The clinical obesity definition marks a major change in how healthcare professionals approach this condition. Experts from the Lancet Diabetes & Endocrinology Commission introduced this framework to address the weaknesses of body mass index (BMI) as a sole measure. This new approach focuses on excess adiposity and its impact on organ function, rather than just body size. It helps doctors make better decisions and shapes policies for treatment access. In this article, we examine the background, key elements, and wider effects on health systems and economics, with data from recent studies.

Understanding the Roots of Change

Obesity affects more than one billion adults worldwide, and costs healthcare systems around £1.6 trillion each year, as reported by the McKinsey Global Institute in 2022. For years, doctors used BMI—a simple calculation of weight divided by height squared—to spot obesity. A BMI over 30 kg/m² signalled the condition. Yet this method often misses the mark. It ignores differences in muscle mass, fat placement, and ethnic backgrounds. For instance, people of Asian descent face higher risks at lower BMI levels, around 25 kg/m².

The Commission, which included 58 specialists from various fields and countries, plus those with personal experience of obesity, developed the clinical obesity definition through detailed discussions and evidence review. They achieved full agreement on their proposals, with support from over 76 global groups. This definition views obesity as a result of excess fat that stems from multiple causes, including genetics and environment. It stresses that BMI works best for broad population studies or initial checks, not for personal health assessments.

Core Components of the New Framework

To confirm excess adiposity under the clinical obesity definition, clinicians must go beyond BMI. They can use direct tests like dual-energy X-ray absorptiometry scans if available. Otherwise, they combine BMI with at least one other measure, such as waist circumference (over 94 cm for men or 80 cm for women) or waist-to-hip ratio (above 0.90 for men or 0.85 for women). Adjustments account for age, sex, and ethnicity. For those with BMI above 40 kg/m², doctors can assume excess fat without extra steps.

Once excess adiposity appears, the framework splits into two stages. Preclinical obesity describes excess fat without harm to other body parts, yet it raises the chance of issues like type 2 diabetes or heart disease by 1.5 to 2 times, based on long-term data from the Framingham Heart Study. Clinical obesity, on the other hand, signals a serious illness. It occurs when excess fat impairs tissues, organs, or daily life. Signs include lab results showing poor function, such as high blood sugar levels, or limits in activities like walking or dressing, adjusted for age using tools like the WHO Disability Assessment Schedule.

A study by Stein and colleagues in JAMA Internal Medicine, published in October 2025, tested this clinical obesity definition on over 290,000 people from the UK Biobank. Over a follow-up of more than ten years, those with preclinical obesity had an 18% higher risk of death from any cause (hazard ratio 1.18, 95% confidence interval 1.14-1.23). For clinical obesity, the risk jumped to 56% (hazard ratio 1.56, 95% confidence interval 1.50-1.63). These results outpaced the predictions from BMI groups alone, proving the framework’s value.

Effects on Healthcare and Economics

This clinical obesity definition influences how systems handle care and costs. In the UK, where obesity links to 8-10% of health spending, the National Health Service could save £400 billion by 2040 through focused treatments that prevent heart issues, according to Global Burden of Disease estimates from 2024. Drugs like semaglutide, which target GLP-1 receptors, show strong results in clinical cases, adding 0.5 to 1.0 quality-adjusted life years, as per Institute for Clinical and Economic Review reports from 2023. However, applying the definition means more checks, which might raise initial expenses by 5-8% in places with limited resources.

Market access for treatments changes too. In Europe, health technology assessments now stress outcomes over simple metrics. The clinical obesity definition supports deals where payments link to improvements, like better blood sugar control. Globally, the anti-obesity drug market could hit £120 billion by 2030, but it splits into groups: most for lifestyle advice in preclinical stages, and advanced options for clinical needs. Still, without a category for moderate risks, some early actions might slip through, as seen in U.S. programmes that cover advice for BMI over 25.

Practical Steps Forward

Leaders in healthcare should update guidelines to include the clinical obesity definition, such as through the American Diabetes Association or European Society of Cardiology by 2026. Start with pilots that mix BMI screening and quick body measures. Tech like mobile apps can track progress remotely, aiding areas with few doctors.

For economics, companies and insurers need studies on costs using this definition. Set up tiered plans: no-cost talks for preclinical risks and covered medicines for clinical cases. Governments should also match obesity care with other long-term conditions.

Training matters greatly. Providers require education to cut bias and apply the clinical obesity definition fairly. Fund research on diverse groups to check the framework across backgrounds, adding cost measures like lost work time.

Wrapping Up the Shift

The clinical obesity definition from the Lancet Commission fixes BMI’s flaws with a focus on real health effects, backed by strong data on risks and wide support. It guides sharper care, cuts economic strains, and promotes fair treatment. As obesity grows, systems must adapt through better checks, linked payments, and bias training. Decision-makers should review the full report and join efforts to put these ideas into practice.

Source

Richman IB, Inouye SK. Moving Beyond BMI to Define Obesity. JAMA Intern Med. Published online October 27, 2025. doi:10.1001/jamainternmed.2025.4985