Beyond BMI: What the new clinical definition of obesity means for practice

This article features commentary from Stephen McKenzie and Fahad Hanna.

Here’s what the shift means for assessment, diagnosis and multidisciplinary care.

In January 2025, a Lancet Diabetes & Endocrinology Commission published a new framework for diagnosing obesity. Rather than relying primarily on body mass index (BMI), it distinguishes between “preclinical obesity”, where excess body fat increases future health risks, and “clinical obesity”, where it is already causing measurable illness or functional impairment.

The framework has since been endorsed by more than 75 organisations worldwide, and formal adoption processes are underway in health systems internationally, including in Australia. For health professionals and students, the more pressing question is what the shift means in practice: how obesity is assessed, diagnosed and managed day to day.

This kind of lag is typical: new diagnostic frameworks generally take one to three years to move from publication into formal clinical guidelines and routine data collection, regardless of the underlying evidence.

Here’s a closer look at the new framework, what distinguishes preclinical from clinical obesity, and what it could mean for diagnosis, treatment and multidisciplinary care.

What is the new global definition of obesity?

The new definition moves away from using BMI as the main measure of an individual’s health.

BMI can still be useful as a screening and population-level measure, but the Lancet Diabetes & Endocrinology Commission recommends looking more closely at excess body fat, organ and tissue function, symptoms and a person’s ability to carry out everyday activities.

Under the framework, preclinical obesity means a person has excess body fat, but their organs and tissues function normally. Their risk of developing future health problems is higher, so prevention and monitoring are important.

Clinical obesity means excess body fat is already affecting organs, tissues or everyday functioning. It is treated as a chronic illness in its own right.

The Commission was led by Professor Francesco Rubino of King’s College London and published in January 2025. Its central argument is straightforward: two people with the same BMI may have very different health needs.

Formal adoption in Australia could change how clinicians assess patients, moving the conversation beyond a single number on the scales towards evidence of actual health effects.

What adopting the framework could mean for practice

Formal use of the terms “clinical obesity” and “preclinical obesity” is still emerging in Australian practice, and national data collection using these categories doesn’t yet exist. But regardless of formal timelines, the shift has real implications for how health professionals assess and manage patients.

Australia’s delay doesn’t necessarily signal disagreement with the science. Changing a disease definition across a health system has practical consequences.

Associate Professor Fahad Hanna, Public Health academic at Torrens University Australia, describes the new framework as a significant shift “from viewing obesity primarily through a ‘BMI’ lens to assessing whether excess adiposity is already causing illness or functional impairment”.

Australia is generally cautious when adopting new disease definitions because they can affect diagnosis rates, service demand and resource allocation,” Associate Professor Hanna says. Policymakers need confidence that a new framework will improve outcomes “without creating unintended consequences for access, equity or costs.

Australia’s Clinical Practice Guidelines for the Management of Overweight and Obesity are also being updated to reflect this evidence base, in work led by researchers at Deakin University

Implications for weight management care

This shift matters more than ever given the rapid rise of GLP-1 medications in clinical use.

The three main weight-loss drugs in Australia are Ozempic, which contains semaglutide and is approved in Australia to treat type 2 diabetes, but is not Therapeutic Goods Administration (TGA) approved as a weight-management medicine; Wegovy, which also contains semaglutide and is approved for chronic weight management for eligible people; and Mounjaro, which contains tirzepatide and is TGA-approved for both type 2 diabetes and weight management, but not PBS-subsidised for either.

People are nevertheless accessing Ozempic for weight loss through private, off-label prescriptions, including through telehealth services. Australian rules allow a prescriber to make an off-label clinical decision, with responsibility for appropriate prescribing and informed consent. That’s different from Ozempic being approved as a weight loss injection.

Individual eligibility still needs to be assessed by a qualified medical practitioner.

This distinction matters for clinical practice: TGA approval, PBS subsidy, and individual clinical suitability are three separate considerations when advising patients.

Associate Professor Hanna says the new obesity definition could help “align treatment decisions more closely with health need rather than weight alone. This could have important implications for the use of GLP-1 medications, particularly as policymakers seek to balance growing demand, clinical benefit and the sustainability of healthcare spending.”

The psychological side of treatment matters, too.

Associate Professor Stephen McKenzie, Head of Psychology at Torrens University Australia, cautions against presenting drugs for weight loss as a simple “quick fix”.

Medication alone doesn't address the underlying behaviours, habits, and emotions that cause weight gain, and doesn't provide the self-knowledge that people need to help them be fully free of their unhelpful behaviours.

McKenzie compares multidisciplinary obesity treatment with treating physical pain.

Pain killing medication is often very valuably combined with physiotherapy, because without it the patient may not be able to do the physical rehabilitation exercises needed for optimal recovery. People with weight loss and other conditions with strong psychological components may not be able to respond effectively to psychological deep remedies such as behaviour change, without the temporary aid of a drug treatment.

That makes the debate bigger than weight loss drugs like Ozempic or any single weight loss treatment. Effective care can bring pharmacotherapy together with nutrition, physical activity, psychological support and behaviour change.

This is also consistent with World Health Organization (WHO) guidance released in December 2025. WHO conditionally recommends GLP-1 therapies for the long-term treatment of adults with obesity and says structured behavioural interventions involving healthy eating and physical activity may be offered alongside medication.

Research and workforce readiness

As the new framework moves into practice, health professionals will need to understand what it means for assessment, diagnosis and care.

Associate Professor Hanna says implementation would require “workforce training, updated clinical assessment pathways, and funding frameworks that support a more comprehensive evaluation of obesity beyond BMI alone. Clear national guidance would also be essential to ensure consistent identification and management of both obesity risk and clinical obesity, reducing variability in practice and promoting equitable care and access across healthcare settings.”

What this means for practice moving forward

Updated Australian Clinical Practice Guidelines will be an important reference point once finalised, offering clearer national guidance on how current evidence should shape diagnosis and management.

WHO is also continuing to work on implementation guidance for allocating GLP-1 therapies according to expected health benefits.

For health professionals and students, the direction is becoming clear: obesity care is moving towards more individualised assessment, interdisciplinary collaboration, and closer consideration of health outcomes rather than weight alone. These medicines can offer real benefits for appropriately selected patients, but they also carry risks, costs, and longer-term management considerations that warrant careful individual assessment.

“Adopting a new definition of any health issue matters because it shapes who receives care, how resources are allocated and how prevention strategies are designed,” says Associate Professor Hanna. “I feel like any adopted change adds value to both clinical decision-making and health equity.”