Two people can have nearly identical BMIs while experiencing very different effects of obesity on their health, according to a new international study testing a recently proposed way of defining obesity.
Researchers applied new categories known as clinical and preclinical obesity to 2,316 people being evaluated for bariatric surgery. Nearly three-quarters had clinical obesity, meaning excess body fat was accompanied by signs of organ dysfunction or limitations in daily functioning. About one-quarter had preclinical obesity, meaning they had excess body fat but no evidence that it was currently impairing organ function. Despite having similar BMIs, the two groups differed considerably in their overall health.
The study, published in JAMA Network Open, does not establish that the newer definitions are better than BMI for predicting long-term outcomes, nor does it show how well they would work in the general population. Instead, it suggests that looking at how excess body fat is affecting health may reveal information that BMI alone cannot capture among people considering bariatric surgery.
Several authors reported financial relationships outside this study with companies involved in obesity medications or bariatric surgery, including Novo Nordisk, Eli Lilly, Medtronic and Johnson & Johnson Medtech.
BMI, or body mass index, compares a person's weight with their height. It has long been used to classify obesity and has historically played an important role in determining who qualifies for bariatric surgery.
But BMI does not directly measure body fat or tell doctors whether excess fat is already affecting a person's organs, mobility or overall health.
In 2025, an international commission proposed shifting some of the emphasis away from BMI alone. Under that framework, clinical obesity describes excess body fat that is already associated with problems such as organ dysfunction or impaired daily functioning. Preclinical obesity describes excess body fat without those current health effects, although the person may still face an increased risk of developing them in the future.
For the new study, researchers looked back at clinical records from adults who underwent sleeve gastrectomy or gastric bypass between 2014 and 2025 at four bariatric surgery centers in the United Kingdom, France, Spain and Brazil. They then used information already recorded in patients' medical records to classify them under the new framework.
Of the 2,316 patients, 1,709, or 73.8%, met the criteria for clinical obesity. The remaining 607, or 26.2%, were classified as having preclinical obesity.
Yet BMI did not clearly separate the groups.
Within each center, average BMI and the distribution of BMI categories were similar among patients with clinical and preclinical obesity. In the U.K. group, for example, average BMI was about 47.5 among people with clinical obesity and 48.5 among those with preclinical obesity.
Their health profiles, however, looked quite different.
People classified with clinical obesity tended to be older and had a greater burden of chronic health problems. They also generally had higher estimated cardiovascular and long-term mortality risk scores and higher ratings of risk related to surgery.
Common health problems used to identify clinical obesity included high blood pressure, metabolic problems, sleep apnea and limitations involving movement or daily functioning. The specific patterns varied substantially among the four centers.
The findings don't mean that everyone with preclinical obesity is healthy or does not need treatment.
People in that category still have excess body fat and may face an increased risk of developing health problems over time. The distinction is intended to separate people who already have signs that obesity is affecting their health from those whose treatment may be aimed more at preventing future disease.
That difference could eventually matter when doctors and patients are considering treatment options, including medications and surgery.
For someone with clinical obesity, bariatric surgery may be used partly to treat health problems that are already present. For someone with preclinical obesity, the goal may focus more heavily on lowering the risk that those problems develop later.
But the new study did not test whether using these categories to decide who receives surgery leads to better outcomes.
Differences in major complications during the first 30 days after surgery were also less clear than differences in patients' health before surgery. Patients with clinical obesity had significantly more major complications in the French group, but the difference was not statistically significant in the U.K. or Spanish groups. Comparable complication data were not available from the Brazilian center.
There are other limitations.
Because researchers applied the new definitions to existing medical records, the four centers did not always collect health information in exactly the same way. In some cases, researchers had to use available diagnoses and clinical information to determine whether a patient met the newer criteria.
The researchers also could not always establish that a particular health problem was caused by excess body fat, something the new definition of clinical obesity technically requires. That could have led some people to be classified as having clinical obesity when another factor contributed to their condition.
And these were not typical adults with obesity. Everyone in the study was already a candidate for bariatric surgery, and BMI levels were generally high. The findings therefore cannot tell us how well the same categories would distinguish health status among the much broader population of people living with obesity.
Still, the results illustrate why two people represented by the same BMI number may not have the same health needs.
Rather than replacing BMI with another single measurement, the newer framework adds a different question: not only how much excess body fat someone has, but whether it is already interfering with how their body functions.
The published article does not list a study funding source. Several researchers reported financial relationships outside this study with pharmaceutical and medical-device companies involved in obesity treatment, including Johnson & Johnson Medtech, Medtronic, Novo Nordisk, Eli Lilly, Boehringer Ingelheim, AstraZeneca, Amgen, Sanofi and others. These relationships included research grants, consulting or advisory work and speaking fees.