“Obesity is a disease,” Oprah Winfrey declared after disclosing her weight loss with an Ozempic-like drug. “It’s a brain disease,” a prominent obesity doctor explained on a “60 Minutes” episode about the drugs. “Obesity is disease” even has its own discover page on TikTok.
The American Medical Association and the World Health Organization share that view, but whether obesity should be considered a disease has been referred to by health experts as “one of the most polarizing topics in modern medicine.” Even Jens Juul Holst, a discoverer of the hormone that drugs like Ozempic mimic, told me he isn’t sure what to call obesity. “Whether it’s a disease in its own right is a very difficult question,” he said. Finally, this dispute is coming to a head amid soaring demand for new weight loss medicines, as expert groups around the world rush to define what it means to have obesity.
At the heart of the debate: The medical community has never provided a precise definition for obesity as a disease. It’s typically understood as an excess of body fat, using body mass index, or B.M.I., to gauge who has too much. But B.M.I. — a person’s weight divided by the square of their height — was never meant to be used as a diagnostic tool and can’t determine whether someone is healthy or sick. And there’s no consensus on the signs and symptoms that make obesity an illness the way high blood sugar levels are used to diagnose type 2 diabetes, or chest pain and irregular imaging to tell if someone has heart disease.
Diagnosis by B.M.I. was always imprecise; in an era of remarkably effective weight loss drugs, it’s untenable. Consider that 40 percent of American adults are classified as having obesity by having a B.M.I. of 30 or above. With new treatments that cost upward of $1,000 per person per month, along with supply shortages, how to define obesity is more than just a fight over nomenclature. It’s about pinpointing who is sick and will benefit from health care and how to triage that treatment and most effectively allocate resources. It’s about ending the murkiness that has surrounded obesity diagnosis for decades.
Obesity, as it’s currently understood, doesn’t reflect what we now know about body fat. It makes patients out of people who aren’t ill and glosses over those who need health care urgently. In declaring a disease without nailing down what the disease is, the medical community left obesity open to debate among doctors, insurers and everyday people. This in turn left people with obesity vulnerable, their bodies subject to accusations and questioning, overtreatment, undertreatment and mistreatment. That is to say, almost 30 years into what’s referred to as the obesity epidemic, medicine could be doing a better job at figuring out who’s sick.
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In 2013, when the American Medical Association officially recognized obesity as a disease, it was part of a well-meaning effort to improve health insurance coverage for treatment of people sick with obesity and reduce stigma by emphasizing that size is not a personal choice.
Like many clinicians, researchers and advocates, I thought that decision made sense. Covering obesity for a decade as a journalist, I’ve interviewed hundreds of patients, following them through diets and bariatric surgeries, and on (and sometimes off) weight loss medications. I learned how encumbered and unwell many people feel under the weight of excess fat and how its accumulation can send body systems spiraling, making it harder to breathe and move, and increasing the risk of other maladies, including type 2 diabetes and several forms of cancer.
I also learned that when obesity rates began rising fast in many high-income countries around the 1980s, it was not because of a global breakdown of willpower or a sudden change in our genes. Rather, the most sound explanation is that we now live in environments that exploit genetic susceptibilities many people have, just as air pollution or sunlight causes cancer in some but not others.
So, if obesity is not a condition people choose and can make us sick, even cutting life expectancy short, it seemed logical to call it a disease.
Yet the more I know about obesity, the more the D-word gives me pause. Excess fat is the defining characteristic of obesity, linked to sickness and death since antiquity. But modern science tells a more nuanced story. Excess fat can be a symptom of illness, a disease itself or a risk factor — an on-ramp — for other health problems. The ills of excess fat can manifest in people with larger bodies and high B.M.I.s, and in people who aren’t large at all but harbor abnormal body fat — what’s referred to as “normal weight obesity” or “thin fat obesity.” This makes “obesity” a surprisingly convoluted term for a problem that, at least for now, seems to affect more than a billion people worldwide and costs the U.S. health system $173 billion per year.
Much of this ambiguity stems from the reliance on B.M.I., which was developed in the 19th century by a statistician as a means to determine the characteristics of a “normal man.” A powerful tool for studying risk in populations and screening people in the doctor’s office, B.M.I. doesn’t directly quantify body fat. More crucially, it can’t account for how that fat is distributed and whether it’s problematic. The imprecision breeds misdiagnosis.
“If you have a definition of a disease that even slightly over-diagnoses, you’re talking about millions of people,” said Dr. Francesco Rubino, chair of metabolic and bariatric surgery at King’s College London. That makes a “huge problem an intractable one,” he added.
Corpulent healthy people and muscular athletes may be classified as obese because of B.M.I., even though they aren’t necessarily ill or even fat. “We’re telling a lot of people that they’re sick and diseased when they’re healthy,” said Dr. Scott Kahan, an obesity clinician and faculty member at George Washington University School of Medicine.
