Obesity needs a new clinical definition, study finds

Assessing whether someone is overweight or obese using body mass index (BMI) scores is a blunt tool that stands in the way of targeting care effectively, according to UK researchers.

The University College London (UCL) team found that relying only on BMI – a simple ratio of height and weight – fails to capture whether excess body fat is actually harmful, and could be improved with additional measures such as tissue or organ functional impairment or age-adjusted limitations in daily living activities.

While BMI is useful as a quick screening tool for large populations, a more refined measure could help direct individualised treatments, such as weight-loss medications and bariatric surgery, which could in turn help health systems save time and money.

The new study, published in the International Journal of Obesity, looked at data on 7,000 people from the 1970 British Cohort Study at the age of 46, comparing BMI results with a refined measure – the Lancet clinical obesity framework – that incorporates excess adiposity, functional limitations, and organ dysfunction.

Published last year, the new definition and diagnostic criteria distinguish between preclinical obesity – excess body fat that does not yet cause organ dysfunction or health issues but poses a future risk – and clinical obesity that has already led to health complications like high blood pressure, joint pain, and sleep apnoea.

The UCL researchers found that around two-thirds (69%) of people classified as living with obesity because they had a BMI of 30 kg/m2 or more were not considered to have clinical obesity under the Lancet criteria.

At the same time, a smaller number of individuals (4%) who were considered a healthy weight by BMI thresholds showed signs of having clinical obesity.

They conclude that the NHS deliver better care for patients if the newer definition of obesity were used routinely by GPs and hospital doctors alongside the BMI, and may also better identify those people at greater risk of obesity-related illness in the future.

"Midlife is a sensitive period when we consider trajectories of developing negative health outcomes," said lead author Dr Jo Blodgett of UCL's Division of Surgery and Interventional Science.

"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," she added. "This has a lot of value from a policymakers’ perspective and could save the NHS and its staff time and money."