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Body mass index (BMI) is widely used to determine whether someone is over or underweight. For many people, it can provide a reasonable indication of this; however, BMI has several limitations. Because it is calculated using no more information than a person’s height and weight, it provides no actual insight into body composition. Understanding how your weight impacts your health requires information about the relative proportions of fat, bone, and muscle in the body.
How does BMI work?
BMI began as a statistician’s attempt to describe the relationship between height and weight in an average man. In the 1830s, Adolphe Quetelet collected height and weight data from Belgian men. Based on this data, he determined that a man’s weight increases in relationship to the square of his height. As a result, he developed the Quetelet Index to describe this relationship: a man’s weight in kilograms divided by the square of his height in meters.
In 1972, American physiologist Ancel Keys coined the term “body mass index” to refer to this ratio and analyzed data from “healthy men” to support its use in assessing weight in broad populations. Keys, however, never intended BMI to be used in medical evaluations as an indicator of individual health. In fact, he cautioned that the formula was not an accurate measure of body fatness. Nonetheless, BMI has become ubiquitous in medical records, and it’s routinely used to classify people as over or underweight.
BMI ranges are defined as follows:
- Less than 18.5 is considered underweight
- At least 18.5 but less than 25 is considered healthy
- At least 25 but less than 30 is considered overweight
- 30 or more is considered obese
- 40 or more is considered severely obese
By these standards, many people are mistakenly flagged as at risk for serious health conditions like high blood pressure, heart disease, diabetes, stroke, and many types of cancer. On the other hand, some people who are at risk for these conditions can be overlooked because they fall into the “healthy” category.
What does BMI miss?
BMI misses a lot of information. Not only does it fail to provide any insight into body composition, but it also contains inherent bias because it’s based on norms that were developed by measurements of (largely white) men who were not particularly athletic. As a result, BMI can be a highly inaccurate way of assessing an individual’s health.
Body Composition
BMI does not provide any information about the relative proportions of muscle, bone, and fat in the body. Because BMI relies only on a person’s height and weight, a person with strong bones and a lot of muscle mass will have a higher BMI than a person with osteoporosis and weak muscles. As you might imagine, many athletes who are the picture of good health have BMIs that indicate overweight or even obesity.
Variations Across Age, Sex, & Ethnic Groups
Because BMI doesn’t differentiate based on sex, age, or race, it can be especially misleading for certain groups. Women, for example, tend to have 10% higher body fat mass than men with the same BMI. Regardless of sex, people tend to lose muscle as they age, so an unchanged BMI can indicate increased fat mass over time. Additionally, people of Asian ethnicity tend to carry more fat than White people of the same BMI, and Black people, on average, have greater bone mineral density and lean body mass than their White counterparts. As a result, equivalent BMI measurements can suggest different body compositions across ages, sexes, and ancestries.
Metabolic Health
BMI is often used as an indicator of metabolic health, but studies have shown unreliable correlations between “healthy” BMI levels and metabolic health as measured by more precise means. A 2016 study published in the International Journal of Obesity compared subjects’ BMI to their blood pressure, triglycerides, cholesterol, C-reactive protein, glucose, and insulin resistance data. The researchers found that many overweight and obese subjects were metabolically healthy, while many in the normal range were not:
- 29% of obese individuals, including 16% of those with BMI of 35 or more, were metabolically healthy.
- 30% of those with normal BMI were metabolically unhealthy.
Based on the data, researchers estimated that nearly 75 million adults in the U.S. are misclassified based on their BMI.
How can we accurately measure body composition?
In 2023, the American Medical Association released a statement recognizing BMI as a problematic measurement “due to its historical harm, its use for racist exclusion, and because BMI is based primarily on data collected from previous generations of non-Hispanic white populations.” The AMA recommends viewing BMI in the context of other factors such as visceral fat, body composition, and waist circumference.
Dual-energy X-ray absorptiometry body composition analysis (DXA BCA) provides body composition data with a low-dose X-ray scan. DXA BCA illustrates not only the proportions of fat, bone, and muscle in the body but also the location of body fat. Because visceral fat—the deeper level of fat that surrounds the organs—has been linked to obesity-related negative health outcomes like heart disease, stroke, and diabetes, this information is much more indicative of overall health than simple body fat percentage (and far more useful than BMI).
Who can benefit from DXA BCA?
Body composition analysis can be beneficial for a variety of people, including athletes in training, people recovering from injury, and those who are striving to attain and maintain a healthy weight. By receiving a series of DXA scans, you can monitor changes over time to track your progress. This information can be used to adjust training, rehabilitation, or weight loss programs to meet your goals more effectively.
Iowa Radiology offers DXA BCA at our Des Moines-area clinics. The exam is easy and painless, and the scan takes just a few minutes of a 30-minute appointment. Learn more about DXA BCA at Iowa Radiology, or subscribe to our blog for regular updates on important health topics.
Resources
American Medical Association. AMA adopts new policy clarifying role of BMI as a measure in medicine. Published June 14, 2023. Accessed April 15, 2024. https://www.ama-assn.org/press-center/press-releases/ama-adopts-new-policy-clarifying-role-bmi-measure-medicine.
Centers for Disease Control and Prevention. Defining Adult Overweight & Obesity. CDC.gov. Reviewed June 3, 2022. Accessed April 15, 2024. https://www.cdc.gov/obesity/basics/adult-defining.html
Centers for Disease Control and Prevention. Consequences of Obesity. CDC.gov. Reviewed June 3, 2022. Accessed April 15, 2024. https://www.cdc.gov/obesity/basics/consequences.html.
Jeong, SM, Lee, DH, Rezende, LFM, et al. Different correlation of body mass index with body fatness and obesity-related biomarker according to age, sex and race-ethnicity. Sci Rep 2023;13:3472. https://www.nature.com/articles/s41598-023-30527-w.
Muscogiuri, G., Verde, L., Vetrani, C. et al. Obesity: a gender-view. J Endocrinol Invest 2024;47:299–306. dx.doi.org/10.1007/s40618-023-02196-z
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):e48230. https://dx.doi.org/10.7759/cureus.48230.
Tomiyama A, Hunger, J, Nguyen-Cuu, J, et al. Misclassification of cardiometabolic health when using body mass index categories in NHANES 2005–2012. Int J Obes. 2016;40:883–886. dx.doi.org/10.1038/ijo.2016.17.
