Medical Group Says B.M.I. Alone Is Not Enough to Assess Health and Weight

The American Medical Association voted to adopt a new policy on Tuesday encouraging doctors not to rely only on body mass index, a long-used but potentially misleading metric, when assessing weight and health. The policy officially recognizes the “historical harm” of B.M.I. and states that the metric has been used “for racist exclusion.”

“It’s a pretty big shift,” said Dr. Cynthia Romero, director of the M. Foscue Brock Institute for Community and Global Health at Eastern Virginia Medical School, who was involved with the development of the new policy. “Now we have to be truly more mindful and more holistic when it comes to patient care.”

The policy could be a first step in moving away from a model of medicine that urges people above a certain B.M.I. to lose weight, without accounting for the toll those weight loss measures may take, said Dr. Scott Hagan, an assistant professor of medicine at the University of Washington who has studied obesity and was not involved with the decision. “It’s a really big deal,” he said. While the A.M.A. is influential in the medical community, the new recommendation is just a suggestion to doctors, not a strict rule they must comply with.

The association, one of the largest medical groups in the country, will now recommend that doctors not use B.M.I. alone to assess whether a patient is at a healthy weight. The A.M.A. suggested that physicians also consider factors like someone’s visceral fat (the fat stored in the abdominal cavity and around the organs), body adiposity index (a calculation that uses hip circumference and height), the percentage of fat, bone and muscle in one’s body and genetic and metabolic factors, like abnormal blood sugars or thyroid tests.

Body mass index is a simple calculation with a complicated past. You generate it by taking your weight in kilograms and dividing it by the square of your height in meters. Critics have long argued that the metric is an inaccurate measure of health, since someone with a lot of muscle and little fat may have the same B.M.I. as someone with obesity.

“The B.M.I. is just a very poor measure of general health,” Dr. Hagan said. “Someone with an elevated B.M.I. may be perfectly healthy.”

And where you carry weight matters, said Leslie Heinberg, director of the Enterprise Weight Management Center at the Cleveland Clinic. Abdominal fat is associated with higher rates of Type 2 diabetes and cardiovascular disease, compared with fat around the hips, she said, a nuance that the metric fails to capture.

“There are numerous concerns with the way B.M.I. has been used to measure body fat and diagnose obesity, yet some physicians find it to be a helpful measure in certain scenarios,” Dr. Jack Resneck Jr., who stepped down as president of the A.M.A. this month, wrote in a statement, adding that physicians should understand both the benefits and limitations of the metric.

At a population level, B.M.I. is “probably the best we can do” at assessing large groups of people, said Iliya Gutin, a program officer at the National Academies of Sciences, Engineering and Medicine and a research affiliate at the University of Texas at Austin who has also studied the metric and said that he applauds the new policy. It is also inexpensive and efficient.

But B.M.I. is “not this magic or powerful number that dictates how healthy or sick you’re going to be,” said A. Janet Tomiyama, a professor of psychology at the University of California, Los Angeles, who has studied B.M.I. and said she was “in shock” about the new policy. “For the longest time, I’ve been in this emperor-has-no-clothes situation, where I just couldn’t understand why really smart physicians continue to rely on something that was so clearly flawed.”

The new policy also highlights that B.M.I. has been based primarily on data pulled from previous generations of non-Hispanic white people, making it difficult to apply the measure to a broader population.

Despite pushback against the metric, it has remained widely entrenched. 

“A lot of our standards and guidelines were built around it,” Dr. Gutin said. “Once that happens, it’s very hard to shift the inertia.”