This simple number has had an impact on so many of us, but science says it might not actually be a good indicator of your health.
an illustration of 3 woman climbing stairs that are falling apart
Credit: Libby VanderPloeg

Body mass index (BMI) is foundational in health care around the world today. The simple equation of weight relative to height may be used to determine your risk for certain diseases, your insurance premiums and even whether or not you qualify for certain medical procedures. But researchers have been questioning the value of BMI in individual health assessment for a while now—and more recently, body-positivity advocates have implicated its contributing role in weight bias and racism in health care, which has more people than ever asking: should we be using BMI at all?

This formula—used as a proxy for body fat—was actually never intended to measure individual health. It was developed in the early 1800s to study weight variations across entire populations. "BMI is a rigid number based on old data," says Fatima Stanford, M.D., an obesity physician at Massachusetts General Hospital and Harvard Medical School. And it's far from perfect in terms of the cutoffs designed to indicate risk, adds Diana Thomas, Ph.D., a professor of mathematics and obesity researcher at the United States Military Academy at West Point. A study published in the International Journal of Obesity estimated that more than 74 million American adults were miscategorized as "unhealthy" or "healthy" based on their BMI alone. Nearly half of people considered overweight and 29% of those categorized as obese were actually metabolically healthy—meaning their blood pressure, cholesterol and blood glucose were all normal. Meanwhile, 30% of those considered "normal weight" had metabolic or heart issues.

Part of the problem is that BMI can't differentiate between weight from fat and muscle. So bodybuilders and elite athletes with high muscle mass may be categorized as overweight or obese. And even among people who do have higher body fat, BMI doesn't tell physicians anything about how it's distributed—a key connection to disease risk. Subcutaneous fat around the hips, thighs and buttocks is not as dangerous or as tightly linked to conditions like heart disease and type 2 diabetes as visceral fat in the abdomen.

Furthermore, BMI cutoffs are largely based on data from Western populations—predominantly Caucasian males—making it problematic to apply this measurement to the diverse U.S. population. There's growing evidence that body composition varies between races and ethnicities. African Americans, for example, are likely to carry more subcutaneous fat than their white counterparts who tend to have more visceral fat, says Stanford. A study published in the journal Obesity showed that, for Black adults in the U.S., having a higher BMI (25 and over) did not carry the same risk of death as it did in white adults. And there's research showing that people of Asian descent tend to have more body fat—and particularly visceral fat—compared to other racial and ethnic groups with the same BMI, putting them at an increased risk of type 2 diabetes, hypertension and cardiovascular disease at a lower BMI. Some experts argue that the cutoff for normal weight in Asian and South Asian groups should be set as low as 22, says Thomas.

Leaning too heavily on BMI, Stanford says, prevents good, individualized patient care. For instance, some procedures, like bariatric surgery, may only be covered by insurance for people above a certain BMI, even if someone's doctor has determined they're otherwise a good candidate.

Regardless of the BMI category you fall into, it's important to find a health care provider who will go beyond the number to get a holistic picture of your health. Because despite the stigma that people with a higher BMI have a greater risk for a number of diseases than thinner people, the research shows it's not that simple.

This article first appeared in EatingWell magazine, January/February 2022.