Study tests two-step method to diagnose obesity beyond BMI

A large international study published in Nature has tested a new way of diagnosing obesity that does not rely on body mass index (BMI) alone. According to New Scientist, the approach can separate people who are harmed by excess fat and need treatment from those who do not need intervention.

BMI divides weight by height squared. It cannot distinguish fat from muscle, show where fat sits in the body, or show whether excess fat is harming health. The study tested a framework proposed by a 2025 Lancet Commission.

The framework has two steps. The first uses waist circumference and waist-to-height ratio alongside BMI to establish whether someone has excess body fat. The second checks for organ dysfunction across several systems, including heart failure, chronic obstructive sleep apnea, osteoarthritis and chronic kidney disease.

People with excess fat and organ dysfunction are diagnosed with clinical obesity. Those with excess fat but intact organ function are diagnosed with preclinical obesity.

The researchers analysed health data from 844,108 adults across four cohorts in China, the UK, the US and Spain. Between 1.2 and 3.1 per cent of adults were diagnosed with obesity by BMI but do not have it under the new framework. Between 5.2 and 8.6 per cent had excess adiposity that BMI alone missed. Mortality risk was better predicted by excess body fat than by BMI alone.

Clinical obesity made up about 23 to 31 per cent of obesity cases, with a population prevalence of 5.6 to 10.9 per cent. People with clinical obesity had substantially higher mortality than those with preclinical obesity. Over 10 years, around 10 to 30 per cent of people with preclinical obesity developed clinical obesity.

Francesco Rubino of King's College London said: "In countries with large populations, you're talking about misdiagnosing tens of millions of people – this is not a trivial difference." He also said: "These measures of adiposity in the abdomen are more relevant from a clinical standpoint, because we know that fat has an impact on health."

The findings could let clinicians use simple interventions and monitoring for lower-risk people, and intensive treatment for those at greater risk or with organ dysfunction. The study adjusted for lower BMI and waist thresholds used for some ethnic groups, such as South and East Asian populations. Sonia Anand of McMaster University said: "Further validation in South Asian, Indigenous and other underrepresented populations using incident diabetes, cardiovascular disease and organ-specific outcomes is still needed." The researchers acknowledged that their cohorts used diagnostic codes as proxies for organ dysfunction rather than clinical assessments.