Height and weight are usually among the first measurements taken during a medical visit. These values are often used to calculate BMI (body mass index), which falls into the following categories: underweight (less than 18.5), healthy weight (18.5 to less than 25), overweight (25 to less than 30), and obese (30 or greater). BMI is used to assess risk for various medical conditions, from childhood obesity to cardiovascular risk in adults.
Created nearly 200 years ago, the BMI is currently used to determine a range of health factors, most commonly obesity. Despite being ubiquitous in medical settings, the BMI was not originally invented for medical purposes. Belgian statician Adolphe Quetelet first conceptualized the BMI in 1832, desiring to characterize the “normal man” according to quantifiable, numeric criteria. Thus, the Quetelet index was born, rooted in the idea that weight is relative to height, and that ideal weights can be determined by this relationship. Ancel Keys, an American physiologist, furthered the ideas of Quetelet; he renamed it the body mass index and conducted a large-scale study of 7,426 healthy men, providing evidence for its use.
It is important to note that neither Quetelet nor Keys were medical professionals, nor did they develop this measure for the numerous medical applications it is used for today. Quetelet was interested in the weight of an ideal man, whereas Keys was interested in using BMI in research studies to analyze large quantities of data — neither was interested in its efficacy in assessing disease risk. In fact, Keys even acknowledged that though the BMI is convenient and easy to administer, body density is a superior way to assess body fat mass.
Because of its non-medical origins, BMI overlooks important factors that influence body fat such as its location, type, and key variations across racial, ethnic, and sex groups. Fundamentally, the BMI’s major medical flaw is that it groups all types of body fat together. Certain types of fat, such as visceral fat, are associated with metabolic disorders, insulin resistance, and cardiovascular health problems, unlike subcutaneous fat, which is less dangerous and can even have protective functions. BMI cannot differentiate between these types of fat, leading to inaccurate assessments of an individual’s health and potential risks. In addition, BMI does not differentiate between muscle mass and fat tissue. Olympic rugby player Ilona Maher has spoken out about the inaccuracy of BMI, stating, “I do have a BMI of 30 — well, 29.3, to be more exact, and I’ve been considered overweight my whole life.” Despite being classified as obese by BMI standards, Maher is composed of lean muscle and is far more muscular and in better health than the average American. This highlights how BMI often fails to account for variations in body composition and leads to misconceptions about health. Similarly, the Nurses’ Health Study found that abdominal fat is significantly correlated with cancer and cardiovascular mortality in women, even in those who report normal BMI. This highlights how the inability of BMI to differentiate between different types of fat reduces its accuracy in determining risk.
“Because of its non-medical origins, BMI overlooks important factors that influence body fat such as its location, type, and key variations across racial, ethnic, and sex groups.”
Another issue with the BMI is its lack of applicability to diverse populations. In Keys’ study providing evidence of BMI’s utility, the sample consisted of 7,426 healthy men lacking racial diversity and female representation. Different racial groups carry fat in different ways — for example, many Asian races, especially from the Indian subcontinent, carry more fat for a given BMI in comparison to Caucasians. The opposite is generally true for Black people. In this case, Asian patients may report a “normal” BMI but not receive appropriate preventative counseling, although they are at risk. In the Flegal study of 8,821 children, it was found that non-Hispanic Black girls reported significantly higher BMIs than non-Hispanic White girls. However, the amount of body fat was not significantly different between the two groups. As a result, non-Hispanic Black girls may be treated for at-risk conditions in the clinic despite not actually needing it.
BMI is also clinically insignificant in numerous cases. For instance, 25 states in the US require schools to screen students’ BMI, and 11 states mandate that schools inform parents of their children’s BMI to help reduce the risk of childhood obesity. However, research indicates that reporting BMI is not effective in actually decreasing childhood obesity rates. These reporting standards and inaccurate definitions of obesity can potentially harm growing children. Being misclassified as unhealthy or overweight can negatively affect self-esteem and contribute to body image issues and eating disorders among children and adolescents.
So why are we still using an outdated, ineffective, and potentially harmful measure? Part of the reason is that BMI is deeply ingrained in our medical system. To move forward, we need a shift toward more culturally inclusive, scientifically accurate, and representative measures of body mass.
In recent times, promising alternatives have emerged to fill the gap that BMI does not address. The body shape index (ABSI) is a tool that is calculated from waist circumference, weight, height, and BMI. With these few extra measurements, it was shown that ABSI is a better predictor of certain conditions – one study found that ABSI was associated with mortality risk and found that ABSI was a strong predictor of frailty, whereas BMI was not. Similarly, a DEXA scan, a quick and painless scan in a medical visit, can offer in-depth detail about body composition, with precise measures of fat, muscle, and bone. Finally, other measures such as waist-to-hip ratio or body roundness index, take into account estimates of visceral fat, which tends to lie near the abdominal region and is the main predictor of many health conditions. However, despite the availability of new and promising measures, BMI continues to be the primary tool used in medical contexts, limiting our ability to accurately assess health risks and offer personalized care.
The widespread use of BMI also represents an ongoing struggle that the US faces — an inadequate focus on preventative medicine. With so much focus on new and improved treatments for medical ailments, we have neglected to improve existing measures for prevention. Ultimately, shifting focus to preventative measures and funneling research and resources into this pursuit would lead to reduced healthcare costs and better health outcomes for Americans. We must move away from spotlighting reactive treatments to investing in high-quality, accurate measures that can predict, identify, and enable early intervention for conditions like cardiovascular disease and obesity — the very conditions BMI attempts, but fails, to predict.
Debates about the utility of BMI highlight the need for iterative medicine. Beyond being evidence-based and accurate, medical measures should be continuously refined to meet the needs of diverse populations. In addition to advancing medical technology, it is equally important to critically evaluate existing measures and assess their validity in modern contexts. Otherwise, antiquated tools like BMI may continue to pervade daily practice.
