CURRENT BMI-based assessments lack accuracy and can overstate obesity-related health risks in some people while missing others at risk, according to new international research.
Lead by scientists at Kings College London, the study tested a new diagnostic framework proposed by a Lancet Diabetes & Endocrinology Commission in January 2025 that considers more nuanced factors when diagnosing obesity than current BMI measurements.
Designed to improve the identification of body fat as well as potential health risks, researchers found the new framework outperformed BMI, which may be causing millions of people to be misdiagnosed.
BMI, which takes into account only two measurements by dividing a person’s weight by the square of their height, ignores important factors such as body-fat amount or distribution – which are both key to assessing health risks like cardiovascular disease, diabetes or stroke.
The new Lancet diagnostic framework works in two steps, first by confirming excess body fat by supplementing BMI with measurements such as waist circumference and waist-to-height ratio.
In then distinguishes between whether that excess fat is impairing organ function or everyday activities, defined as clinical obesity, or if excess body fat poses increased health risks but organ and whole-body function remain preserved, known as preclinical obesity.
By analysing health data from 844,108 adults in China, the United Kingdom, the United States and Spain and comparing the two models, researchers found that BMI missed some people and overclassified others.
Across the study groups, it missed obesity-level fat in up to 8.6% of participants and classified approximately up to 3.1% as having obesity when they did not meet the combined measurement criteria.
Around a quarter to a third of people with obesity were estimated to have clinical obesity, with the proportion varying between study groups. This illustrates why a single obesity label can cover people with substantially different health needs.
Obesity was linked to a higher risk of death only when identified using waist measurements alongside BMI – not when using BMI alone
The groups also had different death risks during follow-up. Compared with participants without obesity under the combined definition and without organ dysfunction, clinical obesity was associated with a 69% higher mortality hazard, versus 19% higher for preclinical obesity.
Most people with preclinical obesity, approximately 77–91% depending on the cohort did not progress over a ten-year period, supporting the argument for assessing individual risk rather than assuming everyone will develop clinical obesity.
Professor Francesco Rubino, Senior Author of the study and chair of Metabolic and Bariatric Surgery at King’s College London, stressed the importance of the findings for “getting the scope of care right” and being able to offer appropriate, individualised care to those who need it.
He said: “Body mass index alone cannot tell us whether someone is already ill or at risk of illness, or what care they need, leaving people vulnerable to both undertreatment and overtreatment.
“The new definition makes personalised, proportionate care possible whereas a uniform BMI-based definition prevents it. People with obesity, like everyone else, deserve care matched to their needs: no less, and no more.”
Professor An Pan, first author of the study, from Huazhong University of Science and Technology in Wuhan, China, agreed that “meaningful diagnosis” is increasingly important and encouraged “global adoption of the new framework to improve obesity care and policy worldwide”.
“By moving beyond BMI alone, experts hope the new framework could help resolve a longstanding medical debate: whether obesity should always be considered a disease or only when it impairs a person’s health.
Featured image: Dmytro Flisak on AdobeStock
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