This article is written by a student writer from the Her Campus at MSU chapter.

Given the popularity of the body mass index, or BMI, in the U.S. (and many other developed nations), it’s likely that you have at least heard of the term before. Still, let’s go over the basics to make sure that we’re all on the same page. BMI is defined by your weight in kilograms divided by your height in meters squared. Based on the score you get from this equation, you fall into one of four categories: underweight (score of 18.49 or below), normal weight (score of 18.5 – 24.99), overweight (score of 25 – 29.99), or obese (score of 30 or above). As you can see, BMI is simple and therefore convenient, which plays a huge role into why this measure is so widely used today. However, this simplicity is also the reason why BMI is an inaccurate measure for what it is primarily used for: measuring the amount of body fat on an individual and predicting any relevant health risks. The problems seen with how BMI is used today can even be traced back to its origin in the early 1800s.

The first form of what would later be known as the modern BMI was developed by a Belgian astronomer, sociologist, and statistician named Lambert Adolphe Jacque Quetelet. Commonly known as Adolphe Quetelet, he was very interested in what he called “social physics”, or analyzing societies without bias using statistics. While this stemmed into a few different focus areas, one of them became delving deeper into “social averages” — particularly in determining the measurements of “the average man”. It was this quest that led him, in 1832, to discover the first equation to relate weight and height: the Quetelet Index. This is now known as BMI, after American physiologist Ancel Keys described the Quetelet Index as the “best way to identify obese individuals” and changed the name to BMI in 1972. Thirteen years later, in 1985, the U.S. National Institutes of Health (NIH) embraced BMI as a way to identify overweight and underweight individuals.

It should be noted that Adolphe Quetelet was not a doctor and that he had no particular interest in studying obesity at the time of the Quetelet Index. His model of “the average man” was a simple system created with no outstanding technological support, which therefore resulted in a model that is way too simple to be an accurate model for individual people. This is because health is not one-size-fits-all. Health has to do with so many factors other than simply your height and weight, and BMI does not take this into consideration. These factors include the individual’s muscle to fat ratio, bone density, age, sex, race, physical activity, overall general health, and more. Let’s break this down a little further.

A huge problem with BMI is that it does not differentiate fat from muscle or lean mass. This comes into play particularly for individuals with high muscle mass, such as athletes. Because of their high muscle mass, these people tend to have a high BMI, meaning they are classified as overweight or maybe even obese. Yet, when you see these people or ask them about other aspects of their health, you’ll likely find that these so called “overweight” or “obese” people are actually very physically healthy. For example, as of 2017, Tom Brady has a BMI of 27 — classifying him as overweight. This type of miscategorization due to the BMI’s inability to differentiate between fat and muscle stems from the fact that muscle actually weighs 18% more than fat. If we are looking for a system that is going to give us reliable answers as to individual people’s body fat and risk for health problems, BMI is not the right method.

All of that said, it would be wrong to say that there is no merit to BMI. According to Nuttall (2015): “BMI has been useful in population-based studies by virtue of its wide acceptance in defining specific categories of body mass as a health issue”. The problem comes when it is applied to individuals without taking any other factors about them into consideration. This is what leads to the trend discussed earlier, where physically healthy people are being labeled by BMI as overweight or obese. It is also possible for the converse to happen: people getting low scores on the BMI but in other assessments found to be physically unhealthy. For example, in the 2005 – 2012 National Health and Nutrition Examination Survey, researchers found that “nearly half of all overweight individuals, 29% of obese individuals, and even 16% of obesity type 2 and 3 individuals were metabolically healthy. Moreover, over 30% of normal weight individuals were metabolically unhealthy”.

While this is not an exhaustive list of all the inaccuracies of BMI, it scratches the surface as to what the problems are. As more and more studies come out and as we continue to look closer at the limitations of BMI, it is clear that BMI is not a measure of “health”, just as it is not a measure of cardiovascular health, cholesterol, blood pressure, blood sugar, or anything like that. If you ever look up or find out your own BMI, be sure to take it with a grain of salt as it is not factoring in your muscle mass, age, sex, bone density, physical activity, race, or overall general health.