Nearly one billion people worldwide live with obesity1. Yet the scientific community is divided over what obesity means for health.
For decades, obesity was viewed mainly as a health risk, increasing the likelihood of diabetes, cardiovascular disease and premature death2. But in the past few years, it is being increasingly described as a chronic disease, a shift that reflects efforts to reduce stigma and improve access to care long denied to people living with obesity3,4.
If it were to be adopted formally by policymakers and clinicians, a uniform disease label would classify one in three adults in many high-income countries1 as having the same chronic illness. Each would be potentially entitled to lifelong treatments, from weight-loss drugs to bariatric surgery and specialist follow-up.
Yet even people with obesity who have the same body mass index (BMI) — a measurement of weight divided by height squared, used to assess whether someone has obesity — can have radically different health statuses and trajectories2,5. Some people might struggle with heart failure, breathlessness and limited mobility. Others might remain in good health long term or even throughout their lives. The experiences, prognosis and treatment needs of these two groups differ profoundly.
To address this problem, an international commission of 56 global experts (chaired by me) was convened by The Lancet Diabetes & Endocrinology in June 2022. In January 2025, after reviewing the evidence, we concluded that a single, uniform disease label is incompatible with the varied manifestations of obesity6. The commission proposed a distinction to reflect this nuance: clinical and preclinical obesity.
Clinical obesity refers to the presence of excess fat tissue that directly impairs daily activities or causes demonstrable organ dysfunction — such as heart failure, breathing disorders, metabolic dysfunction and limited mobility6. That is unequivocally disease. Preclinical obesity, by contrast, describes a condition of increased body weight and excess fat, but with preserved organ function. Risk is elevated, but there is no established disease. Crucially, both types are defined independently of obesity-associated diseases, such as type 2 diabetes, cancer and mental-health disorders, which might coexist with either state but do not define them. The distinction between preclinical and clinical obesity allows risk to be treated as risk, and disease as disease.
Despite endorsement by 76 medical organizations worldwide6, this framework has been contested by some scientific societies and other specialists7,8. Critics argue that it sets an impractical diagnostic threshold, because of the challenges of demonstrating that excess fat directly causes a person’s organ dysfunction. They worry that the existence of the preclinical category might lead to restricted access to treatment for some people. And they contend that diseases that commonly occur alongside obesity — such as type 2 diabetes — should be included in the diagnosis of clinical obesity7. Much of this pushback stems from the view that obesity should be treated as a disease because, on a population level, it increases the risk of many health conditions. This viewpoint has long held sway in the field, but is at odds with the variable manifestations of obesity in individuals (see ‘A diverse landscape’). And it runs counter to the way diseases are typically diagnosed in the rest of medicine9 — in a person, not a population.
(Nature)
The controversy is sowing confusion among professionals and the public7,8. Division over something as fundamental as how to diagnose disease could distort clinical care, policy, insurance and public-health decisions.
Here, I outline why the commission was right to rule that obesity can be a disease — but not in every case. This view, I argue, better serves patients, policy, advocacy and scientific inquiry.
For millennia, physicians have recognized that excess body weight can have health consequences. Obesity, however, has been understood as a spectrum of condition, not as a uniform disease state. Classical medical scholar Hippocrates, for instance, noted that ‘corpulence’ could signal future illness in some, constitute disease in others, and seem to protect people from the ill effects of other maladies10.
Views of obesity narrowed in the twentieth century. Population-level observational data examining statistical associations between body weight and health outcomes — particularly the ‘ideal weight’ tables developed in the 1940s by the New York City-based Metropolitan Life Insurance Company — linked excess weight to increased risk of early death. Research over the following decades showed that this risk is associated not with body weight itself, but with conditions such as diabetes, high blood pressure and cancer that frequently occur alongside obesity2–4. This reinforced the view of obesity as a risk factor or risk state, rather than as a disease entity in itself.