Should the BMI Measure Be Used in Medicine?
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The American Medical Association (AMA) – the largest association of physicians in the US – announced it has adopted a new policy aimed at clarifying how doctors use the body mass index (BMI) measure in medicine, suggesting it should be adopted in conjunction with other measures. Here, we explore where the BMI calculation came from, why it has a “problematic history” and the AMA’s policy.
Where did the BMI calculation come from?
In a quest to determine the characteristics of the “l’homme moyen” – the average man – Belgian mathematician Adolphe Quetelet devised the Quetelet index in the early 19th century. His analyses of human growth data in Belgian populations led him to conclude that, aside from childhood growth spurts and puberty, “weight increases as the square of the height”. Quetelet coined a formula whereby a person’s body size is calculated as their weight in kilograms divided by their height in meters, a measure that could then be compared across populations.
In a time when calculators and electronic systems were non-existent, this simplistic calculation probably seemed a logical approach to stratify individuals, explore obesity levels and associated health outcomes. But well over a century later, Quetelet’s formula was relabeled the BMI by American physiologist Ancel Keys. In 1972, Keys and colleagues published an article in the Journal of Chronic Diseases promoting the metric as “preferable over other indices of relative weight”. Later endorsements from the National Institutes of Health and the World Health Organization (WHO) led to the BMI calculation finding firm roots in the medical community. Despite several updates regarding the thresholds for categorizing individuals as “healthy” or “obese”, the measure has persisted. Now, it is still used frequently as a tool to quantify health and disease risk, influence public health strategies and even to affect insurance reimbursements.
How is BMI calculated, and what are the categories?
BMI = kg/ m2.
The WHO created an expert consultation group in 1992, which was tasked with developing uniform categories of the BMI. The group published its categories in 1995, which included underweight (BMI in the range of 15 to 19.9), normal weight (BMI in the range of 20 to 24.9) overweight (BMI in the range of 25 to 29.9 and obese (BMI in the range of 30 or above).
The BMI calculation has faced much criticism over the years, with some researchers urging for its clinical applications to be dropped entirely. These calls for change appear to have reached a crescendo now that the largest association of physicians in the US has publicly cautioned its use.
Why is BMI problematic?
The AMA’s new policy was created based on the AMA Council on Science and Public Health’s report, which analyzed the pros and cons of the BMI measure – including its “problematic” history – and proposed new alternatives.
BMI is considered a poor metric for measuring health based on a variety of factors. Firstly, it doesn’t distinguish fat from fat-free mass, which includes bone, muscle and other tissues. Individuals with the same BMI may therefore have very different bodily compositions. Take a lean athlete that carries a lot of muscle. Applying the BMI calculation, they might be deemed overweight. Beyond the adverse effects this classification could have on a physician’s ability to both accurately and fairly treat a patient, the psychological burden could be heavy. The potential glorification of a low BMI can also lead to unhealthy attitudes towards body image, food and self-acceptance in society and culture.
BMI also does not account for how much of a person’s fat composition is made up of visceral fat – fat buried deep inside the body that wraps around the abdominal organs – which is associated with increased disease risk. Consequently, the AMA suggests the BMI should be used “in conjunction” with other validated approaches, including body adiposity index, measurements of visceral fat, body composition, relative fat mass, waist circumference and genetic/metabolic factors.
The AMA also highlights an issue that has led many to consider the BMI metric as racist – it fails to acknowledge that healthy body shape and composition varies across different races and ethnic groups; Quetelet’s original index only considered white European bodies. “Our AMA recognizes the issues with using BMI as a measurement because: (a) of the eugenics behind the history of BMI, (b) of the use of BMI for racist exclusion and BMI cutoffs are based on the imagined ideal Caucasian and does not consider a person’s gender or ethnicity,” the Council on Science and Public Health report states. Beyond race and ethnicity, body shape and composition also vary depending on sex, gender and age-span. As a result, the association advises that it is “essential” to consider when applying BMI as a measure of adiposity that “BMI should not be used as a sole criterion to deny appropriate insurance reimbursement.”
The report dedicates some time to analyzing the benefits of utilizing BMI – though the paragraph is petite compared to the list of disadvantages. The AMA says that BMI can be useful in monitoring the treatment of obesity: “Further, BMI is readily available, inexpensive, can be administered easily and is understood easily by patients. BMI can also be used as an initial screening tool to identify those at an elevated health risk because of excess body weight and poor distribution of fat mass,” the report reads. Though ultimately the association is clear in its statement that BMI loses its predictability when applied at the individual level.
“There are numerous concerns with the way BMI has been used to measure body fat and diagnose obesity, yet some physicians find it to be a helpful measure in certain scenarios,” says AMA Immediate Past President Dr. Jack Resneck. “It is important for physicians to understand the benefits and limitations of using BMI in clinical settings to determine the best care for their patients.”
Molecular alternatives to BMI?
BMI’s failure to consider the person as an individual certainly isn’t “on brand” for society’s move towards personalized medicine. Consequently, research groups are proposing alternative, molecular-based measures.
Earlier this year, scientists from the Institute for Systems Biology (ISB) published their work outlining a “biological BMI” in Nature Medicine. Led by senior research scientist Dr. Noa Rappaport, the research team conducted multiomics profiling on blood samples from 1,000 individuals enrolled in the now closed Arivale wellness program. Using machine learning models, they generated molecular BMI scores, such as a metabolomics- or proteomics-based BMIs. “Biological BMI is a multi-dimensional molecular measure of BMI calculated from blood measurements of proteins, metabolites or clinical labs. It is a more comprehensive and accurate measure of metabolic health compared to the traditional BMI measure, which only considers height and weight,” Rappaport explains in conversation with Technology Networks. Unlike traditional BMI, biological BMI can identify misclassified individuals with a normal weight but disrupted metabolic health, who may not be currently monitored or treated.”
The researchers emphasize that more work is needed to validate the method’s effectiveness and applicability in wider populations but affirm that they see “deep molecular profiling” as the future of precision medicine.