For years, medical experts have defined obesity primarily based on body mass index, which measures stored fat by calculating height and weight, to determine a person’s health risks.
Major public health organizations, including the World Health Organization and the US Centers for Disease Control and Prevention, define adult obesity as a BMI of 30 or greater and overweight as a BMI between 25 and 29.9.
Recently, an international commission proposed a revised definition of obesity that focuses on how excess body fat affects the body, and factors in waist circumference, rather than simply calculating BMI. This new definition, published January 14 in the journal The Lancet Diabetes & Endocrinology, could shift the paradigm of clinical care for the 890 million adults worldwide who are currently considered to have obesity.
What is BMI, and what have been criticisms of its usage? How is the new definition different? In what ways could it change clinical care? Will these changes be forthcoming? And while revisions are being considered, what should people consider asking their health care providers?
Dr. Leana Wen: Body mass index, or BMI, is a calculation performed by dividing a patient’s weight in kilograms by their height in meters squared. For those who are more used to weight in pounds and height in feet and inches, there are a number of online calculators. You can input weight and height and obtain an individual’s BMI.
The main advantage of BMI is ease of calculation. All you need to know someone’s BMI is a scale and ruler. Another advantage of using BMI as a basic indicator of health in a clinical setting is that it has been used for decades in many research studies, which have consistently shown higher BMI is associated with a number of chronic diseases including type 2 diabetes, high blood pressure, high cholesterol, heart disease and cancer. Conversely, lowering BMI is associated with a reduction in these risks.
Despite BMI’s ubiquity of use by health care professionals, it’s far from a perfect measure. To begin with, it measures total weight, rather than the weight attributed to fatty tissue. That means muscular people, including athletes, can have high BMI and could be considered on paper to have obesity when in reality they do not have too much body fat, also known as adiposity.
On the other hand, an individual could have normal BMI but have an unhealthy proportion of adiposity. Another criticism has been that BMI measurements do not provide consistently accurate results across racial and ethnic groups. This is an issue previously raised by multiple medical groups, including the American Medical Association, which in June 2023 issued a report that pointed to BMI as an imperfect way to measure body fat across different demographics. Some critics also think there should be separation between those who have obesity as defined by certain criteria and those who experience health consequences as a result of obesity.
Wen: This commission, composed of 58 experts worldwide, proposes that obesity be thought of differently from the traditional definition in two ways. First, instead of using BMI to define obesity, the authors propose using it as part of an initial screen to determine who should be assessed for excess body fat. These methods include measuring waist circumference, determining the waist-to-hip ratio or using special equipment, such as a DEXA scan, which measures bone density, to figure out body fat percentage.
Second, the commission proposes differentiating those who have obesity into two groups. The first is made up of those who are “clinically obese,” defined as those who already have signs of obesity affecting their body systems. Individuals who have obesity and also have diabetes or heart disease, or who are experiencing joint or back pain, or other obesity-associated medical conditions, would be considered clinically obese. Medical providers should consider these individuals for treatment to specifically address their disease of obesity.
On the other hand, there are those who are “pre-clinically obese.” The commission defines these individuals as those who have obesity, but whose obesity is not yet causing additional disease processes. For these people, obesity is a risk factor and should be addressed as such to reduce the risk of chronic conditions developing, but they do not yet have medical complications caused by obesity.
Wen: I think it’s good to be more precise about defining obesity. On a population level, it may be more convenient to measure BMI for the purpose of research studies, but on an individual patient level, BMI should be an initial screening tool — not the only measure that’s taken into account when determining whether someone has obesity.
Those who support the commission’s change point to the need to differentiate between individuals with obesity who need treatment with medications like the blockbuster GLP-1 agonist drugs approved for weight loss and those who could be managed with lifestyle changes alone. Perhaps the new approach could direct people with downstream consequences of obesity to receive needed medical care sooner. The proposed change in definition could also help reduce stigma around obesity so that it is treated as a chronic medical condition.
At the same time, BMI alone has long been known to be a major risk factor for the development of serious chronic ailments. Moreover, some clinicians may disagree with the classification of “pre-clinical obesity,” as they may wish to treat a serious chronic condition before complications arise. If this shift in definition catches on, one could imagine insurance companies denying certain forms of treatment until complications are seen, which would not be good from a preventive health care standpoint.
Wen: It’s possible, but I don’t think the changes will take place right away. BMI is such an ingrained part of clinical practice and research; I don’t think it will be easily replaced.
That said, I think a lot of clinicians and medical groups have already moved away from using BMI as the sole tool to assess for obesity. Perhaps this report can further push clinicians not to rely on BMI alone.
Wen: I think it’s very reasonable to ask your provider to measure your adiposity, as recommended in this commission’s report. Again, this can be as simple as a measurement of waist circumference. Studies have shown that the amount of belly fat is an important determinant that correlates with the risk of chronic diseases because it is a sign of adipose tissue accumulation around the organs.
I also think everyone should be aware of the health consequences associated with obesity. This is a condition that should not be met with stigma, but rather with compassion and targeted treatment, which includes nutrition, physical activity and, when appropriate, medical interventions — the same that would be considered for other chronic medical conditions. Individuals should discuss all of these options with their providers.