Doctors urged to move beyond BMI alone as a health measure
Roughly 200 years ago, a Belgian mathematician and statistician named Adolphe Quetelet, seeking to characterize a “normal man,” observed that adults’ body weight in kilograms is roughly proportional to the square of their height in meters – a measurement that came to be referred to as the Quetelet index.
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It wasn’t until 1972, when physiologist Dr. Ancel Keys proposed it as an estimate of body fat, that it got its more recognizable name: the body mass index, or BMI.
Now it’s everywhere, most specifically as a screening tool for obesity: A BMI of less than 18.5 is “underweight,” “healthy weight” is up to 24.9, “overweight” is 25 to 29.9, and “obesity” is a BMI of 30 and above.
And for a metric designed to be applied broadly across the general population, it’s taken on outsized significance for individuals, even sometimes called a “scarlet letter.”
“It is noted in every medical record,” said Dr. Jamy Ard, a professor of epidemiology and prevention at the Wake Forest University School of Medicine. “It is part of the consideration for life insurance; your employer may use BMI to offer certain wellness programs to you.”
BMI is used to determine eligibility for weight loss medications, and it can affect access to joint-replacement surgery and fertility treatment.
“It is easy to calculate and essentially costs nothing,” Ard wrote in an email, “so this makes it very hard to replace.”
But a movement to shift away from BMI as a measure of individual health risk alone is gaining steam: Last week, the American Medical Association adopted a new policy on the index, noting “significant limitations associated with widespread use of BMI in clinical settings” and citing its “historical harm” and “use for racist exclusion.”
“BMI is based primarily on data collected from previous generations of non-Hispanic white populations,” the AMA wrote. And while it’s “significantly correlated with the amount of fat mass in the general population,” the association said, it “loses predictability when applied on the individual level.”
To some clinicians, the need to focus on more than BMI for individuals is old news.
“This is almost like somebody announced that we’ve just figured out that people are going to be using cell phones,” said Dr. Ethan Weiss, a preventive cardiologist at the University of California, San Francisco and entrepreneur in residence at venture capital firm Third Rock. “It’s like, ‘Oh, really? That was 20 years ago.’ ”
Dr. Shannon Aymes, an assistant professor of medicine focused on weight management at the UNC School of Medicine, said she uses BMI along with other criteria like the Edmonton Obesity Staging System, combined with up to an hour-long patient visit, to come up with a plan for each patient.
“Obesity, like all disease, is complex and cannot be captured with a single measurement,” she said, pointing out that there are different severity levels. “There are people with elevated BMI who have no evidence of disease typically associated with higher weight such as hypertension, obstructive sleep apnea and type 2 diabetes.
“But there are some people with modestly elevated BMI,” she continued, “who have metabolic disorders that are potentially responsive to weight loss.”
The AMA’s new policy acknowledges those limitations, noting that BMI doesn’t differentiate between lean and fat body mass, and points out that it doesn’t account for differences between racial and ethnic groups, sexes and people at different ages.
Women tend to have more body fat than men, for example, and Asian people have more body fat than white people, according to a report supporting the AMA’s policy change.
BMI also doesn’t account for where people carry fat, something that’s become a clear marker of health risk over the past few decades, Weiss said.
“If you carry a lot of fat in your abdomen and around your organs, or visceral fat, basically, that’s bad,” he explained. “If you carry fat in your hips and your legs, your thighs and your rear end, that’s actually not only not bad, it’s good.”
Put another way, he said, having an “apple-shaped” body “is much more of a risk factor than pear-shaped.”
And different groups tend to carry fat differently: Black women, Ard said, tend to carry more body fat around the hips and thighs compared with white women, who carry fat more centrally around the waist, raising the risk for heart disease and type 2 diabetes, at the same BMI.
This is where dependence on BMI can disadvantage certain groups, he explained.
“If BMI is used to help determine life insurance rates, for example, you might have women of color who have a larger body size but are metabolically healthy get higher premiums compared to other women who might have a lower BMI but have body fat in different regions,” Ard said.
“I do not believe that BMI as a measure itself is racist,” he continued. “In general, it is not good science to extrapolate results from one group to another without validating the science in the target population.”
The AMA suggests that other measurements of health risk be used potentially along with BMI, like waist circumference, measurements of visceral fat, body composition, and genetic and metabolic factors.
“More important than BMI, in my opinion, is whether any individual patient has the metabolic syndrome,” said Dr. Willa Hsueh, a professor of medicine and director of the Diabetes and Metabolism Research Center at Ohio State University’s Wexner Medical Center. She cited elevated triglycerides, low levels of so-called good cholesterol, diabetes or prediabetes, high blood pressure or excess liver fat.
“These components increase cardiovascular risk including heart attack, stroke and heart failure,” she said, noting that a high BMI can prompt a physician to check for those factors and provide treatment.
And although BMI’s limitations are well-understood, some doctors say it will be hard to displace completely.
“There are other ways of assessing body fat,” said Dr. Louis Aronne, director of the Comprehensive Weight Control Center at Weill Cornell Medicine, but “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.
Aronne said BMI shouldn’t be the gatekeeper for weight-loss treatment, though, for people with “overweight” or even “normal” BMI who have high waist circumference or lab findings suggesting metabolic risk. People in that category, he argued, “should qualify for obesity treatment.”
And the AMA’s move shouldn’t be taken as directed for individuals to dismiss BMI completely, Ard said.
“The thing I don’t want to happen as a result of this is that people of color and black people in particular ignore BMI and discussing excess body weight with a health care professional because they misunderstand the intent here,” he said in the email. “The goal is to personalize how BMI is used in medical decision-making and to move away from blanket generalizations that can lead to stigma and bias.”
