Should the Body Roundness Index Replace BMI?

In daily practice, physicians need a quick and simple way to assess whether a patient’s weight presents a health risk. For decades, the body mass index (BMI) has been used for this purpose, with calculations based on height and weight. Despite its convenience, BMI has faced increasing criticism. Recent research suggests that another metric, the body roundness index (BRI), might better gauge the health risks associated with obesity.

According to experts, BRI may more accurately identify people with high levels of visceral fat than BMI. It’s well documented that abdominal fat is strongly linked to higher risks for obesity-related diseases.

Studies Support BRI

Several studies have suggested that BRI could be a valuable tool for assessing health risks. In June of this year, researchers from China reported a significant U-shaped association between BRI and overall mortality in a paper published in JAMA Network Open. People with very low or very high BRI had an increased risk for death, noted Xiaoqian Zhang, MD, from Beijing University of Chinese Medicine, Beijing, China, and his colleagues.

A study published in September in the Journal of the American Heart Association showed that elevated BRI over several years was associated with an increased risk for cardiovascular diseases. “The BRI can be included as a predictive factor for cardiovascular disease incidence,” stated the authors, led by Man Yang, MD, from Nanjing Medical University in Nanjing, China.

Why Replace BMI?

Why is a replacement for BMI necessary? When asked by Medscape Medical News, Manfred Müller, MD, senior professor at the Institute of Human Nutrition and Food Science at the University of Kiel, Kiel, Germany, explained: “BMI was designed to provide a simple value that was as independent of body size as possible, that could detect obesity and estimate related disease risks. But scientifically, BMI has always been a very crude measure to characterize disease risks.”

Müller was part of a research group led by US mathematician Diana Thomas, PhD, who, at the time, worked at Montclair State University, Montclair, New Jersey, and now holds a position at the US Military Academy at West Point, West Point, New York. The group developed and published the BRI in 2013.

BMI Classifies Bodybuilders as Obese

The researchers justified their search for a “better” anthropometric measure with two aspects of BMI that still constitute the main points of criticism of the widely used index today:

1. BMI incorrectly classifies individuals with significant muscle mass, like bodybuilders, as obese, as it doesn’t distinguish between fat and muscle mass. 
2. BMI provides no information about fat distribution in the body — whether it’s concentrated in the hips or the abdomen, for example. 

In practice, this means that a person with a normal BMI could already have prediabetes, high blood pressure, and high cholesterol, which might go undetected if no further investigations are conducted based solely on their BMI.

The BRI aims to solve this problem. As the name suggests, this index seeks to capture a person’s “roundness.” The formula for calculating BRI includes waist circumference and height but excludes body weight:

BRI = 364.2 − 365.5 × √(1 − [Waist circumference in cm/2π]²/[0.5 × Height in cm]²)

In their 2013 article, Thomas, Müller, and colleagues wrote that it still needed to be proven whether their newly developed index correlated with mortality and the risk for cardiovascular and metabolic diseases — and whether it was sufficiently better than BMI to justify the more complex calculation.

Could BRI Replace BMI?

Opinions differ on whether the BRI should replace the BMI. Zhang’s team concluded that the BRI needs to be validated in additional independent cohorts. If it does, it could become a practical screening tool in patient care.

Yang’s research group is optimistic about the BRI’s future: “The longitudinal trajectory of the BRI could be used as a novel indicator of cardiovascular disease risk, which provides a new possibility for cardiovascular disease prevention,” they wrote.

However, even BRI Co-creator Thomas has concerns. “Our entire medical system has been built around the BMI,” she told JAMA, referring to factors such as children’s growth charts and dosage recommendations for medications. That cannot be changed overnight.

Any anthropometric measure intended to replace BMI would need to be rigorously validated across all age groups, genders, and ethnicities. The impact of interventions such as bariatric surgery, diet, and exercise on the new measure would also need to be demonstrated.

Anthropometric Measures Only for Clinical Use

Even if BRI proves to be a “better” metric than BMI for patient care, Müller believes it would be no more suitable for research than BMI. “Regardless of the anthropometric measure, these are practical tools for everyday use,” he stressed.

“A high BRI, like a high BMI, is a risk factor — similar to high blood pressure, high cholesterol levels, or smoking — but it is not a disease,” he added. “In practice, as a physician, I know that a patient with a high BMI or BRI has an increased risk. I need to pay attention to that patient.”

Problems arise when indices like BMI or BRI are used in research. “These ‘invented’ anthropometric measures have no biological basis, which can harm obesity research,” Müller emphasized.

He cited the example of genetic research into obesity, which seeks to identify associations between specific genetic patterns and BMI values. “Why should weight in kilograms divided by height in meters squared be genetically determined?” he asked. “These measures are human-made constructs that have nothing to do with biology.”

Müller believes that the use of BMI has created a “gray area in obesity research” that may account for many of the “unexplained” phenomena in this field.

The BMI Might Be Responsible for the ‘Healthy Obese‘

One such phenomenon is the much-discussed “healthy obese,” referring to individuals with a BMI over 30 who do not have high blood sugar, high blood pressure, metabolic disorders, or elevated uric acid levels. “It’s speculated that it must be due to genetic factors, but in reality, the classification is simply wrong,” Müller said.

According to Müller, research should rely on other methods to determine obesity or relevant fat. For example, to assess diabetes risk, liver fat needs to be measured through enzyme tests, ultrasonography, CT, or MRI.

Visceral fat is also important in assessing cardiometabolic risk. “In the doctor’s office, it’s acceptable to estimate this by looking at waist circumference or even BRI. But for research, that’s inadequate,” noted Müller. Direct measurement of trunk fat with dual-energy X-ray absorptiometry or visceral fat with CT or MRI is needed.

“You always have to distinguish between research and patient care. In daily practice, measures like BRI or BMI are sufficient for assessing cardiometabolic risk. But in research, they are not,” Müller explained. To accurately study the disease risks associated with obesity, one must be aware that “with BMI, you cannot create scientifically valid patient or population groups because this value is far too imprecise.”

This story was translated from Medscape’s German edition using several editorial tools, including AI, as part of the process. Human editors reviewed this content before publication.