But the misdiagnosis of muscular people isn’t the primary issue. Rather, it’s that people may undergo unnecessary treatments and face the psychological burden of being diagnosed with a chronic disease they don’t have. That’s in addition to the stigma larger people already encounter everywhere, including in the doctor’s office. I would know: I was once obese according to my B.M.I., and though I exhibited no signs or symptoms of sickness, I was fat-shamed by doctors, even in childhood.
The problem of underdiagnosis is arguably more pernicious. Some people are more likely to store fat in and around their organs and muscles, which is considered a higher risk for cardiovascular and metabolic health complications than fat stored just beneath the skin in, say, the thighs or the bottom. B.M.I. can’t detect this type of dysfunctional “sick fat.” South Asians are well known to researchers for storing fat this way — and having a relatively high risk of cardiovascular disease and type 2 diabetes at lower B.M.I.s. But because these patients don’t appear overweight, their health problems can go unnoticed.
More fundamentally, B.M.I. helped create an equivalency between a disease and body size, but that’s not how researchers understand obesity today. A person can be fat without illness or “thin fat” and sick. Fat can cause mechanical problems in the body — sore knees, difficulty breathing. It can also be a symptom, rather than the cause of illness, like weight gain associated with a thyroid problem.
“We have this incredible breadth of data to support how it isn’t necessarily the fat that’s the problem but it’s the brain, hormones and our nervous system,” and how the brain understands how much fat we have stored, said Dr. Beverly Tchang, assistant professor of clinical medicine at Weill Cornell. “The problem is that we’ve allowed society, insurance companies and everyone else to decide that obesity is just too much fat, which is an oversimplification.”
Obesity is not alone in its identity crisis. Diseases are squishier concepts than they may seem. Their boundaries shift with evolving science and social norms, as well as market forces. Osteoporosis, formerly viewed as a normal part of aging, is now considered a disease — one that grew more common as a diagnosis thanks to the widespread deployment of bone imaging following the discovery of bone-building medicines. Researchers are working on recategorizing type 2 diabetes to reflect the different ways the disease can progress, while some oncologists argue medicine needs to rethink what we call cancer based on new understandings of how tumors grow.
We then layer diseases with myths and metaphors — cancer as a death sentence and a battle to win. In her book “Illness as Metaphor,” Susan Sontag wrote that these metaphors burden already suffering patients and hinder treatment. People with obesity have arguably been double burdened. Obesity’s metaphors — gluttony, sloth, laziness, greed — all imply self-infliction, which not only hurts patients but also can make it harder to get care.
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When countries and health groups have reached different conclusions on whether obesity is a disease, it’s typically because of conflicting beliefs about how their decision will shape public perceptions and affect medical care, rather than for scientific reasons.
The American Medical Association’s declaration of obesity as a disease in 2013 went against the advice of its own advisory council. The council had been concerned about the limitations of B.M.I. to diagnose obesity and called the disease label “premature,” in the absence of a better diagnostic tool for clinicians.
The medical association proceeded with the designation anyway. It argued that obesity should be considered a disease because it would give the condition more legitimacy and help mobilize resources for prevention, treatment and research while reducing weight stigma and discrimination.
I’ve interviewed several people who said that learning obesity is considered a disease, and one that can be treated with medications, alleviated blame they’d placed on themselves about their body weight. “Calling it a disease took some of the onus off me to think that it was a lack of willpower and that I was wrong for it, just weak willed, and made me realize there are other deeper factors going on,” Aditi Juneja, a New York City lawyer, told me. Ms. Juneja’s mobility problems, high cholesterol levels and family history of type 2 diabetes prompted her to try the drug Mounjaro to treat her obesity in 2023. She eventually discontinued the drug after realizing she’d have to use it for life to keep her weight down, which didn’t sit well with her.
Other expert groups around the world came to different judgments on the disease question. Obesity is not classified as a disease in Denmark, for example. Though the country is home to Novo Nordisk, maker of the blockbuster drugs Ozempic and Wegovy used for it, the Danish government classifies obesity as a risk factor, not a disease in and of itself.
In a paper about why Denmark and other welfare states shouldn’t follow the A.M.A., obesity researchers there argued that labeling obesity a disease in countries with robust, publicly funded health systems wouldn’t improve people’s access to health care. Instead, they were concerned that the disease moniker “would categorically define the obese body as deviant,” potentially increasing stigma for anyone with a high B.M.I.
The Danish and American approaches each has its merit. But the lack of medical consensus fuels public speculation about obesity, its causes and impact.
