Overhaul the way obesity is diagnosed, experts recommend

A global Commission, published in The Lancet Diabetes & Endocrinology and endorsed by 75 medical organisations around the world, suggests that the traditional tool used to diagnose obesity – body mass index or BMI – is unreliable as a measure when used on its own.

The Commission involved 56 world-leading experts including Louise Baur, Professor of Child and Adolescent Health at the University of Sydney and former President of the World Obesity Federation, medical specialists in the US, UK, India and Brazil, as well as people living with obesity.

The report recommends clinicians adopt a new approach to diagnosing obesity, by measuring excess body fat as well as BMI and assessing objective signs and symptoms of individual ill health, such as breathlessness and joint stiffness. 

Two new diagnostic categories of obesity are recommended – “clinical obesity” (a chronic disease associated with organ dysfunction due to excess fat) and “pre-clinical obesity” (a high level of fat but no ongoing illness) – to ensure that people with obesity receive care that is evidence-based and tailored to their individual health status. 

Why is BMI unreliable on its own?

BMI measures a person’s weight relative to their height. For decades it has been used as the standard tool to determine whether a person is living with obesity.

However, the Commission argues that looking at BMI on its own is unreliable. This is because it is not a direct measurement of fat and does not account for its distribution around the body. 

For example, some people with a lower BMI will store excess fat around their waist or organs (such as the liver, heart and muscles), which could lead to illnesses such as cardiovascular disease. By relying on BMI as a measure, it could lead to health problems going unnoticed in some lower BMI individuals. 

Another problem noted by researchers is that BMI was developed as a measure for people of European descent and requires adaptation to take into account the obesity risk-factors of different ethnicities.

Professor Baur said: “Over a billion people across the world live with obesity, which increases the risk of developing serious conditions such as type 2 diabetes and certain types of cancers. But despite this, people are often diagnosed as having obesity using a single, catch-all measure that is not tailored to the individual. Taking a more nuanced approach will allow people to receive care that is proportionate to their needs.”

Rather than using BMI as the sole indicator of obesity, the Commission recommends clinicians should use one of the following methods to identify whether someone is living with obesity: 

• at least one measurement of body size (waist circumference, waist-to-hip ratio or waist-to-height ratio) in addition to BMI;
• at least two measurements of body size (waist circumference, waist-to-hip ratio or waist-to-height ratio) regardless of BMI;
• direct body fat measurement (such as by a bone densitometry scan or DEXA) regardless of BMI;
• in people with very high BMI, excess body fat can be pragmatically assumed.

Clinical obesity and pre-clinical obesity

The Commission has also developed two new evidence-based definitions to help assess obesity: “clinical obesity”; and “pre-clinical obesity”. 

“Clinical obesity” is defined as a chronic disease where people have signs of reduced organ function or significantly reduced ability to conduct standard day-to-day activities such as bathing or dressing due to excess body fat. 

The Commission sets out 18 diagnostic criteria for clinical obesity in adults and 13 specific criteria for children and adolescents, including: 

• breathlessness caused by effects of obesity on the lungs;
• obesity-induced heart failure;
• knee or hip pain, with joint stiffness and reduced range of motion as a direct effect of excess body fat on the joints;
• certain alterations of bones and joints in children and adolescents limiting movement;
• other signs and symptoms caused by dysfunction of organs including kidneys, upper airways, metabolic organs, nervous, urinary and reproductive systems and the lymph system in the lower limbs.

“Pre-clinical obesity” is defined as a person with a high BMI but normal organ function. While they do not have ongoing illness, they have an increased risk of developing clinical obesity and other serious conditions such as cardiovascular disease in the future. The Commission recommends that people who are “pre-clinical” should be supported to reduce the risk of potential disease.

Professor Baur said: “We hope that these new definitions will make it easier for clinicians and patients alike to have discussions about their health, and to get the care that they need without stigma or judgement.” 

Commission chair, Professor Francesco Rubino, from King’s College London said: “Our reframing acknowledges the nuanced reality of obesity and allows for personalised care. This includes timely access to evidence-based treatments for individuals with clinical obesity, as appropriate for people suffering from a chronic disease, as well as risk-reduction management strategies for those with pre-clinical obesity, who have an increased health risk, but no ongoing illness. This will facilitate a rational allocation of healthcare resources and a fair and medically meaningful prioritisation of available treatment options.”