Welcome to Straight Talk on Health. I’m your host Dr. Chet Zelasko. Straight Talk on Health is a joint production with WGVU in Grand Rapids MI. I examine the practical application of health information. Nutrition. Exercise. Diet. Supplementation. I look at the science behind them, and let you know whether it’s something to consider or not. You can check out other things that I do on my website drchet.com and sign up for my free emails.
The Body Mass Index has been used to provide an indication whether someone is underweight, normal weight, or overweight and obese and the degree to which they are obese. If you break the measurement down into its components, it is a measure of surface area. How so? It is the body weight in kilograms divided by the height in meters squared. There is no measure of body fatness implied and yet, physicians, healthcare professionals, and public health researchers have used it for that purpose for a couple of decades.
While it may not indicate the degree of body fatness directly, it can be used to assess the potential risk of degenerative disease like cardiovascular disease, pre-diabetes, and other metabolic conditions. In my experience, many people really dislike it, especially when they cross over into the obese category. How can I be obese? I have big bones!
There has been a recent study published In JAMA Network Online, using the Body Roundness Index or BRI for short. I try to stay on top of body composition assessments but somehow, I completely missed the BRI before this paper. Before I get to that let me give you an idea of what the BRI is all about and then see whether it’s really relevant.
In a recently published commentary in JAMA, the commentary writer suggested that it may be time to exchange the BMI for the Body Roundness Index (BRI). The author interviewed the developer of the BRI. The developer, a mathematician, told the commentator that the BMI is based on a “cylindrical” model and when she looked in the mirror, she didn’t feel like a cylinder. She felt she was more egg shaped. Using her expertise in mathematics, she developed a model for assessing body fatness based on an ellipse. Why? She gave a couple of reasons.
The first reason is that BMI can misclassify individuals because it fails to distinguish between individual amounts of fat-free mass (FFM) and FM. In conjunction with that is that it does not provide information about the distribution of body fat. Specifically, visceral fat versus subcutaneous fat. The first type of fat is around organs and may be more associated with cardiometabolic disease (CMD) while the second is the fat just under the skin. She decided to developed a mathematical model, based on an ellipse model, to assess body fatness which would indicate where the fat is distributed. With some very complicated math, she developed the BRI. It only uses two measurements—height and waist circumference—but avoids that hated scale! The benefit of using the BRI is that it may be a better predictor of body fat than the BMI. As I’ve already established, the BMI is not a predictor of body fatness but surface area.
Based on her model, she demonstrated that BRI is associated with body fat distribution. It makes sense. The waist measurement would provide an indication of fat around the waist. The question after that is would it be a better predictor for CMD than BMI. We’ll see after looking at the study that got my attention to begin with.
A group of researchers decided to use open-source data from the National Health and Nutrition Examination Survey database. The researchers wanted to examine the relationship between BRI and all-cause mortality. The dates chosen coincided with the years that physical assessments were done including height and waist circumference. Weight was collected but not used in the analysis in this instance. The time period began in 1999-2000 and continued every 2 years through 2017-2018.
There were two observations that were significant. First, in every demographic group, regardless of age, gender, or race/ethnicities, the BRI has increased during every examination period. As a country, the US has gotten fatter. That matches every other measure such as body weight or BMI as well.
The second observation was that the hazard ratio (HR) increased as the BRI dropped below normal, the HR became normal when the normal BRI was reached, and then rose again as the BRI increased. Just so you have a reference point, the researchers compared quintiles of BRI. Simply stated, there was an increased risk of mortality when people were too lean or too fat.
Here's something interesting. The BRI more or less lines up with the equivalent numbers for BMI numbers that were provided in the validation study by the developer of BRI.
You may be wondering why I didn’t give you a formula to do calculations for yourself. It’s very complicated and there are BRI calculators available if you search for them on the internet. The main reason is -- it just doesn’t matter. The BRI is no better at predicting mortality than the BMI did. That’s one thing that the researchers did not do, even though they had the data they needed in body weight, was compare the BRI with the BMI in predicting HR. Why not? They had the data. Why not do it? But fortunately the BRI developer did during the validation study. The categories of adiposity associated with BMI matches up quite well with the BRI and thus with body fatness.
The author of the commentary went on to suggest that what we really need is technology that is less expensive than current measures of percent body fat such as the DEXA scan or the BodPod air displacement methods. I don’t think that’s really necessary. It won’t predict who’s at risk for CMD any better than BMI. It won’t give any better precision than the BMI. Just higher expense and remember, I’m a body composition geek and have done almost all of these techniques as part of my research.
Let me throw something else into the mix. The BRI may indicate body fatness but it cannot discern between visceral and subcutaneous fat. I’ve talked about body fat distribution and aging before and used the expression “fluffy” to describe how people can have a normal body weight but still be overfat. Their waist measurement may also be lower because of the lack of muscle underneath. In trying to substitute the BRI for BMI, there is still no solution for that.
On top of that, there is also the dilemma of the jelly belly. Those are people, primarily men, who carry their fat subcutaneously, not viscerally. A simple waist measurement cannot discern where the fat is located. Just because a person has a larger waist measurement does not mean that it is evenly distributed. They may not have any more visceral fat than a person with a normal BRI yet be categorized as at increased risk for CMD. That’s why there is no need for another formula which will be used to predict the risk of CMD just with anthropometric data. It leaves too much to chance. On top of that, once it’s in your health records, it never really leaves.
In my opinion, it’s really the clinical use everyone seems to have a problem. With rare exceptions, the BMI does give an indication of body fatness as well as whether a person is under, normal, or obese. If a physician or other health care professional cannot see the patient before them and realize they are too lean or too muscular to fit the typical interpretation of BMI, the fault lies with them, not the tool they are using. That’s it for this episode. Until next time, this is Dr. Chet Zelasko saying health is a choice. Choose wisely today and every day.
Reference:
1. JAMA. 2024; 332(16):1317-1318. 10.1001/jama.2024.20115
2. JAMA Netw Open. 2024; 7(6):e2415051. 10.1001/jamanetworkopen.2024.15051.
3. https://doi.org/10.1002/oby.20408
4. https://bri-calculator.com/#calculator
