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May 21, 2021

Is BMI an effective health measure? Depends on the context, experts say.

Daily Briefing
    Editor's note: This popular story from the Daily Briefing's archives was republished on Nov. 11, 2021.

    There are "few single measures in health care that seem to carry as much weight as body mass index" (BMI). While some health experts believe the metric is a "very useful measure of a person's health," others actually consider it a "scam," Alice Callahan writes for the New York Times.

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    Why we use BMI

    According to Callahan, BMI became a popular measure of weight in the 1970s, when the Minnesota physiologist Ancel Keys popularized it among insurers. At the time, insurers were estimating people's body fat by comparing their weight to the average weight of others of the same height, age, and gender. Keys thought BMI—which uses people's weight and height to classify them as underweight (less than 18.5), normal weight (18.5 to 24.9), overweight (25.0 to 29.9), or obese (30 or greater)—was a more accurate measure.

    Since then, the measure has become a widely used indicator of overall health everywhere from gyms to doctor's offices. But Callahan—who spoke with an epidemiologist, two obesity medicine physicians, a health psychologist, and sociologist—found that few experts consider the metric an effective measure of health.

    When is BMI useful?

    According to JoAnn Manson, a professor of medicine at Harvard Medical School, BMI is most useful as a "tool in epidemiologic research," particularly when used to track overall rates of obesity. Research suggests that when applied across large populations, BMI is linked to a greater risk of heart disease, type 2 diabetes, and certain cancers, Manson. She also said BMI has a J-shaped correlation with mortality, in which people with very low or very high BMIs have a greater mortality, while those with "normal" or "overweight" BMIs have a lower mortality risk.

    Moreover, as Keys found in his work, Callahan writes, "BMI is also easy and inexpensive to measure, which is why it is still used in research studies and doctor's offices today."

    How effective is BMI as a metric for individual health?

    Although BMI may be an effective metric for research, it's "fairly useless when looking at the individual," according to Yoni Freedhoff, an associate professor of family medicine at the University of Ottawa.

    For instance, Callahan writes, BMI can't distinguish what percentage of an individual's weight is comprised of their bone, fat, or muscle, which is why many very muscular athletes often have high BMIs, despite having little overall body fat. And, as Mason points out, people lose muscle and bone mass as they age, while gaining abdominal fat—a shift in body composition with health care implications that might go unnoticed if the overall BMI remains the same.

    Further, a 2016 study of more than 40,000 U.S. adults suggests that BMI is an ineffective measure of people's metabolic health. In that study, researchers compared participants' BMI with more specific health measures, such as insulin resistance and blood pressure, and found that nearly 50% of those classified as overweight and about 25% of those considered obese "were metabolically healthy by these measures," Callahan writes, while a full 31% of those with "normal" BMIs were considered metabolically unhealthy.

    BMI can "label a huge swath of our population as somehow aberrant because of their weight, even if they're perfectly healthy," said A. Janet Tomiyama, lead author of the study and an associate professor of health psychology at the University of California-Los Angeles.

    And according to Sabrina Strings, an associate professor of sociology at the University of California-Irvine, BMI was primarily developed and tested as a measure among white men—meaning a standard that is used as universal metric has little if any representation for "[w]omen and people of color."

    Can BMI be harmful?

    There are several instances when BMI can be applied harmfully, experts told Callahan. For instance, Rekha Kumar, an associate professor of clinical medicine at Weill Cornell Medical College, said patients who are at their "healthy, happy weight," but who still have high BMIs, may feel as if they must lose more weight to be considered "normal"—even if that weight loss is unrealistic or unnecessary.

    In addition, Freedhoff added, if providers use BMI as the primary measure of someone's health, they may overlook poor diet or exercise habits in people with "normal" BMIs because they assume those people to be healthy. Providers may also miss important diagnoses among people with higher BMIs, incorrectly assuming their weight is the source of any health issues.

    And research has indicated that weight stigma is harmful, Tomiyama said, noting that anti-fat bias can result in lower-quality care. In addition, people who've felt discriminated against based on their weight are more likely not only to have mood or anxiety disorders, but to gain additional weight and have a greater mortality risk.

    According to Strings, that stigma may target Black Americans in particular, given that population tends to have higher BMIs—even though research indicates that higher BMIs among Black Americans "is not as clearly linked to earlier death in Black Americans," Callahan writes. Strings said instead of focusing on BMI, experts should focus on the structural issues that result in poor health, such as "poverty, racism, lack of access to healthy fruits and vegetables," and environmental toxins.

    What should we use instead of BMI?

    According to Callahan, experts suggested several alternative methods of gauging potentially unhealthy body fat.

    For instance, Mason recommended focusing on abdominal fat—which in excess can increase the risk of type 2 diabetes, high blood pressure, and other conditions—by measuring waist circumference. And Freedhoff said his weight management clinic focuses not on BMI, but on patient's "'best weight, which is whatever weight a person reaches when they're living the healthiest life they can actually enjoy."

    Meanwhile, Tomiyama said people should focus less on body size and more on measures of blood glucose, triglyceride, and blood pressure—as well as how they feel physically. To improve their health, she advises people to focus on metrics more controllable than BMI, such as getting "better sleep [and] more exercise," reducing stress, and eating more healthfully (Callahan, New York Times, 5/18).

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