BMI Is a Flawed Measure of Obesity. What Are Alternatives?
Author: Mitchel L. Zoler, PhD / Via MedScape / April 26, 2023
BMI is trash. Full stop.” This controversial tweet, which received thousands of likes and retweets, was cited in a recent Medscape perspective by one doctor on when physicians might stop using body mass index (BMI) to diagnose obesity.
Body mass index (BMI) has for years been the consensus default method for assessing whether a person is overweight or has obesity, and is still widely used as the gatekeeper metric for treatment eligibility for certain weight-loss agents and bariatric surgery.
But growing appreciation of the limitations of BMI is causing many clinicians to consider alternative measures of obesity that can better assess both the amount of adiposity as well as its body location, an important determinant of the cardiometabolic consequences of fat.
Alternative metrics include waist circumference and/or waist-to-height ratio (WHtR); imaging methods such as computed tomography (CT), magnetic resonance imaging (MRI), and dual-energy X-ray absorptiometry (DEXA); and bioelectrical impedance to assess fat volume and location. All have made some inroads on the tight grip BMI has had on obesity assessment.
Chances are, however, that BMI will not fade away anytime soon given how entrenched it has become in clinical practice and for insurance coverage, as well as its relative simplicity and precision.
“BMI is embedded in a wide range of guidelines on the use of medications and surgery. It’s embedded in Food and Drug Administration regulations and for billing and insurance coverage. It would take extremely strong data and years of work to undo the infrastructure built around BMI and replace it with something else. I don’t see that happening [anytime soon],” commented Daniel H. Bessesen, MD, a professor at the University of Colorado in Aurora and chief of endocrinology for Denver Health.
“It would be almost impossible to replace all the studies that have used BMI with investigations using some other measure,” he said.
BMI Is “Imperfect“
The entrenched position of BMI as the go-to metric doesn’t keep detractors from weighing in. As noted in a commentary on current clinical challenges surrounding obesity recently published in Annals of Internal Medicine, the journal’s editor-in-chief, Christine Laine, MD, and senior deputy editor Christina C. Wee, MD, list six top issues clinicians must deal with, one of which, they say, is the need for a better measure of obesity than BMI.
“Unfortunately, BMI is an imperfect measure of body composition that differs with ethnicity, sex, body frame, and muscle mass,” note Laine and Wee.
BMI is based on a person’s weight in kilograms divided by the square of their height in meters. A “healthy” BMI is between 18.5 to 24.9 kg/m2, overweight is 25-29.9 kg/m2, and ≥ 30 kg/m2 is considered to represent obesity. However, certain ethnic groups have lower cutoffs for overweight or obesity because of evidence that such individuals can be at higher risk of obesity-related comorbidities at lower BMIs.
“BMI was chosen as the initial screening tool [for obesity] not because anyone thought it was perfect or the best measure but because of its simplicity. All you need is height, weight, and a calculator,” said Wee in an interview.
Numerous online calculators are available, such as this one from the US Centers for Disease Control and Prevention, where height in feet and inches, and weight in pounds, can be entered to generate the BMI.
BMI is also inherently limited by being “a proxy for adiposity” and not a direct measure, added Wee, who is also director of the Obesity Research Program of Beth Israel Deaconess Medical Center in Boston, Massachusetts.
As such, BMI can’t distinguish between fat and muscle because it relies on weight only to gauge adiposity, noted Tiffany Powell-Wiley, MD, an obesity researcher at the National Heart, Lung, and Blood Institute in Bethesda, Maryland. Another shortcoming of BMI is that it “is good for distinguishing population-level risk for cardiovascular disease and other chronic diseases, but it does not help as much for distinguishing risk at an individual level,” she said in an interview.