For decades, the bathroom scale and a simple formula have determined who carries a disease diagnosis and who does not. A number, body weight divided by height squared, has driven clinical decisions, insurance coverage, and public health campaigns across the globe. Now, some of the world’s most prominent medical experts are arguing that number tells only a fraction of the story, and that the way medicine has long thought about obesity is fundamentally wrong.
The debate erupted into full view this past week, with a major commentary published in Nature by Francesco Rubino, chair of bariatric and metabolic surgery at King’s College London, and simultaneous coverage from leading health outlets examining whether the body mass index, the metric introduced to American medicine in meaningful force by life-insurance actuaries in the 1940s, has outlived its diagnostic usefulness. At the same time, new research surfaced suggesting that BMI may be dramatically undercounting the true prevalence of obesity, particularly in populations where fat distribution matters more than total weight.
The collision of these two arguments has created something unusual in medicine: a scientific community divided not over whether obesity is serious, but over what obesity actually is.
A Disease, a Risk, or Both?
At the heart of the controversy is a framework proposed in January 2025 by a 56-member international commission convened by The Lancet Diabetes & Endocrinology. The commission, chaired by Rubino, concluded that calling all obesity a disease was, in his words, exchanging one oversimplification for another.
The commission introduced two categories. Clinical obesity describes cases where excess fat tissue directly causes organ dysfunction: heart failure, breathing disorders, metabolic impairment, severely limited mobility. That, the panel concluded, is unequivocally disease. Preclinical obesity, by contrast, describes elevated body fat with preserved organ function. Risk is heightened, but no illness has yet taken hold.
Writing in Nature this month, Rubino argued that this distinction aligns obesity with how the rest of medicine actually works. Diseases are typically recognized when clinicians identify consistent patterns of symptoms that recur across many patients, then link those patterns to biological mechanisms and clinical trajectories. Diabetes was defined that way. So was COVID-19.
The framework has drawn endorsement from 76 medical organizations worldwide. It has also drawn sharp pushback.
The Critics
Opponents of the clinical-preclinical split, including several major endocrinology societies, have raised concerns that are partly practical and partly philosophical. The standard for proving that excess fat tissue directly causes a person’s organ dysfunction rather than merely coexisting with conditions like type 2 diabetes or high blood pressure is, they argue, nearly impossible to meet in routine clinical settings.
Critics also worry that the preclinical label will create a two-tier system in which people with high body fat but preserved organ function are denied access to weight-loss treatments, including the GLP-1 receptor agonist medications like semaglutide that have transformed obesity care over the past several years. Those drugs, Rubino acknowledged in Nature, are potent tools, but he argued that their effectiveness demonstrates that body weight is biologically modifiable, not that all excess weight constitutes disease. GLP-1 drugs modulate appetite and satiety pathways shared across individuals, he wrote, rather than correcting a specific pathological defect the way insulin corrects insulin deficiency in type 1 diabetes.
A complementary challenge to BMI is coming from a different direction entirely. Research published this year suggests that the conventional BMI threshold dramatically underestimates how many people carry metabolically dangerous levels of body fat. When researchers applied waist-to-hip and waist-to-height ratios in addition to BMI, the estimated prevalence of obesity among American adults jumped to roughly 75 percent, nearly double the figure produced by BMI alone. A separate study presented at the European Congress on Obesity in 2026 found that BMI incorrectly classifies more than one-third of adults when direct body-fat measurement is used as the standard.
The Measurement Problem
Those findings track with a broader argument in clinical medicine that BMI is simultaneously over-inclusive and under-inclusive: it flags some lean, muscular individuals as overweight while missing large numbers of people who carry metabolically active visceral fat at weights that look normal on a chart.
“BMI is problematic because it does not specifically measure body fat and instead reflects total body weight, which includes muscle and bone,” researchers from Keck Medicine at the University of Southern California noted in recent findings showing that roughly 26 percent of adults with ostensibly healthy BMI readings actually met clinical criteria for obesity when body composition was assessed more precisely.
