Obesity needs new definition, says global report

Calling people obese is medically "flawed" - and the definition should be split into two, a report from global experts says.

The term "clinical obesity" should be used for patients with a medical condition caused by their weight, while "pre-clinically obese" should be applied to those remaining fat but fit, although at risk of disease.

This is better for patients than relying only on body mass index (BMI) - which measures whether they are a healthy weight for their height - to determine obesity.

More than a billion people are estimated to be living with obesity worldwide and prescription weight-loss drugs are in high demand.

The report, published in The Lancet Diabetes & Endocrinology journal, is supported by more than 50 medical experts around the world.

"Some individuals with obesity can maintain normal organ function and overall health, even long term, whereas others display signs and symptoms of severe illness here and now," Prof Francesco Rubino, from King's College London, who chaired the expert group, said.

"Obesity is a spectrum," he added.

The current, blanket definition means too many people are being diagnosed as obese but not receiving the most appropriate care, the report says.

Currently, in many countries, obesity is defined as having a BMI over 30 - a measurement that estimates body fat based on height and weight.

It is calculated by dividing an adult's weight in kilograms by their height in metres squared.

For example, if they are 70kg (about 11 stone) and 1.70m (about 5ft 7in):

Find out what your body mass index (BMI) means on the NHS website

But BMI has limitations.

It measures whether someone is carrying too much weight - but not too much fat.

So very muscular people, such as athletes, tend to have a high BMI but not much fat.

The report says BMI is useful on a large scale, to work out the proportion of a population who are a healthy weight, overweight or obese.

But it reveals nothing about an individual patient's overall health, whether they have heart problems or other illnesses, for example, and fails to distinguish between different types of body fat or measure the more dangerous fat around the waist and organs.

Measuring a patient's waist or the amount of fat in their body, along with a detailed medical history, can give a much clearer picture than BMI, the report says.

"Obesity is a health risk - the difference is it's also an illness for some," Prof Rubino said.

Clinically obese

When obesity is a disease, there will be signs of it affecting organs in the body - through heart disease, breathlessness, type 2 diabetes or joint pain - and a person's day-to-day activities. Treatment with drugs or surgery is likely.

Pre-clinically obese

When obesity is a risk to health - but not yet causing any illnesses, people should be offered weight-loss advice, counselling and monitoring, to reduce the chances of health problems developing.

Doctors should also pay close attention to a patient's family history to see if they are at risk of particular diseases.

At a time when drugs that reduce body weight by up to 20% are being prescribed on a large scale, the report says redefining obesity "is all the more relevant" because it "improves the accuracy of diagnosis".

Access to weight-loss drugs such as Wegovy and Mounjaro is often restricted to patients with a BMI over 30 and a weight-related health condition.

Children's obesity expert Prof Louise Baur, from the University of Sydney, who contributed to the report, said the new approach would allow adults and children with obesity "to receive more appropriate care", while reducing the numbers being over-diagnosed and given unnecessary treatment.

The Royal College of Physicians said the report laid a strong foundation "for treating obesity with the same medical rigour and compassion as other chronic illnesses".

But others worry pressure on health budgets could mean less money for patients in the "pre-obesity" category.