New evidence could transform how doctors assess obesity.
LONDON, United Kingdom. An international study involving 844,108 adults has found that body mass index (BMI) alone can misclassify obesity, potentially leading to inappropriate medical decisions. Published in Nature, the research supports a more personalized diagnostic approach that considers body fat distribution, organ function and individual health risks.
Led by researchers at King’s College London in collaboration with international institutions, the study examined populations in China, the United Kingdom, the United States and Spain. It tested a diagnostic framework proposed in 2025 by a commission published in The Lancet Diabetes & Endocrinology.
Unlike conventional BMI-based classification, the proposed framework distinguishes between clinical obesity, in which excess body fat affects organ function or everyday activities, and preclinical obesity, where excess body fat increases health risks without evidence of existing functional impairment.
The findings revealed significant limitations in relying exclusively on BMI, which calculates body weight relative to height but cannot distinguish muscle from fat or determine how excess adiposity affects health.
Across the four populations, BMI alone underestimated obesity in 5.2% to 8.6% of participants and overestimated it in 1.2% to 3.1%. Adding waist circumference and waist-to-height ratio improved the identification of individuals with excess body fat.
Researchers found that clinical obesity represented approximately 23% to 31% of obesity cases. Importantly, the two categories were associated with different health outcomes, with clinical obesity linked to substantially higher mortality risk.
The study also reported that between 77% and 91% of participants initially classified with preclinical obesity did not progress to clinical obesity over a decade. This finding highlights differences in disease progression, although it does not mean preclinical obesity is harmless or that preventive care is unnecessary.
Professor Francesco Rubino, a senior author from King’s College London, emphasized that medical care should reflect individual needs rather than rely solely on standardized weight classifications.
The implications could extend to clinical guidelines, preventive healthcare and decisions about access to treatment. A more comprehensive assessment may help physicians distinguish patients requiring treatment for existing complications from those who would benefit primarily from risk monitoring and preventive interventions.
However, the findings do not suggest abandoning BMI entirely. Instead, researchers advocate combining conventional measurements with additional clinical indicators to improve diagnostic accuracy.
The study provides evidence for reconsidering how obesity is defined and managed internationally. Its broader significance lies in shifting medical assessment from body weight alone toward measurable effects on health, potentially supporting more precise and individualized care.
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