A new definition of obesity could help treat millions of people

Rethinking the way we define obesity could help millions of people worldwide, claims 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 health professionals assess whether people are obese by calculating their body mass index (BMI), a measure of weight in relation to height. A BMI between 18.5 and 24.9 is considered healthy, while under or over indicates that someone is under or overweight. A BMI over 30 indicates that someone is overweight.

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 increase inflammation, increasing 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 whether the ‘excess’ weight is due to excess body fat or stronger muscle mass or bone mass,” says Francesco Rubino of King’s College London, who led the obesity review.

Even when assessed correctly, through measurement of the waist or, more rarely, X-rays, body fat levels do not completely dictate a person’s health. “No two people react the same way to excess body fat. This is influenced by a person’s race and ethnicity, their age, what foods they eat, and genetics play a huge role,” says Steven Heymsfield of Louisiana State University.

Therefore, Rubino and his colleagues want to introduce more nuances in the definition of obesity and divide cases into preclinical and clinical. Both forms will be characterized as having excess body fat, but only the clinical form will involve symptoms caused by excess fat, such as difficulty breathing, heart problems, or difficulty performing daily activities. Meanwhile, preclinical obesity increases the risk of eventually developing such obesity-related symptoms, Rubino says.

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

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

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

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

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

The updated definitions, which have already been endorsed by 76 health organizations worldwide, may even help reduce the stigma surrounding the condition. “The hope is that defining obesity in a more nuanced way shows that it’s a disease in itself. It’s not just a consequence of behavioral things, there are lots of risk factors, environmental, psychological and genetic,” says Gray .