Hello to BRI, goodbye to BMI?

Body Mass Index (BMI) has long been the standard tool in medicine for classifying individuals based on their weight relative to height. Developed nearly 200 years ago, BMI estimates body fat and categorizes individuals as underweight, healthy weight, overweight, or obese. Despite its widespread use, BMI has been criticized for significant limitations, particularly its failure to distinguish between muscle and fat and its inability to account for variations across race, ethnicity, age, and gender. This often results in misclassification, especially for athletes or muscular individuals. For example, Olympic athlete Ilona Maher was labeled obese by BMI despite being in peak physical condition. Originally not intended for medical use, BMI remains a convenient but imperfect metric.

The Body Roundness Index (BRI) was developed to address BMI’s shortcomings by focusing on fat distribution, particularly around the abdomen (NYTimes). Unlike BMI, which only considers weight and height, BRI incorporates waist circumference to provide a better estimate of body shape and fat distribution. This is crucial because abdominal fat is strongly linked to health risks such as Type 2 diabetes, hypertension, and heart disease. Developed by mathematician Diana Thomas, BRI offers a more nuanced approach to assessing obesity-related health risks, making it a promising predictor of mortality and overall health outcomes.

BRI shares some similarities with the Waist-to-Hip Ratio (WHR), as both focus on central obesity, a critical factor in health risks. However, BRI goes beyond WHR by incorporating height into its calculation, providing a more comprehensive assessment of body shape and fat distribution. While WHR measures the proportion of fat between the abdomen and hips, BRI offers a more holistic view of body roundness, which is more sensitive to central obesity.

In sum, BMI is the ratio of weight to the square of height, WHR is the ratio of the waist circumference to hip circumference, and BRI depends on the ratio of waist circumference to height. Thus BRI is a bit of a hybrid of BMI and WHR attempting to assess body roundness by measuring waist circumference rather than weight before normalizing by height. 

As alluded to above, one of the key advantages of BRI over BMI is its ability to account for variations in body composition and fat distribution. BMI fails to differentiate between muscle mass and fat, often classifying muscular individuals as overweight or obese (see Figure 1). In contrast, BRI’s emphasis on waist circumference and its link to abdominal fat offers a more accurate reflection of health risks associated with obesity, such as cardiovascular disease and metabolic disorders. Studies have shown that higher BRI scores correlate with increased mortality risks, further supporting its potential as a superior predictor of health outcomes.

For example, a 2021 study in explored the predictive power of different anthropometric indices, including BMI, BRI, waist-to-hip ratio (WHR), and waist-to-height ratio (WHtR), in determining cardiovascular risk among a cohort of adults in China. BRI was found to be superior to BMI, especially for identifying individuals at higher risk of coronary heart disease (CHD) .

As the medical community increasingly recognizes the limitations of BMI, BRI offers a more reliable alternative for assessing obesity and its associated health risks. Future directions for BRI include further validation across diverse populations  (e.g. head-to-head comparisons with BMI and WHR)  and its integration into clinical practice for more personalized health assessments. The development of tools to facilitate its widespread use in public health settings could further solidify BRI's role in replacing BMI as a more accurate measure of obesity-related health risks.

Figure 1. Professional rugby player Ilona Maher has a BMI of 29.3 which is on the border between overweight and obese according to the index. BMI has trouble distinguishing weight due to muscle versus weight due to fat.