Body Mass Index vs Body Roundness Index
Body Mass Index (BMI) has long been the standard for assessing weight-related health risks. However, a newer measure, the Body Roundness Index (BRI), may provide a more accurate assessment of body fat distribution and associated health risks.
A healthy weight is the weight at which your health risks are lowest. But how do we determine what a healthy weight is for you? Your health care team probably uses your Body Mass Index (BMI). For over 40 years, this ratio of weight relative to height has been used to define healthy, underweight, overweight, and obese ranges and identify the health risks associated with these.[1] Recently however, the American Medical Association has stated that BMI alone is an imperfect measure.[2] Measuring waist circumference or calculating Body Roundness Index (BRI) have been proposed as better tools for evaluating the risks associated with body weight and fatness.
Body Weight, Body Fat, and Health
We are told to keep our weight within the recommended range to help prevent many of the chronic diseases that are rampant in the modern world. However, it is not actually our weight, but rather how much of our weight is fat, that impacts our health risks. Having too much body fat, which is the definition of obesity, increases the risk of diabetes, high blood pressure, elevated blood cholesterol, heart disease, sleep apnea, and some types of cancer. Where the fat is located also affects the risks; fat that accumulates around the liver and other organs in the abdomen, referred to as visceral fat, is associated with a higher incidence of these disorders than fat in the hips and thighs. These body types have been coined apple- and pear-shaped, respectively: an apple shape carries more risk than a pear shape.[3,4]
So, if it is the amount of body fat that affects our health risk, why don’t we just measure body fat? The reason is that it is difficult.[5] Despite the availability of bathroom scales and monitors that give you a readout of your percent body fat, these devices don’t actually measure body fat. They measure current flow through body water and calculate body fat based on a supposition of the amount of water in the body. The measurements are affected by your age, when you last ate or exercised, whether your bladder is full or empty, and even how dry or moist your skin is, so they are not always accurate. More exact techniques such as underwater weighing and DXA scans are available, but these require trained professionals, invasive measures, and expensive equipment. In the absence of an easy, reliable, cost-effective way to measure body fat, waist circumference measurement, and BRI offer simple, noninvasive ways to assess body fat and associated risks.
Body Mass Index: Advantages and Limitations
BMI is attributed to a 19th century mathematician who used a ratio of weight to height to define a “normal” man. This concept was then picked up by the Metropolitan Life Insurance Company to assess body weight after they noted that more insurance claims were coming from their obese than their thinner policyholders.[1] In 1972, physiologist Ancel Keys refined the equation and coined the term “Body Mass Index” to evaluate body fatness. [6] Today BMI is the standard used by the World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) to assess body weight; a BMI between 18.5 and 24.9 kg/m2 carries the lowest weight-related health risks.[7] But BMI has its drawbacks. The data used to define the healthy and unhealthy weight ranges was primarily from non-Hispanic white men of European descent, so these ranges may not be accurate for women and people of other ethnicities and races. And, although BMI usually correlates with the amount of body fat, it does not actually measure the amount of fat or consider the location of body fat.[7] So, individuals with the same BMI may have dramatically different body compositions. For example, a body builder may have a BMI in the obese range because of a larger-than-average amount of muscle. But they may have little body fat and therefore a low risk of obesity-related diseases. On the other hand, a person with a BMI in the normal range may have little muscle mass and a large percentage of body fat and therefore a higher disease risk than would be presumed by their BMI.
