New obesity definitions reveal ill health more accurately than BMI

An international study led by King’s College London, UK, has found that proposed new obesity definitions give greater insight into the ill health of metabolic bariatric surgery (MBS) candidates, compared with Body Mass Index (BMI). The study, found that people considered for MBS had marked variation in disease burden – the number and severity of diseases – and operation-related risks, despite having similar BMI.

However, by reviewing clinical data from thousands of bariatric surgery candidates and applying new obesity definitions, the researchers were able to better understand their underlying health – which could have important implications for patient treatment and surgery prioritisation in the future.

The research, led by Professor Francesco Rubino senior author and Chair of Metabolic and Bariatric Surgery at King’s College London, reviewed retrospective clinical data from 2,316 surgical candidates across four specialist centres – King’s College Hospital in the UK, and centres in France, Spain and Brazil.

Francesco Rubino

“This study shows that the distinction between clinical and pre-clinical obesity is clinically meaningful even among surgical candidates with very high BMI levels, because BMI alone cannot tell us who has active disease,” said Rubino. “It is now essential that future surgical studies and registries systematically report patients’ clinical or pre-clinical obesity status, so that surgical safety, effectiveness and cost-effectiveness can be interpreted in the appropriate clinical context.”

They applied the new obesity definitions: clinical obesity, where there is clear evidence that excess fat, or adiposity, is causing organ damage, and preclinical obesity, where organ function is preserved despite excess adiposity.

From the surgical candidates, 73.8% had clinical obesity and 26.2% had preclinical obesity. Despite both groups of patients having similar BMI, patients with clinical obesity had far greater surgery-related and cardiovascular risks. They also had an overall high chance of death.

These findings suggest that distinguishing between clinical and preclinical obesity reveals key information about the health status and risks to surgery candidates that BMI alone cannot capture.

Traditionally, obesity has been classified primarily using BMI, a measure of weight relative to height that provides limited information about whether excess fat is actually affecting a person’s health. BMI levels have historically played a central role in determining eligibility and priority for metabolic bariatric surgery – operations of the stomach that help people lose weight and fix health issues, such as type 2 diabetes.

In 2025, the Lancet Diabetes & Endocrinology Commission on Clinical Obesity proposed a new diagnostic framework. This distinguished between clinical obesity, where there is clear evidence that excess fat, or adiposity, is leading to organ dysfunction, from preclinical obesity, where organ function is preserved but future health risk is increased.

In the latest study, the researchers found that these differences in disease status were not reflected in BMI. In the UK centre, for example, BMI was roughly 47.5 among patients with clinical obesity and 48.5 among those with preclinical obesity. Yet those with clinical obesity were approximately 10 years older and had substantially higher risk of death, cardiovascular risk and operation-related risk.

The researchers argue that for patients with clinical obesity, surgery primarily represents treatment of established disease. On the other hand, for those with preclinical obesity, its key goal may instead be to reduce future health risk. Recognising this distinction could help clinicians plan surgery, as well as select and prioritise surgical candidates.

The findings were published in the paper, ‘New Obesity Definition and Clinical Obesity Prevalence for Global Metabolic Bariatric Surgery’, published in JAMA Network Open. To access this article, please click here