Some people quibble about whether obesity is self-inflicted, the result of poor willpower and a lack of personal responsibility. On the other end of the spectrum, movements such as “health at every size” and “fat acceptance” call for an embrace of fat, arguing that obesity’s health risks are overblown and that designating obesity as a disease unnecessarily medicalizes and stigmatizes healthy people. Both arguments reduce body fat to a cosmetic issue and minimize the very real health impact it can have in small and large bodies alike. Meanwhile, people who need health care are left without access or coverage, because their condition lacks legitimacy or because it’s invisible. Too often, they turn to profiteers peddling weight-loss quackery.
A better definition of obesity could help resolve the debate, at least in medicine. That’s why Dr. Rubino has organized a Lancet commission — the medical journal’s program of scientific reviews about urgent health issues — to bring together health experts from around the world to finally define obesity the illness. One of several efforts to push the medical community beyond size metrics like B.M.I., the commission’s report, coming this fall, will for the first time describe what it is calling “clinical obesity,” naming signs and symptoms of sickness, organ by organ, tissue by tissue with plausible mechanisms for each one.
The commission’s diagnostic approach won’t involve diagnoses of other diseases, Dr. Rubino said. Instead, it will require an evaluation of a patient’s medical history, a physical examination, standard laboratory tests and additional testing as needed. B.M.I. will be relegated to its rightful place as a tool for screening, not diagnosis. As a result, clinicians should be able to differentiate who is just heavy, with excess fat or a high B.M.I.; who has obesity as a symptom of another condition; and who has obesity the actual illness, irrespective of body size.
While this may sound cumbersome, Dr. Rubino noted it’s no more than what’s required to diagnose any other chronic ailment. “There is hardly any diagnosis in medicine that is as quick, simple and inexpensive as the current B.M.I.-based measure of obesity,” he said. “But a B.M.I.-based definition of obesity is not a disease diagnosis.”
Dr. Rubino’s idea to tackle this topic predated the widespread popularity of medications like Ozempic, known as GLP-1 drugs, stemming instead from his practice as a bariatric surgeon. He frequently encountered people who were sick with “full-blown problems that would normally characterize a chronic disease state.” Some couldn’t walk. Others lived with labored breathing. But because they didn’t have diabetes or another obesity-related disease, and their body weight wasn’t high enough, they couldn’t get the cost of their bariatric surgeries covered by insurance, which meant they didn’t get care at all.
Even among those who had access, Dr. Rubino noticed that priority was granted on a first-come-first-served basis, instead of reflecting who most urgently needed care. During periods of limited hospital capacity, bariatric surgeries were considered elective, “the first to be shut down and the last ones to restart,” he said, “regardless of the fact that many of the patients who need bariatric surgery may have a life-threatening, full-blown illness.”
Today, he sees history repeating with GLP-1 drugs. In Britain, as in many other countries, the drugs are covered only for patients with what Dr. Rubino called “obesity plus” — a high B.M.I. as well as other diseases, such as diabetes.
Even so, the overarching goal of defining clinical obesity shouldn’t be to expand health care access, Dr. Rubino cautioned. There’s an importance to maintaining objectivity here. Medicine should define diseases for what they are, he said, not based on any purpose, no matter how good or desirable. “This is crucial,” he said, “because if one defines a disease inaccurately, everything that stems from there — from diagnosis to treatment to policies — will be distorted and biased.”
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Denmark offers a stark look at why a more systematic approach to obesity diagnosis is needed. The public health system faced a crisis during its last fiscal year because of the surge in demand for Ozempic, which is publicly subsidized for diabetes. Wegovy, which is approved for obesity, is not publicly subsidized. Prescriptions for Ozempic soared, a third of them off-label, for people who didn’t have diabetes. The frenzy for the covered drugs alone maxed out the 2023 budget for public health several months ahead of the fiscal year end.
Doctors were asked to prescribe only small amounts of Ozempic to each patient, out of concern that some were sharing or reselling their doses, presumably for weight loss. Some patients were getting their off-label Ozempic reimbursed. Health authorities scrambled to issue advisories about proper prescribing, while health systems across the country had to cut their spending by doing things like delaying elective surgeries and enacting hiring freezes.
For this year and the next, Denmark got a health services budget increase — thanks to Ozempic and Wegovy. As drug sales boomed, so too did Novo Nordisk, now Europe’s most valuable company, bolstering Denmark’s economy — and the country’s health care spending. Crisis averted for now, but only in lucky Denmark.
Just calling obesity a disease won’t solve the resource constraints, in Denmark or anywhere else. But if health systems adopt an evidence-based diagnosis, deciding when it’s appropriate to intervene medically, perhaps treatments can be more targeted and prescribing more rational. The era of quibbling about personal responsibility, fat-shaming and the limits of B.M.I. can end. People who are diagnosed with clinical obesity deserve health care without blame, just as those who have diabetes, cancer or clinical depression do. But first, we have to agree on what obesity is.
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