Waist Circumference: Advantages and Limitations
A measurement of waist circumference can be used as a quick assessment of the amount of visceral fat. Visceral fat accumulation has been found to be a good predictor of metabolic syndrome, a cluster of conditions, including abdominal obesity, high blood pressure, high fasting blood sugar, and abnormal blood lipid levels that increase the risk of heart disease, stroke, and diabetes.[8] Visceral fat releases fatty acids, hormones, and other proinflammatory molecules into the blood stream. These are believed to cause low-grade inflammation that leads to other components of metabolic syndrome and the diseases associated with it.[9] Excess visceral fat, called abdominal obesity, has been associated with an increased risk of heart disease, type 2 diabetes, and all-cause mortality, independent of BMI.[10,11,12]
Based on this correlation, measuring waist circumference would seem to be an easy way to assess the risk associated with body weight. But, there no set standard for where to take the measurement; some protocols measure at the narrowest point of the waist, some measure at the umbilicus, some measure at the top of the iliac crest, and others use the midpoint between the lowest rib and the iliac crest.[13]The classification of abdominal obesity might differ depending on the measurement used.[12] In addition, the accuracy of measurements can be affected by factors such as stomach contents and breathing patterns and it can be particularly difficult to identify where to take measurements in those with a large amount of body fat. [13,14] A final concern is the use if a single cut off point for risk assessment; abdominal obesity is defined as a waist circumference greater than 40 inches for men and 35 inches for women. This does not correct for the fact that taller individuals have larger waist circumferences, even when they do not have excess fat.[15]
Body Roundness Index: Advantages and Limitations
Body Roundness Index (BRI) was developed to provide a measure of body shape that would more accurately reflect the potential health risks associated with body fat. [15] The data used to formulate BRI included men and women of a variety of races and ethnicities. The BRI equation uses waist circumference and height to evaluate body shape and calculate body roundness, which is indicative of the amount of visceral body fat. People with more visceral body fat will have a rounder body shape and a higher BRI. Those with narrow, lean bodies will have lower BRI values.[15] BRI has been found to be better than other anthropometric measures at assessing the risk of cardiovascular disease, metabolic syndrome, type 2 diabetes, and colorectal cancer.[16-19] A specific BRI goal has not yet been defined but a BRI of 4.5 to 5.5 has been associated with the lowest all-cause mortality. Health risks increase by almost 50% in individuals with a BRI score of 6.9 or greater compared to those with a BRI between 4.5 to 5.5.[20] While the calculation for BRI is complicated, there are several accurate online calculators available including the two shown here:
While BRI correlates better with total body fat and body fat distribution than BMI, this is not to say that BRI is a perfect measure of visceral fat or weight-related risks. One problem is that the BRI equation uses waist circumference, which as discussed above, can be difficult to measure accurately. This is a particular problem when doing self-measurements, which tend to underestimate actual waist circumference, particularly in women and those with large amount of body fat. [13,14]. Errors in waist measurement produce errors in the BRI number and thus the estimate of health risk.
Beyond Body Weight and Fat
So, what’s your healthiest weight? We know that how much fat you carry affects your health risks more than what you weigh, but actually measuring body fat is difficult. We also know that having a large waist circumference often indicates more visceral fat, which increases the risk for many chronic diseases. Although measures such as BMI, waist circumference, and BRI can provide important information about your potential health risks, none of these consider other factors that affect these risks such as genetics, diet, and physical activity. While we can’t change our genetics, we can make choices that affect our risks. A healthy diet and regular exercise may reduce the risks associated with excess visceral fat, while poor dietary choices combined with a lack of exercise may increase risk even in someone with a lower BRI. Regardless of your weight and body shape, the lifestyle choices you make can reduce your risks of diabetes, heart disease, and a host of other chronic conditions.
References
