Doctors are finally starting to agree on when obesity is a disease
It might seem like semantics. But whether a health condition qualifies as a disease can have a huge impact on patient experiences and treatment options.
Eat less. Move more. It’s how many doctors have treated obesity for decades. That approach fails to account for a key realization among scientists over the last decade—obesity is a disease.
Recognition of the condition as a true “disease” has taken a long time to gain acceptance by both the medical community and the public. The American Medical Association’s determination in 2013 that obesity is a disease, followed by the Canadian Medical Association two years later, catapulted the issue into the national conversation, and though the classification still has critics today, the North American medical community acknowledges that obesity is a chronic disease independent of diseases that can cause or result from it. Last year, a report from The Lancet Diabetes and Endocrinology Commission attempted to more carefully formalize how to distinguish obesity “as a disease” from simply living in a larger body.
Persuading doctors that obesity is a disease has been challenging, explains Carel le Roux, an obesity physician at the University College Dublin and one of the dozens of Commission members who authored the report. But convincing patients has been even harder because of how much internalized stigma, shame, and blame people carry as a result of society’s treatment of people in large bodies.
“I’ve had grown men in my clinic crying,” he says, because it was the first time they had been told that their weight was not their fault, even if addressing the way their obesity affects their health is their responsibility. “By you telling me it’s not my fault, you give me agency,” he says. “If my child has epilepsy, it’s not my child’s fault, but I need to treat it.”
What is a disease?
Part of that increased recognition is helping people understand how and why obesity is a disease. The concept of defining a disease has always been fraught and nebulous in medicine. But it comes down to identifying signs, symptoms, causes, and complications.
The signs of obesity—fat mass or adiposity—are fairly straightforward. Symptoms, le Roux says, include “appetitive behavior,” or the constant feeling of hunger and “food noise” in the brain. The complications of obesity are well understood, such as increased risks of hypertension, type 2 diabetes, cancer, sleep apnea, liver disease, and other conditions. It’s been identifying the causes of obesity—beyond simply consuming more energy than the body needs—that has been so challenging.
One of the biggest challenges, Le Roux says, is that no one fully understands which organ is responsible for the disease of obesity. “Our scientific understanding of the disease process itself, the biology of why people gain adiposity, is not clear,” he says.
So far, it appears that a substantial proportion of obesity originates in the brain, specifically in the areas that control the number of fat cells the body wants to carry, called adipocyte mass, Le Roux says. The brain regulates the mass certain organs “should” have and will instruct the body to do what’s necessary to maintain that mass. Obesity, then, appears to be a mismatch between what a healthy body’s mass should be and what the brain thinks it should be.
For example, someone who donates the left lobe of their liver will eventually grow back their liver to the same mass it was before, though perhaps not in the same shape. In the same way, if someone whose brain expects the body to carry a set amount of adipocyte mass goes on a calorie-restrictive diet to slim down, “the body will just defend that mass,” causing them to regain the weight, Le Roux says.
Targeting the brain, whether with a medication, bariatric surgery or even nutritional therapy, can decrease the mass to a point where it can be adequately regulated and maintained. “If we can affect that organ, we can reverse the symptoms, reverse the signs, reverse the complications, and it goes away,” le Roux says. “That’s why we’re pretty confident we can call it a disease.”
(Why eating less food can actually make weight loss harder)
Changing how obesity is diagnosed
While researchers are on the same page about obesity being a disease, they’re still arguing over when someone officially has it. Until the Lancet report last year, obesity has been solely defined based on body mass index (BMI), a ratio of a person’s weight to height: Those with a BMI between 25 and 29.9 are currently classified as having overweight, and those with a BMI of 30 or greater are classified has having obesity.
This crude calculation would classify a heavily muscled bodybuilder as having obesity because, as the Commission’s paper points out, BMI does not measure fat mass, fat distribution in the body, the type of fat a person carries, or whether excess body fat is causing harm to a person’s health. It also leaves out individuals who may have a BMI below 30 but who carry excess fat that is causing health problems.
“We all agree that BMI is terrible, but we can’t come up with anything better,” Le Roux says. But that’s precisely what the Commission attempted to do, albeit imperfectly, by defining clinical obesity based on two factors: excess fat and presence of organ dysfunction.
Instead of using only BMI as a proxy for excess fat, the Commission suggested three assessment methods: a DEXA scan, or body composition scan that identifies a person’s fat, muscle and bone mass; BMI along with an additional measurement of body size; or two measurements of body size independent of BMI. The three options for measuring body size are waist circumference (greater than 102 cm for men or 88 cm for women), waist-to-hip ratio (over 0.9 for men and 0.85 for women) or waist-to-height ratio (greater than 0.5 for anyone).
