How we diagnose and define obesity is set to change – here’s why, and what it means for treatment

Obesity is linked to many common diseases, such as type 2 diabetes, heart disease, fatty liver disease and knee osteoarthritis.

Obesity is currently defined using a person’s body mass index, or BMI. This is calculated as weight (in kilograms) divided by the square of height (in metres). In people of European descent, the BMI for obesity is 30 kg/m² and over.

But the risk to health and wellbeing is not determined by weight – and therefore BMI – alone. We’ve been part of a global collaboration that has spent the past two years discussing how this should change. Today we publish how we think obesity should be defined and why.

As we outline in The Lancet, having a larger body shouldn’t mean you’re diagnosed with “clinical obesity”. Such a diagnosis should depend on the level and location of body fat – and whether there are associated health problems.

What’s wrong with BMI?

The risk of ill health depends on the relative percentage of fat, bone and muscle making up a person’s body weight, as well as where the fat is distributed.

Athletes with a relatively high muscle mass, for example, may have a higher BMI. Even when that athlete has a BMI over 30 kg/m², their higher weight is due to excess muscle rather than excess fatty tissue.

People who carry their excess fatty tissue around their waist are at greatest risk of the health problems associated with obesity.

Fat stored deep in the abdomen and around the internal organs can release damaging molecules into the blood. These can then cause problems in other parts of the body.

But BMI alone does not tell us whether a person has health problems related to excess body fat. People with excess body fat don’t always have a BMI over 30, meaning they are not investigated for health problems associated with excess body fat. This might occur in a very tall person or in someone who tends to store body fat in the abdomen but who is of a “healthy” weight.

On the other hand, others who aren’t athletes but have excess fat may have a high BMI but no associated health problems.

BMI is therefore an imperfect tool to help us diagnose obesity.

What is the new definition?

The goal of the Lancet Diabetes & Endocrinology Commission on the Definition and Diagnosis of Clinical Obesity was to develop an approach to this definition and diagnosis. The commission, established in 2022 and led from King’s College London, has brought together 56 experts on aspects of obesity, including people with lived experience.

The commission’s definition and new diagnostic criteria shifts the focus from BMI alone. It incorporates other measurements, such as waist circumference, to confirm an excess or unhealthy distribution of body fat.

We define two categories of obesity based on objective signs and symptoms of poor health due to excess body fat.

1. Clinical obesity

A person with clinical obesity has signs and symptoms of ongoing organ dysfunction and/or difficulty with day-to-day activities of daily living (such as bathing, going to the toilet or dressing).

There are 18 diagnostic criteria for clinical obesity in adults and 13 in children and adolescents. These include:

• breathlessness caused by the effect of obesity on the lungs

• obesity-induced heart failure

• raised blood pressure

• fatty liver disease

• abnormalities in bones and joints that limit movement in children.

2. Pre-clinical obesity

A person with pre-clinical obesity has high levels of body fat that are not causing any illness.

People with pre-clinical obesity do not have any evidence of reduced tissue or organ function due to obesity and can complete day-to-day activities unhindered.

However, people with pre-clinical obesity are generally at higher risk of developing diseases such as heart disease, some cancers and type 2 diabetes.

What does this mean for obesity treatment?

Clinical obesity is a disease requiring access to effective health care.

For those with clinical obesity, the focus of health care should be on improving the health problems caused by obesity. People should be offered evidence-based treatment options after discussion with their health-care practitioner.

Treatment will include management of obesity-associated complications and may include specific obesity treatment aiming at decreasing fat mass, such as:

• support for behaviour change around diet, physical activity, sleep and screen use

• obesity-management medications to reduce appetite, lower weight and improve health outcomes such as blood glucose (sugar) and blood pressure

• metabolic bariatric surgery to treat obesity or reduce weight-related health complications.

Should pre-clinical obesity be treated?

For those with pre-clinical obesity, health care should be about risk-reduction and prevention of health problems related to obesity.

This may require health counselling, including support for health behaviour change, and monitoring over time.

Depending on the person’s individual risk – such as a family history of disease, level of body fat and changes over time – they may opt for one of the obesity treatments above.

Distinguishing people who don’t have illness from those who already have ongoing illness will enable personalised approaches to obesity prevention, management and treatment with more appropriate and cost-effective allocation of resources.

What happens next?

These new criteria for the diagnosis of clinical obesity will need to be adopted into national and international clinical practice guidelines and a range of obesity strategies.

Once adopted, training health professionals and health service managers, and educating the general public, will be vital.

Reframing the narrative of obesity may help eradicate misconceptions that contribute to stigma, including making false assumptions about the health status of people in larger bodies. A better understanding of the biology and health effects of obesity should also mean people in larger bodies are not blamed for their condition.

People with obesity or who have larger bodies should expect personalised, evidence-based assessments and advice, free of stigma and blame.

Louise Baur receives funding from the National Health & Medical Research Council in the form of competitive research grants on projects related to child and adolescent obesity; these funds are administered by The University of Sydney. She has also received honoraria from Novo Nordisk and Lilly for speaking on topics related to adolescent obesity; these funds have been directed to her institutional research cost centre. Louise Baur was also on the Steering Committee of the ACTION Teens study, sponsored by Novo Nordisk, and has been on an Advisory Committee for Lilly. She is also a consultant paediatrician in Weight Management Services at The Children's Hospital at Westmead.

John Dixon has received honoraria from Reshape Lifesciences and Nestle Health Science Australia for consultancy, advisory board, and speaker engagements. He has also received personal honoraria from Novo Nordisk, Eli Lilly, iNova, and Eurodrug Laboratories for advisory boards and speaker engagements. He has also received Personal Honoraria for educational presentations for HealthED. Past research funding from NHMRC grants and fellowships, was paid to Monash University and the Baker Heart and Diabetes Institute. Currently, John is Vice President of the National Association of Clinical Obesity Services (unpaid).

Priya Sumithran receives funding from the National Health and Medical Research Council paid to her institution. She is was a council member of the Australian and New Zealand Obesity Society (ANZOS) 2017-2022 and is a member of The Obesity Collective leadership group. She has been a co-author on manuscripts with a medical writer provided by Novo Nordisk and Eli Lilly, and received payment to her institution from Eli Lilly for participation in advisory boards.

Wendy A. Brown has received honoraria from Johnson and Johnson, GORE, Novo Nordisc, Pfizer, Medtronic, Eli Lilly and Merck Sharpe and Dohme for lectures and advisory boards. She receives research grants from Johnson and Johnson, Medtronic, GORE, Applied Medical and the Australian Commonwealth Government for the Australian Bariatric Surgery Registry. She has also received research grants for investigator initiated projects from Novo Nordisc, NHMRC and Myerton. She is a practicing Metabolic Bariatric Surgeon and as such part of her income is derived from performing these surgeries.