[1]Pray R, Riskin S. The History and Faults of the Body Mass Index and Where to Look Next: A Literature Review. Cureus. 2023;15(11). doi:https://doi.org/10.7759/cureus.48230
[2]Berg S. AMA: Use of BMI alone is an imperfect clinical measure. American Medical Association. Published June 14, 2023. https://www.ama-assn.org/delivering-care/public-health/ama-use-bmi-alone-imperfect-clinical-measure
[3] Hamer M, O’Donovan G, Stensel D, Stamatakis E. Normal-Weight Central Obesity and Risk for Mortality. Ann Intern Med. 2017;166(12):917. doi:https://doi.org/10.7326/l17-0022
[4] Correa-Rodríguez M, González-Ruíz K, Rincón-Pabón D, et al. Normal-Weight Obesity Is Associated with Increased Cardiometabolic Risk in Young Adults. Nutrients. 2020;12(4):1106. doi:https://doi.org/10.3390/nu12041106
[5] Tinsley G. The 10 Best Ways to Measure Your Body Fat Percentage. Healthline. Published April 29, 2018. https://www.healthline.com/nutrition/ways-to-measure-body-fat
[6] Blackburn H, Jacobs D. Commentary: Origins and evolution of body mass index (BMI): continuing saga. International Journal of Epidemiology. 2014;43(3):665-669. doi:https://doi.org/10.1093/ije/dyu061
[7] Solan M. Body roundness may beat body mass for identifying risky fat. Harvard Health. Published September 2024. https://www.health.harvard.edu/staying-healthy/body-roundness-may-beat-body-mass-for-identifying-risky-fat
[8] Lee YH, Park J, Min S, Kang O, Kwon H, Oh SW. Impact of Visceral Obesity on the Risk of Incident Metabolic Syndrome in Metabolically Healthy Normal Weight and Overweight Groups: A Longitudinal Cohort Study in Korea. Korean Journal of Family Medicine. 2020;41(4):229-236. doi:https://doi.org/10.4082/kjfm.18.0122
[9]Varra FN, Michail Varras, Varra VK, Panagiotis Theodosis‑Nobelos. Molecular and pathophysiological relationship between obesity and chronic inflammation in the manifestation of metabolic dysfunctions and their inflammation‑mediating treatment options (Review). Molecular Medicine Reports. 2024;29(6). doi:https://doi.org/10.3892/mmr.2024.13219
[10]Zhang C, Rexrode KM, van Dam RM, Li TY, Hu FB. Abdominal obesity and the risk of all-cause, cardiovascular, and cancer mortality: sixteen years of follow-up in US women. Circulation. 2008;117(13):1658-1667. doi:https://doi.org/10.1161/CIRCULATIONAHA.107.739714
[11]Warren TY. Independent Association of Waist Circumference With Hypertension and Diabetes in African American Women, South Carolina, 2007–2009. Preventing Chronic Disease. 2012;9. doi:https://doi.org/10.5888/pcd9.110170
[12] Ross R, Neeland IJ, Yamashita S, et al. Waist circumference as a vital sign in clinical practice: A consensus statement from the IAS and ICCR working group on visceral obesity. Nature Reviews Endocrinology. 2020;16(3):177-189. doi:https://doi.org/10.1038/s41574-019-0310-7
[13] Verweij LM, Terwee CB, Proper KI, Hulshof CT, van Mechelen W. Measurement error of waist circumference: gaps in knowledge. Public Health Nutrition. 2012;16(2):281-288. doi:https://doi.org/10.1017/s1368980012002741
[14] Bigaard J, Spanggaard I, Thomsen BL, Overvad K, Tjønneland; A. Self-Reported and Technician-Measured Waist Circumferences Differ in Middle-Aged Men and Women. The Journal of Nutrition. 2005;135(9):2263-2270. doi:https://doi.org/10.1093/jn/135.9.2263
[15]Thomas DM, Bredlau C, Bosy-Westphal A, et al. Relationships between body roundness with body fat and visceral adipose tissue emerging from a new geometrical model. Obesity. 2013;21(11):2264-2271. doi:https://doi.org/10.1002/oby.20408
[16] Yang M, Liu J, Shen Q, et al. Body Roundness Index Trajectories and the Incidence of Cardiovascular Disease: Evidence From the China Health and Retirement Longitudinal Study. J Am Heart Assoc. Published online September 25, 2024. doi:https://doi.org/10.1161/jaha.124.034768
[17] Rico-Martín S, Calderon-Garcia JF, Purificación Sánchez‐Rey, Franco-Antonio C, Mariana Martínez Álvarez, Francisco J. Effectiveness of body roundness index in predicting metabolic syndrome: A systematic review and meta‐analysis. 2020;21(7). doi:https://doi.org/10.1111/obr.13023
[18] Wu L, Pu H, Zhang M, Hu H, Wan Q. Non-linear relationship between the body roundness index and incident type 2 diabetes in Japan: a secondary retrospective analysis. Journal of Translational Medicine. 2022;20(1). doi:https://doi.org/10.1186/s12967-022-03321-x
[19] Gao W, Jin L, Li D, et al. The association between the body roundness index and the risk of colorectal cancer: a cross-sectional study. Lipids in Health and Disease. 2023;22(1):53. doi:https://doi.org/10.1186/s12944-023-01814-2
[20] Zhang X, Ma N, Lin Q, et al. Body Roundness Index and All-Cause Mortality Among US Adults. JAMA Network Open. 2024;7(6):e2415051. doi:https://doi.org/10.1001/jamanetworkopen.2024.15051
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