Is B.M.I. a Scam?

There are few single measures in health care that seem to carry as much weight as body mass index, or B.M.I. We encounter it not just at doctor’s offices, but with online calculators and smart scales, at gyms and even when determining eligibility for the Covid vaccine.

Its formula is simple: Take your weight (in kilograms), and divide by the square of your height (in meters). The result, which slots you into one of four main categories, is meant to describe your body in a single word or two: underweight (B.M.I. less than 18.5), normal weight (18.5 to 24.9), overweight (25.0 to 29.9) or obese (30 or greater).

Many feel judged by these categories, given that only about a quarter of adults in the United States can call themselves “normal” on the B.M.I. scale. But after talking with an epidemiologist, two obesity medicine physicians, a health psychologist and a sociologist, none claimed that B.M.I. was a very useful measure of a person’s health. And, in fact, some said they would indeed call it a scam.

How is B.M.I. useful?

Introduced in the 1830s by a Belgian statistician who wanted to quantitatively describe the “average man,” the calculation was named the body mass index and popularized in the 1970s by the Minnesota physiologist Ancel Keys. At the time, Dr. Keys was irritated that life insurance companies were estimating people’s body fat — and hence, their risk of dying — by comparing their weights with the average weights of others of the same height, age and gender. In a 1972 study of more than 7,000 healthy, mostly middle-aged men, Dr. Keys and his colleagues showed that the body mass index was a more accurate — and far simpler — predictor of body fat than the methods used by the insurance industry.

“It is a very useful tool in epidemiologic research,” said Dr. JoAnn Manson, a professor of medicine at Harvard Medical School. Research has shown that across large groups of people, higher B.M.I. is generally associated with greater risk of heart disease, Type 2 diabetes and some types of cancer, she said. On average, people with a higher body mass index have more body fat, so it can be useful for tracking rates of obesity, which have nearly tripled globally over the last several decades. It also has a “J-shaped” relationship with mortality; very low and very high B.M.I.s are associated with greater risk of dying sooner, whereas the “normal” to “overweight” range is associated with lower mortality risk.

As Dr. Keys found, B.M.I. is also easy and inexpensive to measure, which is why it is still used in research studies and doctor’s offices today.