A new clinical definition of obesity is a step in the right direction, writes WA lead for the RACGP obesity special interest group Dr Mark Mellor.
Obesity is globally prevalent, affecting one in eight people in the world in 2022. In Australia, around two-third of adults and one-fifth of children have overweight or obesity.
The prevalence of obesity has risen over the decades, so much so that the World Health Organisation (WHO) classifies it as a ‘global epidemic’.
The WHO first recognised obesity as a disease in 1948. It was defined as ‘an abnormal or excessive fat accumulation that presents a risk to health’. This simple definition, based on phenotype and health risk, has persisted for decades but is recognised as insufficient for describing the chronic systemic illness that some people with obesity experience.
Historically, our understanding of obesity has been mired by moral viewpoints, with assumptions made about its cause, like eating too much or exercising too little. This leads to weight bias and stigma, like people with obesity are lazy and lack motivation.
Unfortunately, these erroneous beliefs remain prevalent in society, political and healthcare settings. Obesity certainly is not a lifestyle choice. When viewed through a clinical lens, genetic/epigenetic, environmental, nutritional, psychological and metabolic factors are all implicated.
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The dichotomy of obesity as a risk factor for disease has been the focus of many previous discussions. On the one hand, classifying all obesity as a disease risks overdiagnosis and over-medicalisation. On the other hand, classifying obesity as a risk factor for disease only diminishes the negative health effects that obesity causes.
Reconciling this dichotomy has been challenging because a robust clinical diagnosis for obesity has been lacking and its aetiology and pathophysiology incompletely understood.
The WHO definition does not describe the causes of excess adiposity and ignores how it might progress to severe organ dysfunction and chronic illness. Further, body mass index (BMI), the most common metric used to diagnose obesity, falls short for measuring its complex and heterogenous nature. BMI is not a direct measure of adiposity, does not determine body fat distribution nor define when excess adiposity is a health issue.
Taking these issues into account, in 2022 The Lancet Diabetes and Endocrinology Commission sought to address the lack of an accurate definition of obesity and formulate a comprehensive approach to diagnosis. The commission comprised 58 international experts from various medical specialties and people of lived experience for balanced representation. It published its findings in January 2025.
The Lancet commission’s new definition of obesity is simply ‘a state of excess adiposity (body fat)’ preferably measured directly, for example using dual energy x-ray absorptiometry or bioimpedance, or indirectly using anthropometric measures of central adiposity like waist circumference, waist-to-hip ratio or waist-to-height ratio.
BMI should not be relied upon as the sole measure to diagnose obesity, although those with a very high BMI (over 40 kg/m2) can be assumed to have excess adiposity.
The Lancet commission also sets out two new obesity categories: pre-clinical obesity and clinical obesity.
Pre-clinical obesity defines a state of excess adiposity with preserved tissue and organ function. People with pre-clinical obesity are not limited in their activities because of obesity but do have a higher risk of developing clinical obesity and several other non-communicable diseases such as diabetes, cardiovascular disease and some cancers.
Clinical obesity is a state of excess adiposity with reduced tissue or organ function. People with clinical obesity have a chronic disease due to obesity alone. They may have various symptoms and signs directly related to obesity such as breathlessness, hip or knee pain, various metabolic abnormalities, and dysfunction related to abnormalities in other organ systems.
The commission sets out 18 adult and 13 child and adolescent diagnostic criteria for clinical obesity. These are conditions associated with tissue, organ or body dysfunction caused by obesity, such as raised arterial blood pressure and venous thromboembolism.
This pivotal undertaking marks a change in conceptualisation of obesity from a BMI-based classification to a model underpinned by body fat measurement and the presence or absence of tissue or organ dysfunction and/or reduced functional capacity.
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Importantly, the model considers that some conditions are causally related to obesity (same aetiopathogenesis) and not co-morbidities (different aetiopathogenesis).
Of course, there are other frameworks for defining and diagnosing obesity – the European Association for the Study of Obesity (EASO) and Edmonton Obesity Staging System (EOSS) frameworks, to name but a few. These models are highly regarded and agree that BMI is insufficient for diagnosing obesity.
The Lancet commission report is also not without criticism. What constitutes ‘excess adiposity’ is debated and the list of criteria meeting the definition of clinical obesity might be difficult to implement in real-world clinical practice – psychosocial drivers and complications of obesity are not included, and no comment is made on the causative factors for obesity (albeit this was outside of the commission’s scope).
Nevertheless, the model paves a way for clinicians, policymakers, and health authorities to target treatment to those with the negative health effects of obesity and its associated chronic illness.
Further work is needed to address criticisms, and in time, the model may adapt to reflect changes in thinking and understanding as we learn more about obesity as a disease.
It is a much-needed step in the right direction that combats weight bias and stigma, promotes empathy and understanding of those living with obesity, and frames obesity as a disease to support treatment prioritisation.
ED: Dr Mellor is the lead clinician at the Perth Weight Clinic.
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