Provided by King's College London

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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 on the stomach that help people lose weight and treat health conditions, such as type 2 diabetes.

A study published in JAMA Network Open found that people considered for metabolic bariatric surgery 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 prioritization in the future.

New definitions reveal hidden differences

Led by Professor Francesco Rubino, chair of metabolic and bariatric surgery at King's College London, the researchers reviewed retrospective clinical data from 2,316 surgical candidates across four specialist centers—King's College Hospital in the UK and centers in France, Spain and Brazil.

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.

Of the surgical candidates, 73.8% had clinical obesity and 26.2% had preclinical obesity. Despite having similar BMI, patients with clinical obesity had far greater surgery-related and cardiovascular risks. They also had a higher overall risk of death.

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

"This study shows that the distinction between clinical and preclinical 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, senior author and chair of metabolic and bariatric surgery at King's College London.

He continued, "It is now essential that future surgical studies and registries systematically report patients' clinical or preclinical obesity status, so that surgical safety, effectiveness and cost-effectiveness can be interpreted in the appropriate clinical context."

A framework beyond BMI

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, and 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 center, 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 risks of death, cardiovascular disease and surgery-related complications.

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. Recognizing this distinction could help clinicians plan surgery, as well as select and prioritize surgical candidates.

More information: Simone Cremona et al, New Obesity Definition and Clinical Obesity Prevalence for Global Metabolic Bariatric Surgery, JAMA Network Open (2026). DOI: 10.1001/jamanetworkopen.2026.28395