BMI has its flaws. Is ‘body roundness index’ better?

New research points to a better way to measure obesity than body mass index.

Body mass index was first developed in 1832 and has been the standard way of estimating a person’s body fat since the 1980s. The calculation, however, has come under increasing scrutiny in recent years.

An important criticism of BMI is that it does not analyze how much of a person’s weight is fat and where the fat is distributed throughout the body. It also does not take into account the other elements that make up a person’s body composition besides fat, including muscles, bones, water and organs.

“Fat distribution and body composition can vary dramatically among different people with the same BMI,” Wenquan Niu, a professor at the Capital Institute of Pediatrics in Beijing, wrote in an email.

Because muscle is much denser than fat, BMI is higher in people who are very muscular but have less body fat, such as athletes, Niu said. At the other end of the spectrum, BMI may be underestimated in older people with much less muscle mass and more body fat.

On a to study published last week in JAMA Network Open, Niu and his colleagues showed that a different measure, called the body roundness index, is a more accurate way of estimating obesity.

While BMI estimates a person’s body fat using just two measurements, height and weight, BRI also incorporates hip and waist circumferences to estimate the amount of total and visceral fat someone has. Visceral fat is a type of deep belly fat that surrounds your organs and can be more harmful to your health.

BMI “also doesn’t differentiate body composition, how much of your weight is water weight versus fat versus muscle or bone mass,” said Dr. Aayush Visaria, an internal medicine resident at Rutgers Robert Wood Johnson Medical School who was not involved. in the study.

Where BRI makes a big difference is in allowing for more variation, which can better reveal how much of a person’s body is fat.

The new study used data from the National Health and Nutrition Examination Survey, collected from about 33,000 Americans over nearly 20 years. The researchers calculated a person’s risk of death from any cause – which is typically higher if a person is underweight, overweight or obese – based on BMI and BRI.

They found that the BRI did a better job of calculating a person’s body composition and therefore what weight category they fell into.

When they graphed their data, the BRI points created a bell-shaped curve, with a higher risk of death – an indicator of poor health – at each end. When mapped in the same way, BMI also shows these two extremes, but creates a flatter, rather than rounded, middle part. This flat part masks variation, which is a big part of the problem.

“Variation is a part of life,” said Diana Thomas, a mathematics professor at the U.S. Military Academy at West Point who developed the BRI calculation when she was director of the Center for Quantitative Obesity Research at Montclair State University in New Jersey. .

This variation is more important in people who are close to the cutoff points for each category. BMI puts people into groups, with strict limits, such as a BMI of less than 25 being the limit for a healthy weight.

“When you draw these hard and fast lines, you don’t take variation into account. Some people who are above the limits may actually need to be below the limits,” Thomas said.

How does the BRI work?

Both measurements are rooted in geometry, said Thomas, who was not involved in the new study. BMI views the body as a cylinder.

“I looked in the mirror and said, ‘I don’t look like a cylinder, I look more like an egg,’” she said.

To account for more variations in body types, including egg-shaped bodies, Thomas compared a person’s waist or hip circumference to their height, but did not include weight in his calculations. Centering her theory around the body having an oval shape rather than a cylinder, she used a mathematical concept called eccentricity to develop the BRI.

Essentially, the closer someone is to the shape of a circle, the closer their BRI is to zero. The closer they are to being shaped like a straight line, the closer your BRI will be to 1.

“It allows for all variations of body shapes,” Thomas said.

The BRI is not a perfect measure. It’s not yet possible to calculate a person’s muscle mass, which plays an important role in health, “but I definitely think we’re moving in the right direction,” Visaria said. “BRI is one of the methods that have emerged trying to measure body composition in a way that is accessible to many people.”

This is critical, as specialized scales that measure a person’s body composition are not accessible to everyone – and are not always accurate – and it is also unrealistic to use other methods, such as scans, to do this.

“The goal is to have something that can be used by as many sites as possible,” Thomas said. “You don’t need a scanner or a special scale. The BRI only requires a tape measure.”