Move over, body mass index. Make room for roundness — to be precise, the body roundness index.

The body mass index, or BMI, is a ratio of height to weight that has long been used as a medical screening tool. It is one of the most widely used health metrics but also one of the most reviled, because it is used to label people overweight, obese or extremely obese.

The classifications have been questioned by athletes including American Olympic rugby player Ilona Maher, whose BMI of 30 technically puts her on the cusp of obesity. “But alas,” she said on Instagram, addressing online trolls who tried to shame her about her weight, “I’m going to the Olympics and you’re not.”

Advocates for overweight individuals and people of color note that the formula was developed nearly 200 years ago and based exclusively on data from men, most of them white, and that it was never intended for medical screening.

Even physicians have weighed in on the shortcomings of BMI. The American Medical Association warned last year that BMI is an imperfect metric that doesn’t account for racial, ethnic, age, sex and gender diversity. It can’t differentiate between individuals who carry a lot of muscle and those with fat in all the wrong places.

“Based on BMI, Arnold Schwarzenegger when he was a bodybuilder would have been categorized as obese and needing to lose weight,” said Dr. Wajahat Mehal, director of the Metabolic Health and Weight Loss Program at Yale University. “But as soon as you measured his waist, you’d see, ‘Oh, it’s 32 inches.’”

So, welcome a new metric: the body roundness index. BRI is just what it sounds like — a measure of how round or circlelike you are, using a formula that takes into account height and waist, but not weight.

It’s a formula that may provide a better estimate of central obesity and abdominal fat, which are closely linked to an increased risk of developing Type 2 diabetes, hypertension and heart disease, unlike fat stored on the buttocks and thighs.

A paper published in JAMA Network Open in June was the latest in a string of studies to report that BRI is a promising predictor of mortality. BRI scores generally run from 1 to 15; most people rank between 1 and 10. Among a nationally representative sample of 33,000 Americans, BRI scores rose between 1999 and 2018, the new study found.

Those with BRI scores of 6.9 and up — indicating the roundest bodies — were at the highest risk of dying from cancer, heart disease and other illnesses.

Their overall mortality risk was almost 50% greater than those with BRIs of 4.5 to 5.5, which were in the midrange of the sample, while those with BRI scores of 5.46 to 6.9 faced a risk that was 25% higher than those in the midrange.

But those who were least round were also at elevated risk of death: People with BRI scores under 3.41 also faced a mortality risk that was 25% higher than those in the midrange, the study found.

The paper’s authors suggested that the lower scores, seen mostly in those 65 and older, might have reflected malnutrition, muscle atrophy or inactivity.

“BMI cannot distinguish body fat from muscle mass,” Wenquan Niu, who works at the Center for Evidence-Based Medicine at the Capital Institute of Pediatrics in Beijing and was a senior author of the paper, wrote in an email. “For any given BMI, fat distribution and body composition can vary dramatically.”

Indeed, Niu wrote, “When BMI is used to frame risk, it often overestimates risk for muscular athletes, whereas it underestimates risk for older persons with muscle mass that’s been replaced by fat.”

The fat stored in the abdominal cavity is critically important, as it surrounds internal organs such as the liver, and contributes to insulin resistance and glucose intolerance that often precede Type 2 diabetes. It also fosters high blood pressure and lipid abnormalities that can lead to heart disease and death.

“Excessive visceral fat deposition is just like a silent killer lurking in our body, which can sneak up on a person over years with few noticeable symptoms, especially among apparently lean people,” Niu said.

BRI is the brainchild of mathematician Diana Thomas, now a professor at the U.S. Military Academy in West Point, New York. She first described it in a 2013 paper in the journal Obesity.

Although BMI is grounded in the geometry of a cylinder, Thomas said, she was looking in the mirror one day and thought to herself,: “I’m not a cylinder — I’m kind of more an egg. I have hips that make me more like an egg. How do I capture that?”

“In pre-calculus, you learn about eccentricity — how close to a circle you are,” she said. “Different people can be categorized as different ellipses. Some of us are more spherical. Some are closer to a circle. Someone who’s lean and mean is less of a circle.”

Thomas’ paper included a graphic illustration worth the proverbial 1,000 words. It depicted three figures: a lean man who is 5-foot-8 and has a 27-inch waist; a muscular man who is 5-foot-6 with a 29-inch waist; and one with more fat than the others who is 5-foot-6 with a 36.6-inch waist. Each had a BMI of 27.

Other researchers investigated the idea. A 2016 study that compared BMI with blood pressure and blood work, for example, found that the measure was a poor match for health status.

Nearly half of people deemed overweight, as defined by having a BMI between 25 and 29.9, and nearly one-third of those said to be obese — a BMI of 30 or above — were actually in good metabolic health.

Thirty percent of those with a BMI of 18.5 to 24.9, which is deemed a healthy weight, were actually in poor metabolic health.

Ethnic variation is increasingly recognized as a wild card by medical experts. Asians and those with Asian ancestry, for example, have a pattern of central obesity that puts them at high risk for Type 2 diabetes, even at lower BMI scores.

Several medical organizations have recommended considering these patients overweight at a BMI of 23 rather than 25, and obese at a BMI of 27 rather than 30.

Physicians tend to agree that BMI is a crude measure that doesn’t capture variations in body shape, composition, muscle mass and bone density.

“There’s extreme variation in the population in the degree of muscularity of people, even those of the same weight,” said Dr. Steven Heymsfield, a professor of metabolism and body composition at Louisiana State University’s Pennington Biomedical Research Center.

“The percentage of fat can vary from 10% to 40% among people of the same BMI, age and sex,” Heymsfield said. “The BRI is a way of capturing these variations.”

New York Times News Service