Obesity is often described as one of the most pressing public health challenges facing society. Defining it, however, is far less clear.

Even among experts, there is no single, universally accepted way to diagnose the condition—whether it should be based on body size, fat distribution, metabolic consequences, or some combination of all 3.

But as global prevalence continues to rise, with more than a billion people now estimated to be living with obesity, and as increasingly effective weight loss medications reshape the treatment landscape, accurate diagnosis is more important than ever. How obesity is defined can influence who qualifies for treatment, what insurers cover, and how clinicians prioritize care.

Last year, a global commission convened by editors of The Lancet Diabetes & Endocrinology published a new diagnostic framework that moves beyond body mass index (BMI) by incorporating more refined assessments of excess adiposity and distinguishing between preclinical and clinical obesity. Rather than advancing the field toward consensus, however, the proposal has reignited debate among experts.

At the center of the discord is, interestingly, an area of agreement: a broad recognition that BMI, a metric that has been used for decades to estimate a person’s body fat based on weight and height alone, is insufficient.

Although the simplicity of BMI has cemented its position as the dominant obesity screening tool, some experts have deemed it too blunt an instrument for the examination room, where clinical care demands greater precision based on a patient’s specific profile.

“As a clinician, I treat one patient at a time, but when you look at it from a population perspective, it’s an entirely different ask that requires us to think about the availability of resources, treatment costs, and who should receive those interventions,” said endocrinologist Beverly Tchang, MD, an assistant professor at Weill Cornell Medicine who is also a spokesperson for The Obesity Society. “What we’ve done historically has been to prioritize more severe disease, but we don’t have any validated metrics to fairly compare a person with a BMI of 26 and complications to a person with a BMI of 32 and no complications.”

Whereas the commission suggests a more nuanced approach to such variability, critics caution that adding further complexity to an already heterogeneous disease with more than 200 complications could inadvertently narrow who is identified and lead to delays in care.

“We all agree that BMI is problematic in many ways,” said Sohail Zahid, MD, PhD, a cardiology fellow at Johns Hopkins University School of Medicine who coauthored a Viewpoint in JAMA on translating new obesity definitions, including the commission’s, into practice. “But we need to come together with a consensus, so we can recommend to patients, providers, clinicians, and insurance companies the best way forward. The more we debate these issues, the more we delay making real changes.”

Obesity as a Risk or a Disease?
Central to its framework, the commission proposed a shift in how obesity is defined.

“Most diseases are approached from the bottom up, but with obesity, it’s the reverse…and has been understood from the top down as a spectrum of manifestations at the population level,” said Francesco Rubino, MD, commission chair and professor of metabolic and bariatric surgery at King’s College London. He acknowledged that those within the commission—a global group of experts that includes people with obesity—disagreed over whether to define it as a risk factor or a disease.

The framework ultimately landed on 2 new categories: preclinical obesity, in which a threshold of excess body fat is met but complications have not yet developed, and clinical obesity, characterized by chronic illness—such as heart failure, hypertension, or obstructive sleep apnea—in which adiposity directly contributes to ongoing organ dysfunction.

It’s in this distinction that multiple organizations took immediate issue. The Obesity Medicine Association argued that reclassifying individuals as preclinical could undermine their ability to receive treatment, and the European Association for the Study of Obesity voiced concern that the preclinical category promoted a “watchful waiting” approach that may worsen long-term health outcomes. Further, the Endocrine Society, an international group of scientists and clinicians, published an article in the Journal of Clinical Endocrinology & Metabolism that criticized the framework’s causation requirement for clinical obesity as well as its exclusion of type 2 diabetes, one of the most common and consequential complications of excess adiposity, from the criteria for clinical obesity.

To Rubino, it was imperative to “not conflate the diagnosis of 2 distinct diseases,” but Ranganath Muniyappa, MD, PhD, a coauthor of the Endocrine Society article, disagreed.

“If there is one disorder we know is strongly linked to obesity, it’s type 2 diabetes,” said Muniyappa, a senior clinician who studies endocrine disorders at the National Institute of Diabetes and Digestive and Kidney Diseases at the National Institutes of Health. “If you lose weight, you get remission of diabetes, so if you are basing a framework on causal attribution, I don’t think there is a more causally related disorder than this, and they left it out.”

Muniyappa also pointed to other societies that have “gotten into trouble” with preclinical labels, citing how the American College of Cardiology and the American Heart Association ultimately replaced the term prehypertension with stage 1 hypertension to promote earlier interventions. However, terms like prediabetes are still widely used.

“If you call something preclinical,” he cautioned, it might translate to delays in care. “We have medications that can actually treat preclinical obesity transitioning to diabetes that prevent all the complications associated with obesity, but they may say, ‘This is not something that needs to be acutely addressed yet.’”

Rubino considered such rebuttals a “complete misrepresentation” of the commission’s intent.

“It’s not saying ‘Don’t treat the risk’—it’s saying ‘Treat the risk as risk and treat the disease as a disease,’” said Rubino, who noted that such care is often discordant. A person with preclinical obesity may be pushed toward bariatric surgery while a person with clinical obesity and chest pains may be tasked with making lifestyle changes.

Ultimately, he explained, the distinction is similar to polyps: “Nobody says polyps are innocent or you should disregard them. You need to investigate each one to decide if you will want to remove it because it will likely progress to cancer or take a different approach because it’s stable and benign. Just because there is a risk of cancer doesn’t mean you should treat everything as cancer.”

