Modern medicine now offers safe, effective tools to treat obesity—from life-saving bariatric surgery to powerful new medications. Yet most people with obesity can’t access these treatments because obesity isn’t widely recognized as a disease. In many countries, including the U.S. and U.K., access to treatment is granted only when another related condition, like hypertension or type 2 diabetes, is already present, even though there is ample evidence that obesity alone can cause serious harm.

To improve access to care, many professional organizations have advocated for classifying obesity as a disease. However, the idea remains controversial, fueling one of the most polarizing debates in medicine and society at large. 

Reflecting on this in 2019, I wondered why such a basic medical question provokes divided opinions across medicine and society. Determining whether a condition is a disease should be a matter of science—not opinion. Obesity has been known to humanity for thousands of years and both ancient and modern physicians have recognized its health risks. So why wasn’t this question already settled?

As I explored this issue further, it became clear that obesity had never been coherently described as an illness with its own specific signs and symptoms. Instead, it was historically measured by surrogate markers of body size and health risk, like BMI. Without a clear clinical identity, and thus no true diagnosis, it’s no surprise that the idea of obesity as a disease has remained so contentious.

The result has been ineffective prevention, fragmented care, widespread stigma, and confusion around the nature of obesity. 

With more than 1 billion people now living with obesity, effective therapies available but out of reach for many, and no coherent or ethical way to prioritize care and limited resources, an objective clinical diagnosis of obesity was more urgent than ever. 

A conversation about the issue with Dr. Marta Koch, editor-in-chief of The Lancet Diabetes & Endocrinology, led to the formation of the Commission on Clinical Obesity, which I had the privilege of chairing. We convened 56 global experts and spent over three years pursuing a challenging goal: aligning obesity diagnosis with established medical principles. 

The task wasn’t easy. At the start, our opinions varied widely. When pressed to describe obesity as a disease, we’d fall back on portraying it as a harbinger of other diseases. We got stuck in a loop, saying obesity qualifies as a disease if two—or maybe three—other related diseases and conditions are also present. But those conditions have their own distinct causes and mechanisms beyond obesity. This arbitrary reasoning has no parallel elsewhere in medicine.

Accustomed to measuring obesity merely by BMI, which is a proxy for body size and associated health risks, rather than by objective signs of direct harm to health, we specialist care providers had, over time, become so detached from the core principles of clinical diagnosis that our idea of disease itself had become distorted.

Disease diagnosis, by definition, identifies a distinct disease entity rather than an overall state of ill health. In stark contrast, for decades we quantified the health impact of obesity using risk scoring systems and management frameworks that grade obesity’s severity based on the risk of mortality from related conditions like hypertension or diabetes, thereby reflecting a state of overall ill health, but not a distinct pathological entity. This practice has fostered a tendency to conflate diagnosis with management frameworks, confuse co-morbidities with symptoms, and ultimately blur the distinction between obesity as a risk factor and as a disease state.

To move forward, we had to return to the drawing board and recall the foundational principles of disease diagnosis we learned in medical school: diseases are pathological conditions that directly impair organ function, producing measurable signs and symptoms—regardless of other conditions.

Viewing obesity through this lens required us to reverse-engineer its traditional medical framing. Doing so, the conceptual challenges began to resolve, and we reached strong consensus that obesity can indeed be a disease—when it leads to illness by directly causing harm to the body’s organs— though this is not always the case. We proposed a two-part diagnostic framework:

1. Clinical obesity: a chronic disease where excess fat causes impairment in the function of organs (e.g., heart, lungs, joints, metabolism, the reproductive system) or the whole organism.
2. Pre-clinical obesity: a condition where excess fat increases future health risks but hasn’t yet caused detectable harm.

This framework helps identify who needs care aimed at risk reduction and who needs interventions for disease treatment. It moves beyond BMI to focus on real health impact—just as we do with diabetes, cancer, and other diseases.

Over many years as a bariatric surgeon, I’ve treated many patients with severe symptoms of clinical obesity. I’ve seen some people struggle to breathe, walk, or work, but still be denied care because their obesity wasn’t recognized as a disease. This long overdue diagnostic clarity was thus not just a medical milestone—it was a step toward correcting a longstanding injustice and improving access to care.

Although the new diagnostic framework was endorsed by 79 professional societies and praised by many for its clarity, some expert’s reactions echoed the same conceptual barriers we faced during the work of the Commission. A few critics questioned why related diseases like diabetes weren’t included in the definition of clinical obesity, revealing the entrenched habit of defining obesity through other conditions. Others were frustrated—some even outright angry—that the Commission fell short of declaring all forms of obesity a disease and instead recommended a more nuanced clinical diagnosis. Such critics are concerned that our approach might dilute the sense of urgency necessary and undermine public messaging.

But diagnoses are not meant to serve as narratives or slogans—they must reflect medical reality. Long-established medical evidence shows that obesity exists on a spectrum—a fact already recognized since Hippocrates and Galen over 2,000 years ago.

The health impact of obesity, in fact, varies widely at the individual level. Some people experience organ damage due to obesity alone, others remain healthy for years, even lifelong. In some contexts, extra body fat can even offer protection—such as during recovery from surgery or critical illness, a phenomenon called the “obesity paradox.”

Recognizing the evidence of obesity’s spectrum isn’t a weakness of the new diagnosis—it’s a strength. The categories of clinical and pre-clinical obesity reflect real differences in health status and allow for personalised care.

Some worry that people with pre-clinical obesity will not be considered a priority and could lose access to care. But doctors already intervene in other pre-clinical conditions when there is a high health risk—prescribing statins for high cholesterol, metformin for pre-diabetes, even performing surgery for pre-cancerous polyps. The same logic should apply to pre-clinical obesity. No health system can afford to offer surgery or costly medications to every person with obesity—nor should it, if some don’t need them. Tailoring care based on need—not size—is far more coherent and sustainable than applying the same treatment to everyone.

Obesity is not just a clinical spectrum—it’s also a biological one. Excess fat can result from other medical conditions, side effects of medication, genetic disorders, or adaptation to modern environments, and likely other causes that are still unknown. Yet society still frames obesity as a failure of willpower—a harmful misconception that fuels stigma and misleads approaches to care and policy.

An objective clinical diagnosis of obesity finally allows us to separate risk from disease and biology from blame. 

But adopting this diagnosis will require a fundamental shift in mindset, within medicine and across society. It’s time for healthcare professionals, policymakers, and the public to recognize that obesity can be a disease for some, though not for all. And that we must not diagnose illness—nor judge character—based on body size alone.