BMI is BS. Here's Why.
Written by Isabel Vasquez RD, LDN
From health insurance to doctor’s visits, BMI is widely used in our medical system. It’s where the terms “overweight” and “obese” come from, and it’s often used to indicate someone’s health status.
However, its roots are far more problematic than you may assume, and (surprise, surprise!) it’s not a great indicator of health. In this blog, you’ll learn what BMI is, its origins, how it’s used today, and the American Medical Association’s latest stance on BMI.
What is BMI?
Before we get into the problems with BMI, let’s define it. BMI stands for body mass index. It’s calculated with the following formula, using your height and weight:
Weight (kg) / [height (m)] squared
or
Weight (lb) / [height (in)]squared x 703
How is BMI used in the medical system?
While I generally avoid using these terms, for the sake of this article I’ll share that BMI is used to classify someone as underweight, normal weight, overweight, or obese. Per the CDC, it’s meant to determine someone’s level of fatness (although it doesn’t even do this very accurately). Think about someone who is super muscular, like a bodybuilder. They may have a high BMI, but that doesn’t mean they have a high body fat percentage.
Most of the time, BMI is used to quickly and easily determine someone’s health risk (since these BMI categories have become associated with health status); however, there are a multitude of issues with this that we’ll get into soon. In fact, BMI wasn’t even created to be used for this purpose!
A brief history of BMI
Here’s where it gets interesting. Adolphe Quetelet, a mathematician, statistician, astronomer, and sociologist, created Quetelet’s index, which later became known as BMI (you may notice he is not a doctor or medical provider).
As Brittanica explains, “He presented his conception of the homme moyen (‘average man’) as the central value about which measurements of a human trait are grouped according to the normal distribution.” With this mentality, he created his index to determine the “ideal” weight for someone’s height.
He took a sample of French and Scottish men and concluded that their average was not only an average, but the ideal. Those who were above or below the average became labeled as “overweight” or “underweight” rather than simply above or below average.
It’s worth noting that Quetelet was also involved with eugenics, and his work became used to justify scientific racism.
However, Quetelet never meant for his index to be used as a measure of individual health or body fat. In fact, it wasn’t meant to be used for individuals at all; he meant for it to be used for measuring populations.
In the 1970’s, Ancel Keys came along and revived Quetelet’s index. Life insurance companies were already looking for a means of quickly assessing someone’s health risk using their weight to determine what to charge prospective policyholders. They had tried creating tables on their own, some of which considered frame size. These tables were even picked up by doctors (even though the life insurance workers creating these tables were not medical providers).
Keys, a researcher, used a sample of mostly white men to determine the best assessment for weight and body fat. He tried three different methods and found Quetelet’s index (which Keys renamed BMI) to be the most accurate. In this case, accurate doesn’t mean much; it had only a 50% success rate for diagnosing “obesity”. A 2011 study also found poor accuracy of BMI for white, Hispanic, and black women.
Despite its flaws, the National Institutes of Health added BMI to their definition of “obesity” in 1985. Then, in 1998, they lowered the BMI cutoff for “overweight” and “obese”, pushing millions of Americans into these categories who were previously categorized as “normal” even if they hadn’t gained any weight.
The American Medical Association’s new guidelines on BMI
In June 2023 (shortly before this article was written), the American Medical Association (AMA) released new guidelines on the use of BMI, calling it an imperfect clinical measure. They even talked about the problematic history I shared above more in depth. Yay!
But esperate. We still have a long way to go before the medical system stops perpetuating weight stigma. Below, I share the positives and the problems with the AMA’s latest guidelines.
The positives
They say that BMI is inaccurate when it comes to measuring fat mass and mortality rates. For example, women tend to have higher body fat, while athletes tend to have lower body fat. Two people could be the same weight and height and have very different amounts of lean body mass and body fat.
When it comes to mortality rates, they say, “Even when some comorbidities are considered, the correlation of mortality rates with BMI often does not take into consideration such factors as family history of diabetes, hypertension, coronary heart disease, metabolic syndrome, dyslipidemias, familial longevity or the family prevalence of carcinomas, and other genetic factors.” They go on to list other factors BMI doesn’t account for, including smoking history, alcohol use, mental illness, and more.
They highlight that using BMI for eating disorder diagnosis creates issues. In the report, the AMA says, “the use of BMI is problematic when used to diagnose and treat individuals with eating disorders, because it does not capture the full range of abnormal eating disorders.” This is a big win because many people are left without an eating disorder diagnosis because their BMI doesn’t meet the criteria, specifically for anorexia nervosa. The AMA says, “Utilizing BMI can lead to substandard treatment, typically due to the use of BMI by insurance companies to cover inpatient treatment.” They also encourage teachers, nurses, counselors, coaches, and others to become educated in recognizing signs of disordered eating patterns.
They acknowledge that BMI doesn’t account for normal body diversity amongst different races, ages, and sexes. They say, “BMI is inaccurate in measuring body fat in multiple groups because it does not account for the heterogeneity across race/ethnic groups, sexes, and age-span.”
They acknowledge the racist roots of BMI. The AMA noted that BMI is rooted in eugenics and racism. This isn’t talked about enough so it’s a huge step forward to see the AMA putting this out there in such a public way.
The problems
They proposed numerous other metrics for determining body fat. While the AMA notes the problems with BMI, they propose a number of other metrics to assess health risk, including visceral fat, body adiposity index, body composition, relative fat mass, waist circumference and last, genetic/metabolic factors. As a result, they’re still pathologizing fatness and centering it as a measure of health.
They still advise using BMI-for-age in children. Unfortunately, despite a lot of traumatic experiences with weight stigma happening in childhood, the AMA says, “In children, however, there is no good reference data for waist circumference, so BMI-for-age is currently the gold standard.” For reference, BMI-for-age is one of the growth charts used to measure growth and development for kids and teens up to 20 years old. This is very concerning because kids and teens go through a lot of body changes that can spark body dissatisfaction and even eating disorders. Having their weight closely monitored (like with the BMI report cards done at school) could cause real harm.
They’re perpetuating weight stigma. At the end of the day, the AMA did not acknowledge the harms of weight stigma in their guidelines and the guidelines themselves perpetuate fat bias. We know that weight bias causes harm, just like racism or other systemic forms of discrimination do.
Fighting for fat liberation
While these guidelines are a big step forward, there is still a long way to go before we reach fat liberation. Lindley Ashline, a photographer and author who celebrates the beauty of bodies who fall outside conventional beauty standards, defines fat liberation as, “the deliberate work of tearing down the systems that have created a world where fat people are denied full participation in society and life, from apparel to healthcare.”
These standards continue perpetuating the idea that fatness is a disease and something to be feared. Medical providers are some of the worst perpetrators of weight stigma, and these guidelines only seek other ways to diagnose fatness.
They ignore the harms of weight stigma and instead, place a focus on getting more accurate measures of someone’s fatness versus their weight alone.
Final thoughts
BMI is a problematic measure that has racist roots. It’s been used to assess health risk using height and weight, but the AMA recently critiqued it, calling it an imperfect clinical measure.
While their statement had some great points, like the problematic use of BMI for eating disorder diagnosis and mortality risk, and an acknowledgement of BMI’s racist origins, it still perpetuates weight stigma and seeks “better” ways to measure fat mass.
It’s definitely some forward progress, but there’s far more work to be done to dismantle fatphobia.
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