Weighty subject: Trouble with the metrics of body mass index
There are few single measures in health care that seem to carry as much weight as body mass index, or BMI 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. The result, which slots you into one of four main categories, is meant to describe your body in a single word or two: underweight (BMI under 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 US can call themselves “normal” on the BMI scale. But after talking with an epidemiologist, two obesity medicine physicians, a health psychologist and a sociologist, none claimed that BMI was a very useful measure of a person’s health. And, in fact, some said they would indeed call it a scam. 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 popularised 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 BMI 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 BMIs are associated with greater risk of dying sooner, whereas the “normal” to “overweight” range is associated with lower mortality risk.
As Dr. Keys found, BMI is also easy and inexpensive to measure, which is why it is still used in research studies and doctor’s offices today. BMI is “fairly useless when looking at the individual,” said Dr. Yoni Freedhoff, an associate professor of family medicine at the University of Ottawa. BMI can’t tell what percentage of a person’s weight is from their fat, muscle or bone. This explains why muscular athletes often have high BMIs despite having little body fat. And as people age, it’s common to lose muscle and bone mass but gain abdominal fat, a change in body composition that would be concerning for health but might go unnoticed if it didn’t change a person’s BMI, Dr. Manson said.
The measure also does a poor job of predicting a person’s metabolic health. In a 2016 study of more than 40,000 adults in the US, researchers compared people’s BMIs with more specific measurements of their health, like their insulin resistance, markers of inflammation and blood pressure, triglyceride, cholesterol and glucose levels. Nearly half of those classified as overweight and about a quarter of those classified as obese were metabolically healthy by these measures. On the other hand, 31 percent of those with a “normal” body mass index were metabolically unhealthy.