Experts Call for a Major Shift in How We Measure Obesity

Traditional conceptions of obesity might be in for a radical shift. In a new report published this week, a widely-supported group of scientists is pushing for substantial changes in how obesity is diagnosed and classified.

Over 50 obesity experts from across the globe conducted the report, as part of a commission backed by The Lancet. Among other recommendations, the group is calling for body mass index (BMI) to be phased out as the sole criteria for diagnosing obesity. They also argue that obesity should be categorized into two distinct types, depending on how much harm a person’s condition is likely causing them.

The commission’s specific goal was to establish objective but nuanced criteria for diagnosing obesity. Currently, obesity is diagnosed strictly by a person’s BMI, which is calculated using a person’s weight and their height. People with a BMI over 30 are considered to have obesity, with severe obesity seen as having a BMI over 40 (some parts of the world use lower BMI cut-offs to account for population differences in the average size of residents).

Though BMI is an easily obtained and trackable measurement, however, it often fails to convey the whole picture, the commission experts say—a critique that plenty of other public health experts have expressed in the past. The harm associated with obesity is primarily caused by having excess body fat, and BMI can sometimes fail to adequately correlate with that. A very fit person could have high BMI but low body fat, for example, whereas someone with so-called “normal” BMI could be carrying a risky amount of body fat. The distribution of excess body fat can differ from person to person, too, as can the health risks associated with that fat. Too much fat around a person’s waist or around vital internal organs like the liver and heart tends to be more dangerous than excess body fat in the skin underneath our arms or legs, for instance.

The experts aren’t calling for doctors to completely abandon BMI as a tool for diagnosing obesity, but rather for it to be used alongside other bodily measurements. These include measuring people’s waist circumference, their waist-to-hip ratio, or their waist-to-height ratio. Medical professionals should use at least two body size measurements for diagnosing people suspected to have obesity, the researchers say, and one other measurement besides BMI. Alternatively, doctors can instead collect a direct measurement of a person’s body fat, such as by conducting a bone density test, commonly known as a DEXA scan. People with very high BMI (over 40) can still be assumed to have excess body fat, they added.

“If implemented—people with obesity (a BMI near or at or above obesity) should have at least one more measure of body size (e.g. waist circumference in most cases or DEXA if available) to confirm accurate detection of excess body fat—this would confirm one has indeed obesity, and is not just, for instance, a muscular person with high BMI,” Francesco Rubino, an obesity researcher at Kings College London and chair of the commission, told Gizmodo in a email.

Rubino and his commission are also recommending that doctors group obesity into two broad categories: preclinical and clinical obesity. Having high body fat alone might not negatively affect your health, the experts note, so they’ve created a list of criteria (18 for adults, 13 for children) for identifying when someone’s obesity is likely causing other bodily issues. Someone who has obstructive sleep apnea, severe knee pain, or poor cardiovascular health suspected of being linked to their excess body fat would be classified as having clinical obesity, for instance, while someone with obesity but no signs of abnormal organ function would be classified as having preclinical obesity.

“A reframing of the clinical effect of obesity is warranted, to explain how obesity can be both a risk factor for other diseases and a direct cause of illness. The definition of clinical obesity therefore addresses a gap in the characterization of obesity as a direct cause of ill health, and can be an effective way to address widespread misperceptions and bias that misguide decision making among patients, healthcare professionals, and policy makers,” the  authors wrote in their report, published Tuesday in The Lancet Diabetes & Endocrinology.

People with preclinical obesity may still be at higher risk of health problems in the future, but the distinction allows for more personalized obesity care, Rubino says. Whereas people with clinical obesity should be immediately treated with effective treatments to reduce their weight, which can include newer drugs like semaglutide (the active ingredient in Ozempic and Wegovy) or bariatric surgery, doctors can use a less intrusive approach with someone who has preclinical obesity, depending on their level of risk.

“These strategies may be as simple as suggesting monitoring over time and lifestyle changes aimed at feasible weight loss for people whose risk is low (even modest weight loss can go a long way to prevent diseases associated with obesity), or include more active forms of intervention should the risk be assessed as particularly elevated (due to other factors in addition to obesity itself, like family history, abdominal disposition of fat, other conditions, an extreme weight, and especially the combination of the above),” Rubino said.

The group’s recommendations are ultimately just that. But their conclusions are being widely endorsed by health-related organizations across the world—76 in total, including the American Heart Association in the U.S., the Royal College of Physicians in the UK, and the World Obesity Federation. There are still important big questions left to be answered about the nature of obesity, such as the exact prevalence of preclinical to clinical obesity (under the current BMI-only definition, more than 1 billion people worldwide are thought to have obesity). But according to report co-author Robert Eckel, an endocrinologist at the University of Colorado School of Medicine, the new guidelines should go a long way in helping both doctors and people living with obesity.

“The intent of our work was to optimize and individualize patient diagnosis, and risk vs. care,” he told Gizmodo. “We think this characterization will benefit patients, health care providers, and health care to follow.”