An international study co-led by the Girona Biomedical Research Institute (IDIBGI) and the Jordi Gol Institute for Primary Care Research (IDIAPJGol) shows that body mass index (BMI), commonly used to classify obesity, may overlook people with excess body fat while identifying others who do not meet the traditional criteria for obesity. The findings indicate that incorporating measures such as waist circumference and waist-to-height ratio can improve classification and help identify people at greater risk to their health.
The research was jointly co-led with King’s College London (United Kingdom), Huazhong University of Science and Technology, Kailuan General Hospital and North China University of Science and Technology (China); Pennington Biomedical Research Center and the University of Colorado Anschutz Medical Campus (United States); and Università Cattolica del Sacro Cuore (Italy).
The study analysed data from 844,108 adults across four large cohorts in China, the United Kingdom, the United States and Spain. The Spanish sample included 51,269 participants from the Information System for the Development of Research in Primary Care (SIDIAP), a database containing clinical information from primary care in Catalonia. The study therefore highlights the contribution of Catalan health data to international research into the diagnosis and progression of obesity.
The findings show that classifying obesity solely on the basis of BMI can lead to both underdiagnosis and overdiagnosis. Depending on the cohort analysed, between 5.2% and 8.6% of participants had a BMI within the overweight range but showed waist measurements or waist-to-height ratios consistent with excess body fat, allowing them to be classified as having obesity. Conversely, between 1.2% and 3.1% were classified as having obesity according to BMI but did not meet the criteria based on the measurements considered in the study.
In the Catalan SIDIAP cohort, the estimated prevalence of obesity rises from 18.7% to 26.1% when the definition combining BMI with waist circumference and waist-to-height ratio is applied. This change highlights the importance of considering indicators beyond weight and height to identify excess body fat accurately.
The methodology follows the criteria proposed by a commission of The Lancet Diabetes & Endocrinology, which recommends supplementing BMI with other anthropometric measurements to confirm the presence of obesity.
Another key finding is the validation of the distinction between clinical and preclinical obesity proposed by this The Lancet Diabetes & Endocrinology commission. Clinical obesity is defined by the presence of excess body fat associated with impaired organ function or limitations in everyday activities. In preclinical obesity, by contrast, excess body fat is present, but organ and overall bodily functions remain intact.
A pooled analysis of the cohorts with available mortality data shows that people with clinical obesity have a 69% higher risk of all-cause mortality than the reference group. Among people with preclinical obesity, the increase is more moderate, at 19%.
Furthermore, longitudinal follow-up indicates that between 76.9% and 91.3% of people with preclinical obesity did not progress to clinical obesity over ten years of follow-up. These findings show that obesity does not have the same consequences for everyone and highlight the importance of distinguishing between the presence of excess body fat and the clinical impact it may have.
The findings reinforce the need for an approach to obesity that takes into account not only body measurements but also functional status and each individual’s risk. This approach could help improve the identification of people at greater risk of complications and guide prevention, monitoring and treatment strategies.
“In recent years, there has been considerable debate about whether obesity should be considered a disease. This study helps us better understand when excess body fat actually affects health and which signs can help us identify this impact. Recognising these signs can help us identify people who need priority medical attention and prevent potential health complications”, explains Dr Fernandez-Real, head of the Nutrition, Eumetabolism and Health Research Group at IDIBGI and CIBERObn, head of the Endocrinology Department at Dr Josep Trueta University Hospital, and professor and dean of the Faculty of Medicine at the University of Girona.
“Distinguishing between clinical and preclinical obesity should not mean delaying interventions for people who have not yet developed functional impairments. Classification should help guide personalised care, tailored to the risk of progression and the needs of each patient”, explains Dr Ramos, head of the Vascular Health Research Group at IDIBGI, researcher at IDIAPJGol, coordinator of the Girona IDIAPJGol-ICS Research Support Unit, professor at the Faculty of Medicine of the University of Girona, and coordinator of RICAPPS.
Pan, A., Yang, S., Ramos, R. et al. Validation of clinical and preclinical obesity across global cohorts. Nature (2026). https://doi.org/10.1038/s41586-026-11237-5
