8 Reasons Why BMI is BS
Despite its shortcomings, the Body Mass Index (BMI) is still used in many healthcare standards as a proxy for health. Here’s why that’s potentially harmful:
It’s outdated and wasn’t even created by a doctor or other healthcare professional. The BMI is a 200-year-old metric that was created by a mathematician, not a physician, to help the government allocate resources by estimating population level obesity. It was never meant to be a factor used on an individual level.[1]
It’s inherently racist and sexist. When the BMI was created, it was based on the bodies of white European adult men. That means it does not account for body size and shape differences on the axes of age, gender, race, or ethnicity, which we know can vary greatly among and within populations.[2] In fact, some evidence has shown that using BMI can be especially detrimental to people of color, since Black, Asian, and Hispanic individuals tend to have different body compositions than White individuals.[3] University of California, Irvine professor Sabrina Strings puts it best: “It is racist, and also sexist, to use mostly white men within your study population and then try to extrapolate that and create norms and expectations for women and people of color… They have not been included in the initial clinical analyses, and therefore their actual health outcomes cannot be determined by these findings.”[4]
Higher BMI doesn’t cause poor health. From a statistical perspective, we can’t prove that higher BMI causes higher risk of disease or death from the data alone. To make that claim, we would have to be able to control for each and every variable that might affect a person’s weight - including genetics and environment - which is virtually impossible. But even if we just look at the correlation between BMI and mortality, research (linked here and here) has found that mortality is actually lowest among those with “overweight” BMIs between 25 to 30, while mortality risk is comparable between those with “normal” BMIs between 18.5 and 25 and “obese I” category BMIs of 30 to 35.[5] [6]
In fact, there may be a protective effect for the “overweight” BMI category, as shown by several studies across representative populations in the U.S., Canada, Japan, Korea, China, India and other countries.[7] Researchers have coined this finding the “obesity paradox” - that is, the notion that those with overweight BMIs seem to be at lower risk of death, particularly among elderly populations. While the mechanisms remain unclear, it’s suggested that being overweight may help compensate for greater risk of malnutrition and other diseases common in old age, like cancer, that can lead to unintentional weight loss.[8]
The link between BMI and health becomes weak when we control for other measurable health factors. The data show that, rather than BMI or weight alone, exercise, nutrition, insulin resistance, and even weight stigma are among the measurable factors that may partially or fully explain the links between BMI and health. Once researchers control for these factors, the relationship between BMI and health is weak.[9]
Resources
[1] https://www.npr.org/templates/story/story.php?storyId=106268439
[2] https://www.scientificamerican.com/article/is-bmi-an-accurate-way-to-measure-body-fat/
[3] https://www.washingtonpost.com/lifestyle/wellness/healthy-bmi-obesity-race-/2021/05/04/655390f0-ad0d-11eb-acd3-24b44a57093a_story.html
[4] https://www.huffpost.com/entry/bmi-scale-racist-health_l_5f15a8a8c5b6d14c336a43b0
[5] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4132299/
[6] https://nutritionj.biomedcentral.com/articles/10.1186/1475-2891-10-9
[7] https://journals.sagepub.com/doi/full/10.1177/2158244018772888
[8] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6801595/pdf/ijerph-16-03656.pdf