Relying solely on body mass index (BMI) to diagnose obesity can lead to both over- and under-diagnosis, according to a large international study published in Nature. The research, which analysed health data from 844,108 adults in China, the United Kingdom, the United States, and Spain, adds weight to calls for a more nuanced approach to obesity care.
The study tested a framework proposed in January 2025 by a Lancet Diabetes & Endocrinology commission, which distinguishes between clinical obesity—where excess body fat impairs organ function or daily activities—and preclinical obesity, where excess fat increases health risks but normal bodily functions are preserved.
Using BMI alone, the researchers found that it missed obesity-level body fat in up to 8.6% of participants and incorrectly classified up to 3.1% as having obesity. Adding waist circumference and waist-to-height ratio to BMI assessments reduced these errors, but the authors argue that a broader diagnostic shift is needed.
“Body mass index alone cannot tell us whether someone is already ill or at risk of illness, or what care they need, leaving people vulnerable to both undertreatment and overtreatment,” said Professor Francesco Rubino, senior author of the study and chair of Metabolic and Bariatric Surgery at King’s College London.
Clinical vs. preclinical obesity: different outcomes
The study found that clinical obesity accounted for between 23% and 31% of all obesity cases, with an overall population prevalence of 5.6% to 10.9%. Importantly, the two categories were associated with markedly different health outcomes: clinical obesity was linked to a substantially higher risk of death, while preclinical obesity carried a more modest increase in risk.
Most people classified as having preclinical obesity at the start of the study—between 77% and 91% across the cohorts—did not progress to clinical obesity over a 10-year period. This suggests that many individuals with high BMI but no current health problems may not require immediate medical intervention.
“Preclinical obesity should therefore be neither treated automatically as active disease nor dismissed as harmless. Some people may need preventive treatment, but its intensity should match their individual risk,” Rubino added.
The findings have significant implications for healthcare systems across Europe, where obesity rates have been rising for decades. In countries like the UK, Germany, and France, BMI is often the primary tool used by general practitioners to assess obesity and decide on treatment pathways, from lifestyle advice to bariatric surgery.
The researchers argue that adopting the new framework could help clinicians better identify who needs treatment for existing disease and who may benefit from preventive interventions, rather than applying the same approach to everyone with a high BMI.
“The marked differences in prognosis between clinical and pre-clinical obesity, consistently observed across countries and ethnic groups, support global adoption of the new framework to improve obesity care and policy worldwide,” said Professor An Pan, first author of the study from Huazhong University of Science and Technology in Wuhan, China.
The study’s authors acknowledge that implementing the new framework will require changes in clinical guidelines and possibly new diagnostic tools. However, they stress that the potential benefits—more accurate diagnosis, better-targeted treatment, and reduced risk of both undertreatment and overtreatment—are substantial.
For European health systems already grappling with the burden of obesity-related diseases, the study offers a clearer path forward. As Rubino put it: “People with obesity, like everyone else, deserve care matched to their needs: no less, and no more.”
