January 17, 20254 min readBMI Sidelined in New Obesity Definition That Favors Health EvaluationInstead of using the controversial body mass index, or BMI, to assess weight, an international group of scientists proposes an approach that looks at how excess body fat affects healthBy Giorgia Guglielmi & Nature magazine A healthcare professional administers a cardiology test with electrodes and blood pressure cuff. Current methods for diagnosing obesity dont capture the complexity of the condition, say researchers. Goran Bogicevic/Alamy Stock PhotoAmid the rising buzz around Ozempic and similar weight-loss drugs, a group of 58 researchers is challenging the way obesity is defined and diagnosed, arguing that current methods fail to capture the complexity of the condition. They offer a more nuanced approach.The groups revised definition, published in The Lancet Diabetes & Endocrinology on 14 January, focuses on how excess body fat, a measure called adiposity, affects the body, rather than relying only on body mass index (BMI), which links a persons weight to their height. They propose two categories: preclinical obesity, when a person has extra body fat but their organs work normally, and clinical obesity, when excess fat harms the bodys organs and tissues.This shift could improve clinical care, public-health policies and societal attitudes toward obesity, says Elisabeth van Rossum, an endocrinologist at the Erasmus University Medical Center Rotterdam in the Netherlands.On supporting science journalismIf you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.Now the idea is, eat less, move more, and youll lose weight, says van Rossum, who wasnt involved in the work. Although a healthy lifestyle is important, she adds, if it would be so simple, we wouldnt have an epidemic, and this paper is an excellent contribution to the discussion about the complexity of obesity.Global problemMore than 1 billion people worldwide live with obesity, and the condition is linked to about 5 million deaths every year from disorders such as diabetes and cardiovascular disease.Because it is easy to measure and compare, BMI has long been used as a tool to diagnose obesity. But it doesnt offer a full picture of a persons health, because it doesnt account for differences in body composition, such as muscle versus fat.For people of European descent, obesity is typically defined by a BMI of 30 or higher, which correlates with a high level of body fat. However, a muscular athlete might be labelled obese on the basis of BMI, whereas someone with a normal BMI might have excess fat that increases their risk of heart problems or other serious health issues, says Francesco Rubino, a bariatric surgeon at Kings College London, who led the group proposing the new approach.Conventional methods lead to unnecessary treatment for some people while missing others who need help, he says. To address this, Rubino and his colleagues propose a system for diagnosing obesity that goes beyond BMI, combining it with other methods such as measuring waist circumference, which is a proxy for adiposity, or body scans using low-level X-rays, which can directly measure fat mass.Although there isnt a fixed threshold for obesity, body fat is typically considered to be in excess when it is above 25% in men and 3038% in women. Because measuring adiposity directly might be impractical or costly, alternative health markers such as waist size, waist-to-hip ratio or waist-to-height ratio are important, the researchers say. However, they add, its safe to assume that a person with a BMI above 40 has high body fat.Diagnosing obesity should also consider the results of standard laboratory tests, medical history, and information on daily activities to assess how excess body fat might affect a persons health, says study co-author Robert Eckel, an endocrinologist at the University of Colorado Anschutz Medical Campus in Aurora. These are objective diagnostic criteria, theyre standardized across global health systems, he says.Personalized assessments that consider age, gender and ethnicity are equally important, because certain groups might face health risks at lower BMI thresholds than others, says study co-author Louise Baur, a paediatrician at the University of Sydney, Australia.Two categoriesAlthough people with preclinical obesity have tissues that work normally, they are at a higher risk of developing health problems than are people without obesity. They can benefit from counselling and preventive measures such as lifestyle changes to reduce their risk of developing more serious health issues, the group says.Clinical obesity occurs when excess fat harms organs or it seriously limits daily activities such as walking or getting dressed. People with clinical obesity might require treatments aimed at improving health and preventing complications.Rubino notes that this approach is especially important with the growing use of weight-loss drugs, because it helps to provide more accurate diagnoses and make treatment effective and cost-efficient.Stigma challengeThe approach, which has been supported by dozens of scientific and patient-advocacy groups worldwide, reflects increasing evidence of obesitys effects on health, van Rossum says. Another framework, published last year, also recognized that the obesity diagnosis should go beyond BMI to include an assessment of its impacts on health.However, many health-care providers still see obesity as an issue of willpower rather than a disease driven by excess fat, regardless of its underlying causes which may include hormonal changes and genetic factors, van Rossum says. In the Netherlands, for example, a study she co-authored found that most people with obesity had experienced stigma in a health-care setting. This stigma often prevents people from seeking care, highlighting the need for better education and communication about the condition.Although the latest approach aims to give a more accurate picture of obesity, its unclear whether it will lead to more or fewer diagnoses, or how it will affect management of the condition in the clinic. With time, we hope that this sort of assessment will be included in clinical practice guidelines, Baur says.This article is reproduced with permission and was first published on January 14, 2025.