The Body Mass Index Is a Bust
It’s time for this impractical metric to leave the stage.
Posted Dec 04, 2020
I’m sure many of us know a thin or “average weight” person who has had a heart attack, or who has high blood pressure and cholesterol. We also know people living in larger bodies who are extremely healthy.
So why do we continue to believe that being at a higher weight is inherently unhealthy? For starters, because we’ve been trusting an overly simplistic, biased, and archaic metric to tell us who is and isn’t healthy: the Body Mass Index (BMI).
I had a professor in graduate school who would fire off questions anytime a classmate would make a claim. "How was that study designed? How big was the sample? How diverse was the sample? Has it been replicated? Who funded the research?" It made participating in class pretty intimidating, but it also instilled in me the strong conviction that research must be consumed critically.
The more I see the Body Mass Index (BMI) continue to occupy a pedestal in health and medicine, the more frustrated I am that nobody’s wearing their critical consumer hat. The BMI has been used for nearly two centuries to categorize people by their size and presumed “health risks,” everywhere from the diagnosis of eating disorders to the premiums set by health insurance companies.
The BMI charts have influenced public policies and shaped our medical system, causing major harm in the process. When we use BMI as a shortcut for determining health, we miss the full picture, and we perpetuate toxic stereotypes about body size.1 Next time you head to your physical exam, take this knowledge along with you.
Where did the BMI come from?
The Body Mass Index was developed by mathematician and astronomer Adolphe Quetelet in the 1830s. He created a formula of dividing someone’s weight in kilograms by their height squared in meters, and believed that this number indicated how close or far that person was from the “ideal man,” whose height-to-weight ratio fell at the 50th percentile.
In the 1800s, it was popular to treat the arithmetic mean of something as the most desirable place to be. Back then, being “average” was sought-after. These days, we’re much wiser than that. We know that humans are diverse, and that it’s both irrational and inequitable to think we could standardize any aspect of an individual’s wellbeing.
Now, I wasn’t exactly a star student in my statistics classes, but it’s clear we’re fighting an impossible battle if we think we can get all human beings to fall at the 50th percentile on the BMI chart, or on any other measure. Even if it were possible for people to achieve and sustain significant weight loss for the long-term (which it isn’t), contorting the entire population to fit into the bottom half of a normal curve would simply change the values on the normal curve. That’s literally how a normal curve works. The 50th percentile is only the 50th percentile if half of the population falls above it, and half falls below it.
Even referring to someone as “overweight” is irrational, considering it really just means “above average on the normal curve.” If we’re getting technical, what is this weight that people shouldn't be “over”? It’s like calling someone who is six-and-a-half feet tall “overheight" because they're taller than the average person.
Additionally, the BMI categories were based on data from Quetelet’s sample of white European men in their early 20s. Just looking around us, it’s clear that not all bodies match the metrics of young white men, nor should they be expected to! Using this formula to determine anything, especially health, discounts the body diversity that we know exists across different genders, races, ethnicities, and ages. It promotes racism, sexism, and ageism, and reinforces the oppressive message that white males are the standard-bearers.
The BMI gets it wrong in at least one out of three people.
There are serious problems with using a single piece of data to draw conclusions about entire groups of people. To understand the full picture of someone's health, you need to know much more than their weight in kilograms divided by their height squared in meters.
This equation doesn’t reflect body composition, meaning that many professional athletes are squarely in the “Obese” range. It also doesn’t tell us anything about someone’s blood pressure, resting heart rate, blood glucose, cholesterol, heart disease risk, family history, eating habits, exercise habits, alcohol or drug use, tobacco use, or any other indication of physical or mental health.
In using the BMI as a shortcut to determine who is and isn’t healthy, doctors are misdiagnosing millions of people (and in the process, misallocating substantial money and resources). The research shows that using BMI as a shortcut to determine health causes doctors to overlook millions of “normal weight” patients who are actually not healthy, while flagging many millions more as “unhealthy” and needing treatment just because their BMI falls in the “overweight” or “obese” category, when they actually are quite healthy according to their biomarkers.2
In fact, longevity data demonstrates that people in the “overweight” and the “mildly obese” BMI categories actually live the longest! The National Health and Nutrition Examination Surveys, which followed the largest nationally representative cohort of U.S. adults, found the greatest longevity was in the “overweight” BMI category.3
Remember, weight is not a behavior.
There are multi-marathoners who fall at the higher percentiles on the BMI chart, while plenty of people in the “average” and “underweight” categories sit around all day.
As an eating disorder therapist, I see my fair share of larger-bodied patients with restrictive eating disorders. Atypical anorexia, which is a diagnostic category for individuals who meet all criteria for anorexia nervosa (AN) except for being low body-weight, is actually far more common in the population than “regular” AN. Assuming that someone’s weight reflects their eating and exercise habits is both inaccurate and reductionist.
Setting the record straight is vital for combatting the weight stigma and body shame so many people experience. In 2020, we know far too much about our species to continue operating under the assumptions of an astronomer from the 1830s. Let's fight for healthcare that sees humans as complex individuals, not just categories on an arbitrary chart.
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Bacon, L., & Aphramor, L. (2011). Weight science: evaluating the evidence for a paradigm shift. Nutrition journal, 10, 9. https://doi.org/10.1186/1475-2891-10-9
Burgard, D. (2010). What's Weight Got to Do with It?: Weight Neutrality in the Health at Every Size Paradigm and Its Implications for Clinical Practice. In 1093508422 828818374 M. Maine, 1093508423 828818374 B. H. McGilley, & 1093508424 828818374 D. W. Bunnell (Eds.), Treatment of eating disorders: Bridging the research-practice gap (pp. 17-35). Amsterdam: Academic Press/Elsevier. doi:https://doi.org/10.1016/B978-0-12-375668-8.10002-6
Matheson, E., King, D., & Everett, C. (2012). Healthy Lifestyle Habits and Mortality in Overweight and Obese Individuals. Journal of the American Board of Family Medicine : JABFM, 25, 9-15. 10.3122/jabfm.2012.01.110164.