New Delhi: Body mass index (BMI), the measurement widely used to assess obesity, could be misclassifying a significant number of people, according to a new study that found a more comprehensive approach was better at identifying those at greater risk of death.
Published in Nature on 11 November, the study analysed health records from 844,108 adults across China, the UK, the US and Spain.
The researchers found that BMI alone overestimated obesity in 1.2 — 3.1 per cent of participants and underestimated it in 5.2 — 8.6 per cent, depending on the population studied.
The findings lend support to a framework proposed in 2025 by an international commission of experts, which recommends assessing waist measurements and obesity-related health problems alongside BMI.
Why BMI may be misleading
BMI is calculated by dividing a person’s weight in kilograms by the square of their height in metres. A BMI of 30 or above is generally classified as obesity.
However, BMI cannot distinguish between fat and muscle or show where fat is stored in the body. This means it may not always provide an accurate picture of an individual’s body fat or associated health risks.
The new approach, called the combined anthropometric definition of obesity (CAD-obesity), uses BMI alongside waist circumference and waist-to-height ratio to identify excess body fat.
The latest findings “reinforce that BMI is not a good measure of obesity at the individual level and is not good in terms of predicting who is going to develop cardiometabolic disease in the long term,” Cátia Martins, an obesity researcher at the University of Alabama at Birmingham, told Nature.
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Clinical and preclinical obesity
The framework also distinguishes between two forms of obesity, depending on whether excess body fat has affected the body’s ability to function.
Clinical obesity refers to cases in which excess body fat is associated with impaired organ function or other health complications, including heart failure and sleep apnoea.
Preclinical obesity, on the other hand, describes people with excess body fat whose organs continue to function normally, although they may be at risk of developing obesity-related health problems.
“We did not know until now whether or not this new framework had implications in the real world and a prognostic value,” Francesco Rubino, a weight-loss surgeon at King’s College London and co-author of the study, told Nature.
The researchers found that people with clinical obesity had a 69 per cent higher risk of death during the follow-up period than those without obesity or organ dysfunction. Those with preclinical obesity had a comparatively modest 19 per cent higher risk.
What the findings mean for treatment
Applying the combined measurements increased the estimated prevalence of obesity by 3.3–7.4 percentage points across the four populations.
However, the researchers argued that distinguishing between clinical and preclinical obesity could help doctors prioritise treatment according to patients’ health needs.
People with clinical obesity may require more intensive treatment, while those with preclinical obesity could benefit from preventive interventions and regular monitoring.
The study was observational and relied partly on medical records, limiting researchers’ ability to establish whether organ dysfunction was directly caused by obesity.
Further research will be needed to determine how well the framework performs in routine clinical practice.
(Edited by Maryam Hassan)