New clinical definition of obesity could help target care more effectively, saving time and money

The traditional way of assessing whether someone is overweight using body mass index (BMI) may fail to capture whether excess body fat is harmful and could be supplemented with a second measure when assessing patients, a new UCL study suggests.

In a study published in the International Journal of Obesity, the researchers used data from the 1970 British Cohort Study to see how nearly 7,000 46-year-olds compared in terms of their obesity levels according to both the traditional Body Mass Index (BMI) (an adult’s weight divided by their height squared) and the newer Lancet Commission on Clinical Obesity definition¹ which considers excess body fat alongside either tissue/organ functional impairment or substantial, age-adjusted limitations in daily living activities.

They found that about two thirds 69%) of people classified as living with obesity (>=30+ kg/m2) according to the BMI were not considered to have clinical obesity by the newer definition; by contrast, a smaller number (4%) who were considered a healthy weight by BMI thresholds showed signs of having clinical obesity.

Their study is the first to look at a wide group of people of a specific age, thereby ruling out factors that might be accounted for by age (such as that older people are naturally more likely to have organ damage).

The researchers are calling for clinicians and policymakers to consider “moving beyond BMI alone” when it comes to assessing the health implications of an individual’s weight, arguing that the traditional BMI measure does not give the full picture – and can even be very misleading.

They say the NHS could save time and money were the newer definition of obesity to used routinely by GPs and hospital doctors alongside the BMI and the former may also better identify those people at greater risk of obesity-related illness in the future.

Lead author Dr Jo Blodgett (UCL Division of Surgery and Interventional Science) said: “Midlife is a sensitive period when we consider trajectories of developing negative health outcomes. Using the newer Lancet classification in this age group may help identify those at greatest risk of poor health and who would benefit most from intervention. This has a lot of value from a policymakers’ perspective and could save the NHS and its staff time and money.

“Moving beyond BMI alone could better distinguish healthy people, who have high BMI due to their body type, from those who have underlying and clinically relevant health limitations or relevant organ dysfunction.

Co-author Dr John Mitchell (UCL Division of Surgery and Interventional Science) said: “The Lancet Commission definition also provides a more meaningful way for researchers to study who develops clinical obesity, how people transition from pre-clinical to clinical obesity, and which groups are at greatest risk of subsequent disease.”

The Lancet Diabetes & Endocrinology Commission on the Definition and Diagnostic Criteria of Clinical Obesity was established to resolve long-standing controversies over whether obesity is a distinct disease or merely a risk factor for other illnesses, and to move away from the limitations of using Body Mass Index (BMI) alone.

In January 2025, the Commission published its new Clinical Obesity definition in a report which was endorsed by 76 global medical and patient-advocacy organisations.

The UCL team applied the new 2025 clinical definition of obesity to 6,978 46-year-old UK adults and found substantial differences compared with traditional BMI categories.

The new framework classified 30% of their study’s participants as not having obesity, 53% as having a pre-clinical level of obesity and 18% as having clinical obesity. By comparison, BMI classified 29% as of a healthy weight, 38% as overweight and 32% as having obesity.

Importantly, the researchers said, among people classified as being obese using BMI (≥30 kg/m²), only 31% were deemed to have clinical obesity by the new definition, with the remaining 69% being classified as having pre-clinical obesity. The mismatch also worked in the other direction; among people considered overweight by BMI (25-30kg/m2), only 16.6% of those met the criteria for having clinical obesity, while 18% had no obesity, according to the new definition.

There was also some early evidence that the clinical definition may better distinguish future health risk. Over five years, people with pre-clinical and clinical obesity had approximately three and five times greater odds of death, respectively, than those without obesity, whereas BMI-defined overweight and obesity categorisations were not associated with mortality. However, there were only 27 deaths, so these mortality findings need to be interpreted cautiously and replicated in larger studies with longer follow-up.

Notes to Editors

For more information, including a copy of the research paper or to speak to the UCL authors, please contact Nick Hodgson, UCL Media Relations. T: +44 (0)7769 240209, E: nick.hodgson@ucl.ac.uk  

Joanna M Blodgett, John J Mitchell, Janine Makaronidis and Mark Hamer, Rethinking obesity: comparison of the new clinical obesity framework with traditional BMI-based classifications in midlife UK adults, will be published in the International Journal of Obesity at 01:00 Wednesday 11th November UK time/20:00 Tuesday 10th November US Eastern time and is strictly embargoed until that time.

The DOI number is 10.1038/s41366-026-02257-2.

¹ Cf Definition and diagnostic criteria of clinical obesity, The Lancet Diabetes and Endocrinology, January 14, 2025.

About University College London (UCL)

UCL is a global top 10 university, set up in London 200 years ago to offer education for all. Today, we gather 60,000 staff and students, from over 150 countries, to create a unique city within a city – a research and innovation powerhouse that leads the world in subjects spanning the arts, sciences, technology and the humanities. We’ve nurtured 33 Nobel Prize winners, because here, brave ideas have the scale and the support they need to succeed. We are University College London. And here, it can happen. 

UCL turns 200 in 2026. Join us for a year of bicentennial events and celebration. 

www.ucl.ac.uk