Stepping onto the scale at the doctor’s office may not be most people’s idea of a good time, but it’s an accepted part of the routine checkup. We’ve long considered body weight to be one important factor in overall health, and doctors regularly use it to calculate patients’ body mass index, or BMI.

You’re probably familiar with BMI, one of the few terms in medical jargon that’s penetrated everyday language. Put briefly, it’s a measure of weight in relation to height, and your BMI defines whether you’re in a healthy weight range, are under- or overweight, or have obesity. That’s, at least, how BMI has been used for decades.

Recently, there’s been a push to replace BMI with a new concept: BRI, or the body roundness index. As its name implies, BRI considers a person’s body shape, based on a formula that includes height and waist circumference. And in a big change from convention, it takes body weight out of the calculation entirely.

The problem with BMI

The origins of BMI are said to go back almost 200 years, but the term itself was coined in the 1970s. The researchers who developed the modern BMI calculation saw it as ideal for use in studies—a simple way to measure body size in large numbers of people and analyze its relationship to health. And it did work pretty well: A raft of studies have found that people with a BMI in the obesity range generally have increased risks of diseases such as diabetes, heart disease, and certain cancers.

But when it comes to gauging an individual patient’s health, critics have long pointed to the flaws of BMI. For one, the formula is based on old research done exclusively in men, mostly white. So it doesn’t account for the diversity in body size and shape among people of different races and ethnicities or between men and women.

BMI also falls short in this critical way: It draws no distinction between weight from body fat and weight from muscle. Muscle is heavier than fat, so an elite athlete can technically be labeled “obese” based on BMI. In fact, when Arnold Schwarzenegger was in his prime bodybuilding years—at a reported 6’2” and 235 pounds—he fell just inside the obese range based on his BMI of 30.2. Since that’s not the issue for most of us, here’s a much more common example: BMI is not a great metric for older adults. Like it or not, we tend to lose muscle and gain fat as we age—which means it’s possible to have an unhealthy amount of body fat even if your BMI is below the obesity threshold.

BMI tells us nothing about body fat distribution, either—that is, where you carry your fat. Where fat is located matters because, as we now well know, not all body fat is the same. Having a large waistline (being “apple-shaped”) is associated with higher disease risks, versus carrying excess body fat in the hips and thighs (being “pear-shaped”).

As I mentioned, we’ve known about these BMI imperfections for some time, and its use in routine medical care is coming under increasing scrutiny. In 2023, the American Medical Association issued a new policy encouraging doctors to use BMI only in conjunction with “other valid measures of risk,” such as waist size.

Is BRI the solution?

The concept of BRI has been around for about a decade, but lately it’s been gaining momentum. A study that came out in June 2024 in JAMA Network Open suggests that BRI does hold promise in gauging people’s health outlook. Researchers found that among nearly 33,000 U.S. adults, those with a BRI in the top 20 percent were about 50 percent more likely to die over the next 20 years, versus those with a BRI in the midrange. (Having a low BRI was bad news, too.) Another study in 2024 found that BRI was a better predictor of gallstone risk than BMI was.

So is it time for doctors to toss their scales in favor of tape measures? I don’t think so. We still need more research to prove BRI is a more accurate tool than BMI. But even if it is, the bigger question is whether routinely measuring BRI has real value for patients.

If I’m perfectly honest, I don’t think BMI calculations have added much to patients’ care. When I was a practicing internist, I always found that my own eyes gave me a great deal of information about a patient’s general weight and body shape. What was most important was seeing each person as a whole. Doctors and patients need to discuss family medical history; other measures of health, including blood pressure, blood sugar, and cholesterol levels; and lifestyle factors such as exercise, diet, and smoking and drinking habits. Weight and body shape do matter, but they are certainly not everything. And having a normal BMI or trim midsection are no guarantees of good health. So I’m not (yet) convinced that a shift from BMI to BRI would have an impact on patients’ lives.

All of that said, if you want more information on how waist size relates to your health (and the correct way to measure it on your own), the National Institutes of Health has resources. And of course, if you do have questions or concerns about your weight, bring them to your doctor.