Why our definition of obesity is flawed

DEEP DIVE

Flawed definitions of obesity can harm health outcomes, research indicates. The medical community is taking notice

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When you arrive at a doctor’s appointment, you likely get your height and weight measured — two numbers used to calculate your body mass index (BMI). From this metric, your physician can diagnose whether you are under or average weight, or whether you have obesity. The thing is: BMI is a metric that was not designed to diagnose someone with a disease, posing a significant problem for the medical community, as well as contributing to stigma for people with higher weights.

In 1832, a Belgian statistician named Adolphe Quetelet created the Quetelet Index to find the average dimensions of the “normal man.” Physiologist Ancel Keys did some more research to validate Quetelet’s Index in 1972 and officially coined the term we use today: BMI, which is a person’s weight divided by their height squared. 

Quetelet’s work was based on mostly white men, even though gender and racial differences exist in how much fat people carry and where they store it, which in turn affects whether it can pose a disease risk or not. Fat stored near the abdomen, for example, is associated with a greater risk of certain conditions like insulin resistance compared to overall body fat. Asian populations have been shown to have a higher risk of cardiovascular disease at lower BMIs compared to white populations. And the BMI index is not designed to capture changes in fat composition that occur among people who menstruate during puberty or after menopause, when muscle mass can substantially shift.

“I think there are problems with using BMI as the only metric,” said Dr. Jody Dushay, an endocrinologist and assistant professor of Medicine at Harvard Medical School. “There are ethnic differences, there are sex differences, there are differences based on muscle mass … Professional football players, for example, are probably almost all obese based on the criteria of BMI.”

As a result of these limitations, there is a growing movement to adopt a new classification system to diagnose obesity that uses more specific measurements that directly assess body fat rather than using body size alone. But fundamental to adopting a new diagnostic system is another decades-old question that continues to divide the medical community: Is obesity itself a disease?

“I agree with the notion that excess body weight in some people is a disease state,” Dushay told Salon in a phone interview. “Yet certainly not everyone with excess body weight has a disease … It’s much more complicated than hypertension or high cholesterol or diabetes.”

Today, more than 40% of U.S. adults have been diagnosed with obesity, a number that is expected to rise. Language matters, and these definitions are not just a matter of semantics. People receive access to weight loss treatments and new drugs like Wegovy based on their BMI measurements, and have also been shown to have reduced access to things like fertility treatment with higher BMIs due to stigma. Refining the definition and diagnosis of obesity could ensure the medicines used to treat it arrive to the people who need them and reduce unnecessary stigma faced by people who are otherwise healthy but have larger body sizes.

“Many insurance companies are saying if your BMI is below 35, you have to start with a less expensive medication, and they won’t go straight to covering the GLP-1 receptor agonists,” Dushay said, referring to drugs like Ozempic and Wegovy, which can be extremely expensive. “I think embedded in that is the assumption that people with lower BMIs don’t have as serious comorbidities and that is not true.”

In 2013, the American Medical Association (AMA) officially declared obesity a disease, going against the recommendation from its Public Health and Science Committee. The decision was made in part to reduce stigma faced by people with obesity and improve the development and accessibility of treatments. 

Whether the classification of obesity as a disease has in fact reduced stigma is unclear, with some research suggesting that weight bias is increasing as obesity rates rise. In 2020, a Patient Advisory Board for a randomized clinical trial of patients with obesity reported their experience of going to the doctor, where they felt dehumanized as if their entire identity was reduced to a number.

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“We want you to know how to look at us, to see each of us as a person,” they wrote in a letter to physicians posted in the Annals of Family Medicine. “We want you to see us as a person as opposed to an obese person with a certain BMI and a series of conditions.”

More than a decade of research has helped illuminate what obesity is since the AMA’s decision. Research has shown, for example, that some people may have a genetic predisposition to obesity. Scientists have also learned that obesity is caused by areas of the brain failing to recognize fullness as well as areas that direct how calories are stored in ways that can increase things like inflammation, said Dr. Beverly Tchang, an assistant professor of clinical medicine at Weill Cornell.

“Our understanding of obesity has matured over decades because of the research we've been doing to understand it better as a medical disease,” Tchang told Salon in a phone interview. “Nowadays we think of it in a really nuanced, complex way, where we see obesity as essentially abnormal energy regulation.”

A disease is defined as a condition, usually with a known cause, that creates a set of symptoms characteristic to the disease that deviates from the rest of the population’s experience. As Tchang put it, obesity "fulfills those three criteria, whether people have downstream consequences of it or not."

However, others disagree that obesity should be a diagnosis at all. Notably, the United Kingdom and Denmark do not formally recognize obesity as a disease. Dr. D. L. Katz, editor-in-chief of the medical journal Childhood Obesity, who has long argued against such a diagnosis, says the large-scale effect of classifying obesity as a disease is that it deters energy from being put toward preventing it. 

“Obesity is not the disease; obesity is a gateway to disease,” Katz told Salon in a phone interview. “If you think in terms of population health, does obesity mean pathology? Not always, but often.”

Yet Tchang argued that on the individual level, there is room for ramping up resources to both prevent and treat patients with obesity.

“If someone develops lung cancer because they smoke, you don't say, ‘No, medication for you. You just need to stop smoking and the lung cancer will go away,’” Tchang said. “We say, ‘Try not to smoke and let's treat the cancer.’”

Some efforts have been taken to create updated definitions of obesity that better capture its complexities, but those haven't been universally adopted. In 2017, for example, the American Association of Clinical Endocrinology proposed a new term to replace "adiposity-based chronic disease (ABCD)," that explicitly defines the condition as a chronic disease with a precise physiological basis. The European Association for the Study of Obesity also issued a position statement that supported the use of ABCD in 2019.

In July, EASO also released a new framework for diagnosing obesity to “better align with the concept of obesity as an adiposity-based chronic disease.” A Lancet commission is also working to define obesity as a disease with detailed descriptions of how the condition affects body systems. 

Last year, the AMA released a new policy acknowledging BMI’s limitations and clarifying that it should be one tool of many used to diagnose obesity. Even before this policy, physicians would take into account a variety of metrics, including fat mass, family histories, and body adiposity index measures in addition to BMI when determining whether someone should be diagnosed with obesity. 

But other definitions remain vague. The World Health Organization defines obesity as “abnormal or excessive fat accumulation that presents a risk to health” with a BMI over 30. 

“It's night and day where you see BMI as this enormous oversimplification of obesity, and then 30 years later, what we actually understand it to be,” Tchang said.