The body mass index is not going away, a new report suggests, but it could be demoted to just a first step in making a more nuanced diagnosis of obesity as a disease, one that distinguishes between what is and isn’t (yet) an illness requiring treatment.
A Lancet Diabetes and Endocrinology global commission on clinical obesity published Tuesday acknowledges that BMI — the familiar metric derived from height and weight — may be useful as a screening tool before more medically relevant indicators, principally excess body fat, are called into play. BMI can tell us about the health of populations, but for the individual sitting in a primary care office, how their fat levels might be affecting organ function is much more telling.
Defining obesity as a disease has been a sea change long in the making, going back to the World Health Organization in 1948, a conclusion more recently echoed by medical societies such as the American Medical Association in 2013.
Ten years later the AMA urged doctors to downgrade BMI in their health assessments of patients. Treatments are getting more scrutiny, too, as recent guidance from the U.S. Food and Drug Administration edged closer to weight loss drugs and further away from lifestyle changes as the first option for doctors and patients to consider.
That shift — recognizing obesity as a disease and not just a risk for other diseases — demands more clarity in diagnosis, the Lancet commission concluded.
“The lack of a clinical diagnosis couldn’t be more consequential because it does not allow at the individual level to actually establish what is the level of health or illness of the person because of obesity,” Francesco Rubino, commission chair and professor of metabolic and bariatric surgery at King’s College London, said in an interview. “Without a medically coherent description of what obesity looks like as a standalone illness, you cannot even answer the question of whether obesity is a disease or not, which has been now a very hot debate for many years.”
Years of accumulating evidence showing how obesity can impair organ function was reviewed by the commission, leading it to develop a new framework for diagnosis and treatment.
The commission, an international group of experts including people with obesity, proposes two new categories of disease: preclinical and clinical obesity. Body fat and where it is distributed around the body, as well as 18 biological markers tests for high blood sugar or cholesterol among others signals, should be evaluated, along with ongoing illness. Thirteen indicators are flagged for children, including musculoskeletal problems that influence their developing bodies.
Shortness of breath, joint pain, and metabolic abnormalities like type 2 diabetes are obvious signs of what the commission classifies as clinical obesity. But high BMI without such symptoms may mean a person — think an athlete — has higher than typical muscle mass.
BMI can flag people who should only be watched, not treated in what it calls preclinical obesity when they are at risk, much like prediabetes or prehypertension. There are also people who don’t meet the 30 BMI threshold for obesity who are nonetheless at risk for serious cardiovascular conditions because their bodies carry excess fat under the BMI radar.
Clinical obesity is defined as a chronic disease associated with ongoing organ dysfunction caused by obesity alone, the commission said. Preclinical obesity is associated with varying degrees of health risk but not with ongoing illness.
To zero in on obesity without solely relying on BMI, the commission recommends direct measures of fat, such as waist circumference, waist-to-hip ratio, or waist-to-height ratio. The more sophisticated DEXA scan of body fat may be more accurate, but it’s not widely available in the United States or many other countries.
Given the recent rise of costly, lifelong obesity drugs and the seriousness of weight loss surgery, assessing obesity correctly is critical for avoiding both under- and overdiagnosis, Rubino said.
The commission’s findings, approved by 76 medical organizations around the world and published in The Lancet Diabetes & Endocrinology, did not rely on industry funding but was supported by King’s Health Partners. The commission’s 58 members declared multiple conflicts of interest, including research grants or consulting fees from companies that make or market obesity drugs.
Under the commission’s approach, the diagnostic process would start with BMI as a vital sign, then take into account individual characteristics including blood biomarkers and behaviors. Can the person carry out typical daily activities? Are kidney, nervous, urinary, and reproductive systems functioning? If not, that points to clinical obesity.
People with excess body fat may not have evidence of such organ problems at what is called the preclinical stage, but they are deemed at heightened risk for cardiovascular problems, type 2 diabetes, and some cancers as well as clinical obesity. In children, family history of type 2 diabetes or hypertension is especially important in assessing the need for treatment at the preclinical stage, Louise Baur, chair of child and adolescent health at the University of Sydney, said at a media briefing Monday.
“Two children may share the same body fat, the same BMI, but one is at much higher risk than the other,” she said.
