Body mass index has for decades been used as a shorthand for assessing body fat and weight-related health risks. But for about just as long, critics have noted the simple calculation is laden with problems; BMI doesn’t actually measure fat mass, account for its distribution, or how those differ by age, gender, ethnicity, race, and how those differences affect health risks. Calculations and cutoffs are largely based on past generations of non-Hispanic white people. And BMI classifications mislead people on their individual risks of disease and death, and can lead to substandard care for eating disorders.

Now, it seems the hefty criticism has finally reached a critical mass. During the annual meeting of the American Medical Association (AMA) this week in Chicago, physicians and medical students voted to adopt a strongly worded policy acknowledging the calculation's "significant limitations" and "historical harms," including "racist exclusion." While the massive medical group acknowledged that BMI remains useful for population-level trends and associations, the policy called for doctors to become familiar with the calculation's problems and explore alternative measurements for diagnosing obesity and assessing health risks.

"There are numerous concerns with the way BMI has been used to measure body fat and diagnose obesity, yet some physicians find it to be a helpful measure in certain scenarios," AMA Immediate Past President Jack Resneck, Jr., said in a statement.  "It is important for physicians to understand the benefits and limitations of using BMI in clinical settings to determine the best care for their patients."

History

The shift will likely inspire cheers from critics, who have been increasing for decades. Even before the rise of BMI— aka the Quetelet Index as it was called in the 1970s—medical researchers had realized that simple features like leg length, bone mass, and body frame influenced weight-to-height ratios.

BMI is a simple calculation of weight in kilograms divided by height in meters squared. In 1993, an expert advisory committee for the World Health Organization came up with four BMI-based weight categories: underweight (below 20), normal weight (20 to 24.9), overweight (25 to 29.9), and obese (30 and up). In the US then, the National Institutes of Health considered the threshold for overweight to be a BMI of 27.8  for men or 27.3 for women. Anything under those cutoffs was "normal." This was based on an 85 percent cutoff point from data in a large national NIH survey. But in 1998, the NIH lowered the cutoff to 25, aligning with WHO categories. And overnight, millions of Americans went from having normal weight to overweight. That cutoff is still used.

BMI is a simple, easy-to-obtain number that has strong, established associations with body fatness, morbidity, and mortality data. But on an individual level, it's loaded with limitations. For one, it doesn't actually measure body fat mass—it can't distinguish lean mass from fat.