, as set by the World Health Organization (WHO), is having excess body fat that poses a risk to health. The WHO recommends that healthcare workers assess whether people have obesity by calculating their body mass index (BMI), a measure of weight relative to height. A BMI between 18.5 and 24.9 is considered healthy, while below or above indicates someone is under or overweight. A BMI above 30 indicates that someone is obese.

It is true that high levels of body fat can cause it to infiltrate organs such as the liver and pancreas, impairing their function. It can also ramp up inflammation, raising the risk of conditions such as cancer, liver disease and heart problems.

But BMI poorly reflects a person’s body fat levels. “With BMI, we don’t know if that ‘excess’ weight is due to excess body fat or stronger muscle mass or bone mass,” says Francesco Rubino at King’s College London, who led the review on obesity.

Even when assessed correctly, through measurement of the waist or, more rarely, X-ray scans, body fat levels don’t fully dictate someone’s health. “No two people respond the same way to excess body fat. This is impacted by a person’s race and ethnicity, their age, what foods they eat, and genetics plays a tremendous role,” says Steven Heymsfield at Louisiana State University.

That is why Rubino and his colleagues want to introduce more nuance into the definition of obesity, splitting cases into preclinical and clinical. Both forms would be characterised as having excess body fat, but only the clinical form would involve symptoms caused by excess fat, such as breathing difficulties, heart problems or difficulty carrying out everyday activities. Meanwhile, preclinical obesity raises the risk of eventually developing such obesity-related symptoms, says Rubino.

This would be akin to how people can have prediabetes, where their blood sugar levels are higher than usual, but not high enough to be diagnosed with full-blown type 2 diabetes, says Rubino.

Under the proposed changes, healthcare staff would directly measure people’s body fat levels using waist width or X-rays in addition to calculating BMI, although someone with a BMI above 40 would always be assumed to have excess fat. They would then use blood tests to assess organ health and ask people whether they are experiencing symptoms. The blood tests are routinely done by many clinicians anyway, but the direct body fat measurements would somewhat increase their workload, says Heymsfield.

If widely adopted by clinicians, the new definitions could mean people are offered advice and treatment that is better tailored to their bodies, says Rubino. Generally, those with preclinical obesity might only need to monitor their health and adopt lifestyle changes, while those with the clinical form are more likely to require treatment with drugs or surgery, says Rubino.

“It would allow us to more appropriately triage people to the right care,” says Adrian Brown at University College London.

Laura Gray at the University of Sheffield, UK, also welcomes the proposed changes. “It’s much needed. These guidelines set out for clinical practice what current research is saying,” she says. “Not everyone with obesity according to BMI is unwell, and not everyone with a lower BMI is healthy.”

The updated definitions, which have already been endorsed by 76 health organisations worldwide, could even help to reduce the stigma surrounding the condition. “The hope is that defining obesity in a more nuanced way shows it is a disease in its own right. It isn’t just a consequence of behavioural things, there’s lots of risk factors, environmental, psychological and genetic,” says Gray.