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New definitions of obesity aim to overcome current limitations that exclude some people from receiving the health care they need, while pushing others towards unnecessary treatments.
Obesity is common, its prevalence is increasing, and it has a substantial impact on individuals and health care systems. Yet, our health care policies don’t seem to regard it with any urgency.
This could be related to the way we conceptualise obesity. Currently, it is defined by body size and the increased risk it confers for other conditions such as type 2 diabetes, cardiovascular disease and cancer. Despite the clear association of obesity with increased morbidity and mortality, there remains debate about whether it is a disease at all, or simply a risk factor for future health problems.
Since 2022, we’ve been part of a global collaboration of 58 experts on various aspects of obesity, including people with lived experience, that sought to move this debate forward. Our proposal, published in The Lancet Diabetes & Endocrinology Commission in January and endorsed by more than 75 scientific, professional and patient organisations around the world, presents a new approach to the diagnosis of obesity, and defines clinical manifestations of ill health caused by excess adiposity.
How do we propose to define obesity?
Obesity is currently defined by a body mass index (BMI) of 30 kg/m2 or greater, with some regional variations to account for differences between ethnic groups in risk of weight-related diseases.
While BMI is useful to estimate health risks at a population level, it does not quantify fat or provide information about the health of an individual. Hence, it can lead to under-detection and false reassurance for people who, despite having a BMI of less than 30, have excess central adiposity that adversely affects their health. Conversely, defining obesity by BMI alone will over-diagnose people who have a high BMI but not excess fat, for example, muscular individuals.
Therefore, the commission recommends that, to diagnose obesity, BMI may be used as a screening tool, with addition of a measurement to confirm the presence of excess adiposity, such as waist circumference. For people with a BMI of 40 kg/m2 or greater, it can be assumed that excess adiposity is present.
What is clinical obesity?
Once it has been determined that a person has obesity (ie, excess adiposity), the next step is to establish whether it is impairing their health or functioning. This is done through history-taking, physical examination, and relevant pathology tests.
We define two new categories of obesity based on clinical manifestations of impaired health due to high body fat:
- Clinical obesity: where the individual is experiencing signs and symptoms of ongoing dysfunction of one or more organ systems, or impairment of day-to-day activities, attributable solely to excess adiposity. This is a chronic disease.
- Preclinical obesity: where there is no evidence of illness or functional impairment related to obesity alone. Preclinical obesity is not in itself a chronic disease, and people with preclinical obesity have a variable level of health risk.
We defined 18 diagnostic criteria for clinical obesity in adults and 13 in children and adolescents. These include raised blood pressure, pauses in breathing while asleep due to upper airway obstruction, recurrent urinary incontinence, and fatty liver.
If other causes have been excluded, a person with obesity and one or more of these features has clinical obesity — a standalone chronic disease.
We debated at length about what was included (and not included) in the diagnostic criteria, and in particular, whether type 2 diabetes should be among them. It was not, because type 2 diabetes is a distinct multiorgan disease and we could not define one chronic disease (clinical obesity) by the presence of another (type 2 diabetes). The current criteria do include “the cluster of hyperglycaemia, high triglyceride levels and low HDL cholesterol levels”.
The diagnostic criteria may be refined over time, as has been the case for many diseases. However, the concepts of preclinical obesity and clinical obesity represent a major advance in acknowledging that the impact of obesity is not the same for everyone.
What does it mean for treatment?
It was not within the commission’s scope to define treatment pathways but our proposal does have implications for clinical practice and policy. The new definitions aim to overcome limitations of a size-based definition that considers obesity as merely a risk factor, excludes some people from receiving the health care they need, while pushing others towards unnecessary treatments. Personalised approaches to management will enable more rational allocation of resources.
People with clinical obesity should receive comprehensive care aimed at restoring health, which may include behavioural, medication or surgical interventions. A person with clinical obesity has a chronic disease causing impaired health or functioning and health care policies should not require them to have additional diseases to access effective treatment.
Although people with preclinical obesity have no apparent illness attributable solely to obesity, we do not suggest preclinical obesity is necessarily a state of good health that should only be monitored until illness develops. All people with obesity should receive assessment and evidence-based advice and care tailored to individual circumstances. People with preclinical obesity may have low or higher risk of progression to clinical obesity. They may be in good health, or have diseases associated with obesity, such as cardiovascular disease or osteoarthritis. Depending on their individual health status, care aimed at risk reduction and maintenance of health will be appropriate for some, while others will need care aimed at improving health. Weight loss may be appropriate and consideration of all obesity management options may be warranted, depending on health status and risk.
The prevalence of obesity continues to grow, as does our understanding of the complexity of its drivers and the damage generated by weight stigma. Although the commission’s report has been widely endorsed by professional societies around the world, ongoing discussion and debate on these issues is expected. This is the beginning of a much-needed conversation.
Louise Baur receives funding from the National Health and 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 NHMRC paid to her institution. She 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 Brown has received honoraria from Johnson and Johnson, GORE, Novo Nordisk, 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 Nordisk, NHMRC and Myerton. She is a practising metabolic bariatric surgeon and as such part of her income is derived from performing these surgeries.
The statements or opinions expressed in this article reflect the views of the authors and do not necessarily represent the official policy of the AMA, the MJA or InSight+ unless so stated.
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