A new study suggests that replacing BMI-based obesity classifications with a broader clinical framework could change the diagnosis for millions of people, in both directions.

Published in Nature on Wednesday, the study analyzed health data from 844,108 adults in China, the United Kingdom, the United States and Spain, finding that BMI alone can misclassify people: it labeled 1.2 to 3.1 per cent of adults as obese without excess adiposity while missing excess adiposity in 5.2 to 8.6 per cent.

Lead author Francesco Rubino argues that BMI does not measure body fat or show whether a person’s organs are functioning properly, even though BMI thresholds can influence treatment access and coverage.

The proposed framework, developed in 2025, uses waist circumference and related measures to identify excess adiposity, then distinguishes clinical obesity—where it is linked to organ dysfunction or meaningful limitations—from preclinical obesity.

Possible clinical criteria include breathlessness caused by excess fat, heart failure, joint pain that limits movement, and difficulty with everyday tasks such as bathing or dressing.

Applying the combined criteria increased measured obesity prevalence by a net 3.3 to 7.4 per cent across the countries studied, while mortality risk was more closely associated with excess body fat than BMI alone.

The mortality findings remain preliminary: only 27 deaths occurred in the relevant analysis, and the researchers say larger studies with longer follow-up are needed to confirm them.

The study was observational, so it cannot establish that excess fat caused the deaths or show that care guided by the framework improves outcomes; differences between cohorts and the volunteer-based U.S. sample also limit how broadly the results may apply.

If adopted, the distinction could help clinicians match monitoring and treatment to individual risks, directing limited resources toward people with established organ dysfunction while offering risk-reduction support and monitoring to those with preclinical obesity.

The framework could affect access to weight-loss medicines or surgery, but the findings do not change current U.S. guidelines or, by themselves, justify changing medication.

The proposed framework includes 18 possible criteria, but assessing whether excess body fat caused a condition can be difficult and make diagnosis more time-consuming; experts say that effort may be worthwhile if it changes care.

The researchers say further work is needed before clinical use, including identifying which people with preclinical obesity are most likely to develop clinical obesity and validating ethnicity-specific thresholds in underrepresented populations.