Writing in MedCity News, Richard Frank, chief medical officer at Vida Health, put the problem in systemic terms. The healthcare industry has built an entire infrastructure of fragmented point solutions separate programs for weight, for blood sugar, for blood pressure each generating its own data and its own definition of success. “Metabolic disease does not operate in silos,” he wrote. “It is a syndrome driven by overlapping physiological, behavioral, and environmental factors.” A BMI number alone, Frank argued, tells clinicians nothing about where a patient sits on a spectrum of metabolic stability or where they are headed.
The practical stakes are real. Treatment access in most healthcare systems is currently tied to BMI thresholds or the presence of comorbidities. People living with obesity who have limited mobility but no qualifying conditions like diabetes or hypertension are frequently denied coverage for weight-loss interventions. The Lancet Commission’s framework was partly designed to correct that inequity by anchoring disease status in organ dysfunction rather than in a number on a scale. But critics worry the new categories will introduce a different kind of exclusion.
Britain’s Hidden Numbers
The debate has immediate relevance in the United Kingdom, where lack of sleep, sedentary lifestyles, and excess weight are deeply intertwined contributors to the country’s chronic disease burden. Government health data shows that nearly two-thirds of adults in England are overweight or living with obesity, a figure that has risen steadily since the early 1990s. Research suggests a meaningful portion of people who consider their weight normal or healthy would be reclassified as overweight or obese under stricter measurement criteria and that those same people may face elevated metabolic risk they are currently unaware of.
The pattern reflects a broader dynamic that researchers have documented across high-income countries: that public understanding of what constitutes a healthy weight lags significantly behind clinical definitions, and that clinical definitions themselves may lag behind the metabolic reality of individual patients.
A More Complicated Biology
Rubino’s Nature commentary also takes aim at a third pillar of the argument for universal disease classification: the claim that genetic and biological evidence justifies treating all obesity as a disease.
The argument has merit, he wrote, particularly in the effort to reduce the persistent stigma that frames excess weight as a failure of willpower. But the available evidence, he contended, does not fully support a universal disease label. There are no genetic abnormalities shared across all people with obesity. Rare single-gene disorders involving mutations in pathways like leptin signaling can properly be understood as genetic diseases of which obesity is a manifestation. For most people with obesity, however, a constellation of gene variants predicts risk of excess body fat, not whether that excess fat will produce clinical disease.
Rubino drew an analogy to fever and elevated red-blood-cell count. Both can be physiological adaptations; fever helps fight infection; high red-cell count can reflect altitude adjustment or pathological manifestations of disease. Increased body fat, he argued, should be understood similarly: as a regulated physiological state that can represent either a response to environmental pressures or a disease process, depending on the individual. Grouping all cases together, he wrote, risks distorting the science that might eventually distinguish between them.
What Comes Next
The controversy is unlikely to resolve quickly. The Endocrine Society, among other critics, has published formal objections arguing that the commission’s framework sets thresholds that are impractical for routine care and may harm patients by restricting treatment. The debate has moved from academic journals to insurance boards and hospital policy committees, where the question of who qualifies for GLP-1 drugs some of the most expensive and consequential medications in modern medicine is already being contested.
The Lancet Diabetes & Endocrinology Commission’s full report runs to more than 40 pages and proposes not just new categories but a new diagnostic sequence: begin with BMI as a screening tool, then confirm with waist circumference, waist-to-hip ratio, or waist-to-height ratio, then assess for signs of organ dysfunction before assigning clinical or preclinical status.
For patients, what this means in the near term is largely institutional uncertainty. The numbers that have governed medical decisions about their weight for decades may be changed, but the system built around those numbers the software in doctors’ offices, the algorithms used by insurers, the thresholds embedded in clinical guidelines will not change overnight.
What does seem clear, across the spectrum of the debate, is that BMI alone is insufficient. Whether the answer is to layer in waist measurements, direct body-fat assessment, organ-function testing, or some combination of all three, the era of the single-number definition of obesity appears to be drawing to a close. What replaces it remains, for now, genuinely unsettled.