Instead, Rubino said, the goal should be to “deploy proportionate care according to individual-level need.”

Beyond BMI
Another common criticism of the new framework, which has been endorsed by more than 75 international medical organizations, is its feasibility in routine clinical practice.

Although the Endocrine Society authors commended the commission for undertaking “the complex task” of moving beyond tools designed for population-level surveillance, Muniyappa is not optimistic that tape measures or body scanners can replace BMI, even in individualized care.

“These other anthropometric measures are not standardized, and most primary care doctors lack the time, training, and infrastructure to implement them,” he said.

To Rubino, such a criticism showcases the “drift” that obesity care has taken from other medical disciplines.

“A neurologist wouldn’t take any tremor as Parkinson’s or any dementia as Alzheimer, but we are used to measuring obesity in a second,” he said. “You enter your weight and height in an app, and it tells you. If you consider that akin to clinical diagnosis, then of course anything else will be cumbersome.”

Although Zahid considers the logistical roadblocks of carrying out an obesity assessment to be “the main limitation in the Lancet definition,” he acknowledges the notion that screening simplicity comes at the expense of diagnostic accuracy.

“In heart failure, we don’t just measure your echocardiogram or quantify how much plaque is in your arteries,” he said. “We ask how much you can do on a daily basis or how much pain you are in.”

But that’s not standard care for obesity, particularly for patients who fall outside traditional BMI thresholds, often based on muscle density or race and ethnicity.

“To capture them, it requires a head-to-toe examination to determine whether it’s causing you headaches or problems with your blood sugar, cholesterol, or blood pressure,” Zahid said. “So, the question is, is that too much work?”

A Billion Implications
The debate over how to define obesity is less abstract for clinicians who must continue to make real-time decisions about screening, diagnosis, and treatment—often with imperfect tools.

In her practice, Tchang said, “Just asking patients ‘How has this affected your life?’ probably captures everything we need.” Zahid similarly tells his patients he cares less about their weight than how it’s causing them problems day to day.

Although he acknowledges that more nuanced diagnostic approaches could better identify misclassified patients, those tools—such as waist circumference, body composition scans, and metabolic blood markers—could also widen disparities if access to advanced testing is uneven.

For Tchang, BMI remains a “pretty good” metric and will likely prevail as a first step.

In fact, Muniyappa isn’t convinced that more nuanced diagnostic protocols will improve clinical decision-making.

“Unless we have evidence to show that it actually improves outcomes, there is no reason” to perform additional tests, he said. Still, studies have shown that anthropometric measures often perform better than BMI at predicting specific complications, especially type 2 diabetes.

What comes next is unclear. Tchang noted that the commission, anchored in diagnosis, does not make recommendations for the management of obesity. “Any practical considerations have largely been left up to us as clinicians or policy makers and public health officials,” she said.

Zahid added that updated terminology is meaningless on its own.

“If we change a definition but don’t make real changes to how we approach obesity as a society, from the perspective of insurance coverage, or at the pharmaceutical level, then I don’t think we’re making a dent,” he said.

Although Muniyappa pointed to current staging systems that already prioritize severity without requiring causal attribution, Rubino argued that a more precise framework could help align treatment with coverage.

“We’re not going to be able to treat everyone who has a bit of elevated cholesterol with the latest-in-class drug that costs $25 000 a year, but if you tell insurance ‘I’m treating a patient with a risk in a way that is proportional to the risk,’ then it will make more sense,” he said.

Others question how much influence a new framework is likely to have on health systems, considering BMI is still relied on to determine coverage. Tchang pointed to a persistent “disconnect between what is clinically appropriate” and payer priorities.

Although one analysis suggests that adopting the commission’s definition would substantially increase obesity prevalence, mainly because of the addition of people with “anthropometric-only” obesity, Rubino emphasized that clinical judgment should take precedence over population-level classifications in which “more than a billion people all have the same illness and need the same treatment”—a health care burden he described as untenable.

“We clinicians have had to act as if we were epidemiologists, but it’s time we put the white coat back on,” said Rubino. “If we treat every case individually, all of a sudden, a billion people becomes a bit more manageable.”

Whether such a shift can be realized in practice remains uncertain. Tchang described the ongoing debate as “healthy and necessary” but said that without consensus, “there is going to be significant confusion in real-world applications.”

She predicts expert groups—the commission and the Endocrine Society in particular—may need to give up a bit of academic rigor to find “a space in between that serves all our patients.”

Conflict of Interest Disclosures: Dr Tchang reported receiving current or past advisor fees from Novo Nordisk, Roman Health Ventures, Skye Bioscience, Circadian Care, Amgen, Lilly, Boehringer Ingelheim, Gelesis, Cloud Health Systems, SAI MedPartners, Tern Pharmaceuticals, and Intellihealth/Flyte Health. Dr Rubino reported receiving research grants from Ethicon and Medtronic; speaking honoraria from Medtronic, Ethicon, Novo Nordisk, Eli Lilly, Amgen, and AstraZeneca; and consulting fees from Kailera. He also reported serving as a member of the data and safety monitoring board for GI Metabolic Solutions Inc and as president of the nonprofit Metabolic Health Institute. Dr Muniyappa reported serving on the editorial board for the American Journal of Physiology and as a clinical guidelines committee member of the Endocrine Society. No other disclosures were reported.

Bron: JAMA / May 15, 2026