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BMI Is Faulty. Why Do We Still Use it?

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It had been a routine visit for an annual physical. After examining the patient, I explained to her that the physical exam was normal and she was healthy. As she stepped out of the exam room, the woman turned around and said, “After all of my visits to the doctor’s office, you’re the only doctor who has told me I was healthy.” She said that every time she came to the clinic, any doctor who saw her fixated on her need to lose weight. Her body mass index put her in the “obese” category.

Unfortunately, she’s not the only person who has shared this experience with me. Other individuals have described how distressing the stigma they experience is for them and how it makes them not want to return for follow-up in primary care clinic. These anecdotes are borne out in the research — for example, obese women are less likely to get breast and cervical cancer screening than non-obese women.

As many people have experienced, weight is routinely measured for clinic visits along with the vital signs. When weight is measured, it is entered into the medical chart with the patient’s height and their BMI is calculated. The BMI is a widely used ratio that the NIH defines as “measure of body fat based on height and weight that applies to adult men and women” and is used to identify whether an individual is overweight or obese. This categorization has wide ranging clinical implications, including whether an individual qualifies for medical treatments ranging from medications for COVID-19 to bariatric surgery. 

While BMI is strongly correlated with obesity at a population level, its usefulness as a clinical tool breaks down at the individual level. This stems from its origin, which lies with the Belgian mathematician Lambert Adolphe Jacques Quetelet. Quetelet sought to define “l’homme moyen,” or the average man. He hypothesized that by taking many measurements, the ideal weight could be identified. Quetelet measured European men in the 1830s and found that weight increased with the square height of the man. An argument commonly used against BMI is an athlete who falls in the “overweight” or “obese” category because of higher muscle mass: BMI does not distinguish between fat, muscle, or bone. 

Nearly a century and a half later, in the 1970s, the American dietitian Ancel Keys and his colleagues measured about 7,500 “healthy” men from 12 sample groups. These included Americans, Italians, Finns, Japanese, and Bantu and they found that what they described as the “body mass index” was distributed in a normal, bell-shaped distribution. Unfortunately, normal BMI has become synonymous with health and an elevated BMI equivalent to ill-health. However, it’s not that simple — large percentages of those individuals classified as obese are metabolically healthy and conversely, a sizable number of those with normal BMIs are metabolically unhealthy. 

Despite limitations of BMI for use in the clinical setting, we still use it to guide clinical care. As a primary care doctor, I’m left to wonder: in an era of evidence-based medicine and guidelines, why do we use such a faulty tool and one that perpetuates stigma, labeling, and ignores body fat variations across race/ethnic groups, sex, gender, and ages? In 2023, delegates at the annual meeting of the American Medical Association recommended educating physicians on the issues with BMI, including its historical harm and racist exclusion since it is based primarily on white populations. 

In the clinic, health care professionals need to stop relying on BMI and focusing on an individual’s weight unless he or she brings it up. We need to end the oversimplified approach that medicine and our society holds in which fat is bad and thin is good. Finally, we need to develop policies that help people live a healthy, active lifestyle, regardless of the effect on weight.  

How do you approach BMI in your practice?

Dr. Bailey Miles is an internal medicine physician who practices primary care in Chinle, Arizona.

Image by GoodStudio / Shutterstock

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