New medical policy finally address the limitations and flaws of BMI

'No single number gives a complete assessment of patient health.'
An old scale used for calculating body weight.
BMI is calculated by dividing a person’s weight by their height squared. Deposit Photos

Earlier this month, the American Medical Association adopted a new policy on the use of body mass index (BMI), which has been widely used since 1972, in clinical settings. The AMA suggests that going forward, physicians should also consider factors like the fat stored in the abdominal cavity and around the organs (visceral fat), a body adiposity index that uses hip circumference and height, along with genetics, metabolic factors, and the percentage of fat, bone, and muscle in a patient’s body.

[Related: There are better ways to measure body fat than BMI.]

BMI has been criticized by patients and some physicians for its inability to distinguish muscle from fat among numerous other factors, making it a flawed indicator of how much body fat a person actually has. Athletes with large amounts of lean muscle can end up labeled as “overweight” since muscle weighs more than fat, and fat distribution, not just quantity, has an effect on people’s health. 

BMI is also steeped in racial exclusion in medicine and “historical harm.” It is based on a roughly 200 year old calculation called the the Quetelet index, which was created by Adolphe Quetelet, a Belgian mathematician and statistician looking to use height and body weight to characterize “normal man.”

“The new policy was part of the AMA Council on Science and Public Health report which evaluated the problematic history with BMI and explored alternatives,” the AMA wrote in a statement on June 14. “BMI is based primarily on data collected from previous generations of non-Hispanic white populations.” 

BMI is calculated by dividing a person’s weight by their height squared. A BMI of less than 18.5 is  considered underweight, whereas a “healthy weight” is up to 24.9. Overweight is considered 25 to 29.9 a patient is considered obese with a BMI of 30 and above.

The AMA said that BMI “loses predictability when applied on the individual level” compared to being a general way to assess correlation of body fat. The new policy addresses BMI’s known limitations, noting that it doesn’t differentiate between lean and fat body mass or account for differences between racial and ethnic groups, age, or biological sex. 

A report supporting this recent policy change cites that women tend to have more body fat than men and Asian people have more body fat than white people—both are examples of differences between demographic groups. The BMI also does not take into account where body fat is distributed on the body, the authors say. 

Carrying more visceral fat (or having a more “apple-shaped” body) is considered a greater risk for heart disease and type-2 diabetes than being “pear-shaped” (carrying weight around the thighs, legs, and butt). Different groups also tend to carry this weight differently.

[Related: Why is it more dangerous to have belly fat?]

The AMA also notes that BMI is used because it is an inexpensive tool. It can be used to determine insurance rates, determine qualifications for surgery, and can even be a factor in adopting a child

“Health care professionals use a variety of types of numerical information to assess patient health, with common ones being blood pressure, blood glucose, body mass index (BMI) etc,” Wajahat Mehal, an MD and Director of the Yale Metabolic Health and Weight Loss Program at Yale Medicine told PopSci. “No single number gives a complete assessment of patient health and the recent AMA guidelines highlight the usefulness and limitations of using the BMI. These are excellent guidelines and emphasize the need to use a variety of numerical indicators, in addition to BMI, to obtain a complete assessment of patient health.”

Some doctors say that it will be difficult to completely erase BMI from your next physical exam. 

“There are other ways of assessing body fat,” Louis Aronne, director of the Comprehensive Weight Control Center at Weill Cornell Medicine, told CNN. However, “they’re not as easy and as inexpensive as BMI. I’m not sure we can throw out BMI until we have other measures that are as easy to use,” he said, adding that BMI should not be a gatekeeper for patients seeking weight loss treatment.