Millions of people in the United Kingdom may have been wrongly classified as overweight or obese because of limitations in the widely used Body Mass Index (BMI) measurement, a new international study has found.

The research suggests that relying on BMI alone could leave some people with potentially dangerous levels of body fat unidentified, while others may be labelled obese despite having no additional health risks associated with excess body fat.

Findings from the study indicate that approximately 2.4 million people in the UK could have obesity-level body fat that standard BMI assessments fail to detect.

At the same time, an estimated 1.3 million people classified as obese using BMI may not have excess body fat or the corresponding health risks.

BMI is commonly used by healthcare professionals to estimate whether an individual’s weight falls within a healthy range based on their height. However, the measurement does not directly establish how much body fat a person carries or where that fat is distributed.

The international research team, led by King’s College London, examined health information from 844,108 adults across the UK, China, the United States and Spain, including more than 430,000 participants from the UK Biobank study.

Researchers compared conventional BMI assessments with a broader approach that incorporated additional measurements, including waist circumference and waist-to-height ratio.

The findings showed that combining these measurements provided a more accurate picture of excess body fat, revealing cases that BMI alone either overlooked or incorrectly classified.

The researchers also distinguished between two forms of obesity. Clinical obesity refers to excess body fat that has begun to interfere with normal organ function or everyday activities, while preclinical obesity describes a condition in which excess body fat presents potential health risks without causing evident impairment of normal bodily functions.

Professor Francesco Rubino of King’s College London, who led the research, argued that BMI thresholds should not be treated as a definitive measure of an individual’s health status.

He said a person’s BMI does not establish their actual body-fat composition or determine whether their organs are functioning properly, raising concerns about the use of the measurement as the sole basis for medical decisions.

The findings could have implications for how obesity is diagnosed and how patients are assessed for treatment, particularly as BMI classifications are sometimes used to determine access to healthcare interventions.

Published in the journal Nature, the research supports a more individualised approach to obesity assessment, with experts suggesting that body-fat distribution and its impact on physical health should be considered alongside traditional weight measurements.