But DEXA scans are not widely available, particularly in rural or low-income parts of the world, and body measurements do not distinguish between fat tissue that’s harming someone’s health and fat tissue that’s not.
Hence the second component to clinical obesity, organ dysfunction, requires signs and symptoms related to obesity that limit a person’s day-to-day activities, such as shortness of breath, knee or hip pain, certain metabolic measures (such as high blood sugar or high blood pressure) or other organ dysfunction. The report identifies 18 specific health problems that qualify for organ dysfunction, though more than 200 complications have been linked to obesity.
Going beyond the use of BMI alone to define obesity may help improve acceptance of obesity as a disease, says Matthew Landry, a diet and nutrition public health researcher at the University of California Irvine. It’s also valuable to start moving beyond the idea that obesity is a dichotomous “yes or no” diagnosis, he adds. That means “acknowledging that someone can have a higher body weight, but if it’s not really impacting their quality of life or contributing to some kind of chronic disease, then maybe we don’t need as severe an intervention with that person.” And on the flip side, “someone could have a more ‘normal’ BMI but be carrying a lot of visceral fat that is metabolically active” and therefore more harmful to their health.
The real issue is that we’re judging people for something they have no control over.Shelly Russell-Mayhew, Psychologist, University of Calgary
How defining obesity affects patient care
Le Roux feels that understanding obesity as a disease is crucial to fighting stigma because it demands that doctors treat the disease properly. “They may not like the person, they may not like their lifestyle or their choices, but it’s our job to [leave] our biases at the door when we treat disease,” le Roux says.
A study published in 2020 found that physicians’ perception of obesity as a disease was associated with less negative weight bias. “If it’s seen as a disease, it’s no longer prioritized as some kind of moral failing on the part of people who live in large bodies,” says Shelly Russell-Mayhew, a psychologist at the University of Calgary who led that work. While she says some studies have found the opposite effect, most have similarly found that perceiving obesity as a disease is linked with lower levels of weight bias. And, in turn, that perception was also linked to greater empathy toward those with obesity.
“The real issue is that we’re judging people for something they have no control over,” Russell-Mayhew says. More than 100 factors may play a role in what someone’s weight is, she says, “yet we’re focusing on the 15 percent that is probably controllable and saying you need to change your lifestyle.”
It’s been particularly in the last two years or so that there has been a turning of tide with more widespread acknowledgement of obesity as a disease, Landry says. Part of that is due to advocacy, he says, but it’s also been research revealing other mechanisms—from hunger hormone fluctuations to genetic components—that contradict the simplistic “energy in, energy out balance” model that has dominated discussions of weight for decades.
(Read more about the role that genetics plays in obesity.)
For patients who have tried and not succeeded in losing weight to improve their health, understanding that “it’s not all on them” can be helpful, Landry says. “They could be doing everything to a T—following every guideline, every recommendation, and not succeed—so this acknowledgment that it’s a disease makes a difference.” At the same time, Landry cautions, that doesn’t mean the disease can be cured with a simple procedure or medication. Treatment still requires taking ownership of other important steps that could improve health.
An evolving definition of obesity
Ultimately, better categorizing obesity has helped open opportunities to more nuanced discussions between patients and their clinicians in considering treatment options and their choices. But debates over refining defintions remain.
For example, the Lancet Comission’s decision to limit organ complications to just 18 conditions erodes clinicians’ autonomy in determining whether someone has obesity beyond those conditions, notes Sean Wharton, a weight management physician in Toronto who co-authored the Canadian guidelines on obesity. That restriction also risks allowing insurance companies to deny coverage for treatment in someone who doesn’t meet that criteria, particularly for those who have what the Commission describes as “preclinical obesity,” a classification that “leans strongly toward lifestyle modification” as an intervention rather than medication.
“If you had ADHD and it wasn’t that bad yet, why wouldn’t you take a medication?” Wharton says. “It’s like you need it to be a shitshow before you can get medication.”
Le Roux acknowledged that the Commission’s work has substantial limitations in how it defines obesity and argued against limiting the complications to just 18 conditions, “but we have to start somewhere,” he says. Though GLP-1 medications have substantially advanced the field of obesity treatment, they remain expensive and inaccessible for the vast majority of people in the world living with obesity. When there are not enough treatments to meet the demand, starting with those who have the most severe health problems makes sense. Treatments will then eventually become cheaper and more accessible over time, he says.
And as scientists get a better handle on the underlying biology of the disease and its treatments, what obesity means will likely change again. “I think five years from now, we will have much better definitions because the science is moving on,” Le Roux says. “It’s a scientific problem rather than a definition problem.”
