A multinational study published on Wednesday in Nature confirms that evaluating obesity using body mass index alone misclassifies millions of individuals worldwide by missing excess harmful body fat in some patients while incorrectly labeling others.
Led by King’s College London, an international research team analyzed health records from 844,108 adults across the United Kingdom, the United States, China, and Spain. The evaluation tested a diagnostic approach originally proposed in a Lancet Commission report.
Traditional screening relies solely on body mass index, calculated as weight in kilograms divided by height in meters squared. Under historical standards, individuals of European descent with a BMI of 30 or higher are categorized as obese. “BMI misses a lot of information,” Dr. Francesco Rubino, a professor of bariatric and metabolic surgery at King's College London, told Live Science.
The validated framework addresses these gaps by combining BMI thresholds with targeted physical measurements. Doctors check patients with lower BMIs for excess adiposity by examining waist circumference, waist-to-hip ratio, and waist-to-height ratio. Because different ethnicities experience distinct fat distribution patterns, the study incorporated ethnicity-specific waist circumference cutoffs.
Once excess body fat is identified through body measurements, the diagnostic model separates patients into two distinct categories based on organ function and daily activities. “Clinical obesity” is defined as an active disease state where excess fat tissue impairs organ function or daily tasks, encompassing up to 18 obesity-related impairments such as heart failure, sleep apnea, and joint pain.
“Preclinical obesity” describes individuals who meet the body size criteria but maintain intact organ function and no daily functional impairments. Researchers tracked participants for up to 15 years to evaluate mortality and progression rates among these cohorts.
The study found that clinical obesity was associated with a 69% greater death risk than people with neither obesity nor organ dysfunction. Conversely, preclinical obesity carried only a modest increase in risk, and 70% to 90% of individuals with preclinical obesity did not progress to active clinical disease over a 10-year period.
When researchers applied the combined measurements to population data, they uncovered substantial discordance with standard BMI assessments. The uniform BMI scale was found to underestimate obesity in 5.2–8.6% of participants while overestimating the condition in 1.2–3.1%.
In the UK alone, researchers estimated that approximately 2.4 million people possess dangerous levels of body fat that standard BMI fails to detect, while an estimated 1.3 million people classed as obese carry no excess fat or excess risk.
“we are definitely missing some people who actually need obesity treatment but just did not meet the BMI criteria,” said Dr. Jennifer Hwang, an instructor of medicine at the University of Chicago.
| Diagnostic Comparison | Underestimated / Hidden | Overdiagnosed |
|---|---|---|
| Global Cohort Range | 5.2–8.6% | 1.2–3.1% |
Proponents of the diagnostic framework argue that personalized assessment allows healthcare providers to allocate resources more efficiently and tailor interventions—ranging from lifestyle modifications to GLP-1 weight-loss medications and bariatric surgery—to individual patient risk profiles.
This misclassification can account for tens of millions of people, so the inaccuracy of BMI is a big problem at scale and also for individuals.
Rubino
While critics have expressed concern that distinguishing a preclinical state could lead to restrictive treatment access or delayed care, study authors emphasized that diagnostic classification determines health status rather than automatically withholding therapy. Researchers suggest that adopting the framework in clinical settings could optimize patient interventions and reduce overall system costs.
Earlier coverage: Nature Study Finds BMI Misclassifies Millions of Adults Globally
