Obesity: Only 27% of Diagnosed Individuals Have Health Issues Associated with the Condition

A diagnosis of obesity should go beyond an assessment based solely on Body Mass Index (BMI), as excess weight does not automatically mean that a person is already suffering from conditions associated with this state. A study published in the journal Nature, which analyzed data from over 844,000 adults in China, the United Kingdom, the United States, and Spain, shows that approximately 27% of individuals with obesity present with the clinical form of the condition, characterized by organ dysfunction. The research advocates for a revision of diagnostic criteria, distinguishing between "preclinical" obesity-where excess fat acts as a risk factor-and "clinical" obesity, where associated health problems are already present. These findings align with a proposal put forward in 2025 by an international commission of experts, which recommends a more comprehensive assessment than simply relating weight to height.

• Body Mass Index does not provide a complete picture

The study published in Nature, titled "Validation of clinical and preclinical obesity across global cohorts," analyzed data from 844,108 adults drawn from four major medical and research databases. The authors compared the traditional classification of obesity-based on BMI-with a system that also incorporates waist circumference, the waist-to-height ratio, and the identification of organ dysfunction. According to researchers, relying exclusively on BMI can lead to both the overestimation and underestimation of the number of individuals with obesity. Depending on the population analyzed, between 1.2% and 3.1% of participants were classified as obese based on BMI, even though additional measurements did not confirm a corresponding level of adiposity. Conversely, between 5.2% and 8.6% of participants had body fat levels consistent with obesity, yet were not identified as such based on criteria relying solely on BMI. When additional measurements were incorporated, the prevalence of obesity increased across all four populations studied-by margins ranging from 3.3% to 7.4% compared to the traditional classification. These findings demonstrate that BMI does not always accurately reflect the distribution of body fat or its impact on the body.

• Preclinical and Clinical Obesity

The new assessment framework proposes two distinct categories. Preclinical obesity is characterized by an excess of body fat, without evidence that it has yet caused organ dysfunction. Clinical obesity, by contrast, entails the presence of health impairments linked to excess adiposity. To establish a diagnosis, researchers recommend combining BMI with measurements such as waist circumference and the waist-to-height ratio. The assessment should also screen for potential organ-related issues-including heart or liver conditions-that may be associated with excess body fat. According to the study, clinical obesity accounts for between 22.7% and 31.3% of all obesity cases across the four populations analyzed, with an average of approximately 27%. Within the entire adult population, the proportion of individuals with clinical obesity was estimated to be between 5.6% and 10.9%. Francesco Rubino, the study's lead author and a professor of metabolic and bariatric surgery at King's College London, told Reuters that the limitations of BMI-based measurements have long been recognized, yet until now, no better assessment system has existed. This new framework could provide a more clinically relevant characterization of obesity, which would be useful both for patient care and for the development of health policies.

• Most preclinical cases did not progress to the clinical form

Another significant finding of the research concerns the progression of individuals diagnosed with preclinical obesity. Across the four populations studied, between 77.4% and 91.3% of those initially classified in this category did not develop clinical obesity during a follow-up period of up to ten years. The average across these findings is around 82%. Progression rates were higher in China and Spain than in the United Kingdom. For the United States population, the available follow-up period was shorter-four years-which necessitates caution when making direct comparisons. These results do not imply that preclinical obesity is free of risks or that it does not require monitoring. However, the study shows that individuals with excess body fat do not all experience the same level of health impairment, and that risk assessment should take into account the functional state of the body.

• A shift with implications for healthcare systems

The World Health Organization estimates that more than one billion people worldwide are living with obesity. The condition is associated with a higher risk type 2 diabetes, cardiovascular diseases, liver and kidney conditions, as well as certain forms of cancer.

Distinguishing between preclinical and clinical obesity could influence how prevention and treatment priorities are set, as well as how medical resources are allocated. Individuals with organ damage might benefit from more rapid and complex interventions, whereas for those without identified dysfunctions, the focus could be on monitoring and preventing the progression of the disease. However, the research does not demonstrate that adopting these new criteria will automatically reduce complications or healthcare system costs. Experts cited by the Science Media Centre point out that while the findings support the value of a more comprehensive assessment, further evidence is required to demonstrate the impact of applying this model on patient treatment and outcomes. The proposed shift moves the focus away from simply placing an individual into a weight category, toward evaluating the concrete effects of excess fat on their health.