“You can have obesity and yet be very healthy and you may never develop a medical problem,” Robert Kushner, commission member, internist, and professor of medicine at Northwestern Feinberg School of Medicine, told STAT in an interview. He has led clinical trials of the obesity drug semaglutide, including STEP and SELECT. “But we called it ‘preclinical obesity’ because even in that situation, we know from other studies that if you wait long enough, there is a higher risk of developing medical problems.”
BMI has long been falling out of favor. When the American Medical Association urged downgrading BMI in 2023, it blamed its “racist exclusion” and “historical harm.” The group also recommended measuring waist circumference as well as body composition.
Rubino acknowledged the wide variability in risk tied to BMI that doesn’t account for race and ethnicity. Criteria for waist circumference as a measure of body fat do vary among different populations around the world.
“If you want to use a BMI-based measure of obesity, you automatically create a mechanism for inequalities and discrimination,” he said. Still, “while a lot of the social determinants of obesity that in the current environment can have and should have some consideration, the question of whether something is a disease or is not a disease is quintessentially a medical question.”
The commission won cautious praise from some experts not involved in its creation.
“I applaud them in spirit,” Ethan Weiss, a cardiologist at the University of California, San Francisco, said. “I also recognize that it’s hard to put toothpaste back in the tube, and that the world exists now in a very BMI-centric way.”
Clinical trials use BMI in their inclusion criteria, as do FDA approvals and labels based on them. Insurance companies make decisions based on those rulings.
“It’s not going to be an overnight thing where the switch gets flipped and we go from talking about BMI to talking about some other measure of obesity,” he said. “I applaud them for suggesting that we try because it certainly makes sense from an intellectual standpoint.”
In the exam room, Weiss said, encouraging doctors to focus on the mechanistic and primary drivers of the risk — meaning adiposity rather than just BMI — is a good start.
Melanie Jay, a professor of medicine and population health at New York University, co-director of the NYU Langone Comprehensive Program on Obesity, and a VA researcher, said she appreciates the commission’s wrangling with questions doctors have been dealing with for a long time.
“I like that you’re not only relying on obesity-related comorbidities to be able to treat their disease,” she said, particularly when insurers require another diagnosis such as hypertension or diabetes even if patients are being treated in a weight management program.
She does wonder about turning to waist circumference as a better way to gauge a person’s unhealthy fat mass. While reasonable, she has concerns about whether it’s stigmatizing — or necessary — if someone’s BMI is 35 or higher. “I feel like patients already don’t love being weighed when they come to the doctor. It’s a lot more invasive to take a measuring tape around your waist.”
Waist measurements wouldn’t be needed if BMI is above 40, a commission member said at Monday’s media briefing. “We can assume that someone with a very high BMI necessarily has additional adipose tissue as part of their body composition,” said Robert Eckel, emeritus professor of medicine at the University of Colorado.
The commissioners do say that people with a BMI of 40 are likely to have clear symptoms and signs of obesity, from limited movement to concerning blood tests, that might not need to be confirmed with a body measurement.
Commission chair Rubino thinks established treatments like bariatric surgery and newer tools like GLP-1 drugs make it more urgent to get diagnosis right. Further research is needed to see if obesity prevalence will change if the preclinical and clinical framework — which Eckel called “completely novel” — takes hold.
NYU’s Jay did bring up similarities to the Edmonton Obesity Staging System and its subclinical classification, but Kushner of the commission said in response that while staging systems identify people with obesity at risk for other illnesses, they do not diagnose or define obesity per se.
Alexandra Cremona, associate professor of human nutrition and dietetics at the University of Limerick, also said the commission’s work echoed some of the Edmonton system’s stages, up to a point.
“The Lancet places stronger emphasis on recognizing obesity as an independent disease entity rather than solely a risk factor for comorbidities,” she said in a statement via the Science Media Centre. “This reframing offers potential to influence public health policies and clinical approaches by shifting focus to diagnosing and managing obesity itself, irrespective of associated conditions.”
Obesity drugs now dominate the conversation, in the exam room and in research.
“The landscape has changed while the commission was at work,” Rubino said at the briefing. “We are calling for a change, a radical change, because obviously in the context of 1 billion people being classified as having obesity in the world today and with a number that is projected to increase, nobody, no country, rich or poor, is rich enough to be able to afford inaccuracy in the diagnosis of obesity.”
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