Many people could be incorrectly classified as having, or not having, obesity because of the widespread measure of body mass index (BMI) as the sole metric, according to a research study.
Led by researchers at King’s College London and published in Nature, the study found that combining BMI with waist measurement could provide a more accurate diagnosis.
This approach could also help healthcare professionals distinguish between people at risk of obesity-related illness and those already experiencing health complications, the researchers argue.
Health records from 844,108 adults across the UK, US, China and Spain, were analysed, comparing conventional BMI-based classifications with a proposed diagnostic framework.
The findings suggested that BMI alone underestimated obesity in 5.2% to 8.6% of participants across the study populations, and incorrectly classifying between 1.2% and 3.1% as having obesity.
Professor Francesco Rubino, senior author of the study and Chair of Metabolic and Bariatric Surgery at King’s College London, highlighted the limitations of using BMI as an individual diagnostic measure.
Combining BMI with waist measurement
Traditionally, a BMI of 25-29.9 is classified as overweight, while a BMI of 30 or above indicates obesity.
Calculated by dividing a person’s weight in kilograms by height in metres squared, BMI does not directly measure body fat or identify where fat is distributed in the body.
However, the combined anthropometric definition of obesity (CAD-obesity) uses BMI alongside waist circumference and waist-to-height ratio to improve the identification of excess body fat.
Under the framework, a BMI of 40 or above indicates obesity. At lower BMI levels, additional waist measurements are used to confirm excess body fat. The framework also accounts for differences in waist measurements across ethnic groups.
Distinguishing clinical and pre-clinical obesity
The diagnostic approach used in the research, first proposed by a Lancet Commission in 2025, divides obesity into two categories: clinical and pre-clinical obesity.
- Clinical obesity refers to a condition in which excess body fat is already impairing organ function or everyday activities.
- Pre-clinical obesity describes excess body fat that increases the risk of future health problems but has not yet caused organ dysfunction.
The study found that 23%-31% of people were classified as having obesity, representing 5.6%-10.9% of the wider study populations.
Between 77% and 91% of people with pre-clinical obesity did not progress to clinical obesity over the following 10 years.
The study found that clinical obesity was associated with a substantially higher risk of death, while the latter carried a more modest increase in risk.
Implications for treatment and patient care
BMI alone cannot establish whether an individual is already unwell, at risk of illness or what level of care they require, said Professor Rubino, saying that the proposed framework could support more personalised treatment decisions, including lifestyle interventions, GLP-1 weight-loss medicines and bariatric surgery, depending on individual health needs.
“We can optimise our interventions and ultimately make better use of our resources with more safety and efficacy,” he said.
“If you ask if I think this should be employed in the clinic, I think absolutely, yes” he added.
Wider adoption of the framework could improve obesity care and health policy internationally, the researchers suggested.
