For decades, doctors have used two numbers to determine whether a person has obesity: their weight divided by their height. The calculation, known as body mass index (BMI), has the benefit of simplicity. But that’s also a flaw, because it doesn’t account for all the factors that determine whether someone’s weight is healthy, including their race or ethnicity.
Now, an international commission of experts is seeking to redefine obesity in a way that accounts for different body types. That’s a worthwhile endeavor, but they should proceed carefully: Overly strict criteria, if widely adopted by the medical community, could limit who is eligible for weight-loss medications such as Wegovy, Zepbound and other popular GLP-1 drugs.
BMI, despite its shortcomings, is not a useless metric. Studies across many decades have demonstrated that a BMI score of 30 and above, the current definition of having obesity, is associated with chronic diseases such as diabetes, heart disease, arthritis, sleep apnea, depression and cancer.
Still, many medical professionals believe that adiposity, or the accumulation of body fat, is a more accurate way of capturing health than BMI. Research increasingly shows that excess adiposity contributes to chronic inflammation, insulin resistance and other long-term problems associated with obesity. This is common sense: Someone who works out a lot and is very muscular might have a higher-than-average BMI, but they are probably healthier than an inactive person with lower BMI but less muscle and more body fat.
Changing the diagnostic paradigm to prioritize adiposity is the basis of the new recommendations. The guidelines state that people with a BMI of 40 or more can be assumed to have high adiposity, but those with lower BMI should undergo a body composition scan (such as dual X-ray absorptiometry, or DEXA). If this is not available, they could use proxy measures for adiposity such as waist circumference and waist-to-height ratio in combination with BMI.
This recommendation is reasonable and in line with the evolution of medical consensus. Even the traditionally conservative American Medical Association has been warning about the limitations of BMI since 2023. I still think BMI can be a reasonable initial screen, especially for those who lack easy access to adiposity measurements. But if patients can find out their body fat proportion, clinicians should start tracking their adiposity along with height, weight, blood pressure and other key metrics.
The commission made another, more controversial, point: Instead of classifying everyone with high adiposity as having obesity, it proposed separating patients into two categories. The first are the “clinically obese,” for whom obesity has already affected their body. Someone who has diabetes and high cholesterol and is suffering from chronic back and knee pain would fall into this category — and should therefore receive obesity treatment to address these related conditions.
The second are those defined as “pre-clinically obese,” meaning they have obesity by body fat measurements, but this condition has not yet triggered other disease processes. For these people, obesity should be regarded as a risk factor and managed with lifestyle modifications rather than as a disease that’s treated with medications.
Such a distinction would have profound implications. It’s not unreasonable to reserve enormously expensive GLP-1 medications (with list prices as high as $1,400 per month) for those with highest need, and the “clinically obese” have objectively greater need than the “pre-clinically obese.” They are more likely to realize substantial short-term benefit, so if medications are in short supply and health insurance coverage is limited, it makes sense to prioritize these patients.
But the concept of withholding treatment until after complications begin flies in the face of sound medical practice. No one would argue that patients with hypertension should wait to start medications to lower blood pressure until after they experience a heart attack or stroke. Nor would anyone say that people with diabetes must have vision or kidney problems before they start insulin. Obesity is not only a risk factor for other diseases but a serious medical condition in its own right. Indeed, many clinicians have adopted an “obesity first” approach, recognizing that treating it early can delay or prevent other conditions.
To be sure, GLP-1s pose many difficult questions, especially how — if ever — one can stop taking them. In the final days of the Biden administration, health officials announced that these medications would be part of Medicare’s next round of price negotiations. The Trump administration shouldn’t abandon these cost-containment efforts. People with obesity should be able to access these treatments as part of a comprehensive approach to optimize metabolic health.
