BMI doesn’t work! (and how it does)

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Obligatory disclaimer: I am not a medical doctor, and the content of this website was created for informational purposes only. Such content is not intended as a substitute for medical advice, treatment or diagnosis.

We’ve previously gone ove the Body Mass Index, BMI, and learned how it’s defined, how we can interpret it, and some of its strong and weak points. In this post our goal with be to answer some of the most common criticisms of BMI that are found online.

Common criticisms of BMI

Through a quick Google search I found the following claims of how BMI is useless. They are in no particular order, and I have limited the number of claims to respond to so that this post doesn’t become repetitive, boring, or a dissertation.

  • BMI doesn’t work for athletes/short people/tall people/, so it’s useless!
  • BMI doesn’t take into account my activity level!
  • BMI doesn’t tell me about problem areas!
  • BMI gets body fat/muscle mass wrong!
  • BMI is lying by scientific authority!
  • BMI is a tool of the insurance industry so that they can charge higher premiums!
  • There are better tools than BMI!

Answering the criticisms

  • As BMI is determined by considering the weight and the height of an individual, what happens when an individual’s height or weight are out of the norm? Remember, BMI is useful when it comes to populations, so some individuals will be outliers and their BMI will make little sense. An example of this are strength athletes who will be categorized as overweight or obese due to their large muscular mass, which makes no sense. The same goes for shorter than average or taller than average people, the former tend to find themselves in the overweight category despite being healthy while the latter tend to find themselves in the underweight category despite, again, being healty. Is this reason enough to do away with BMI? The quick answer would be to ask yourself if you truly are more athletic than the average person, or if you’re taller or shorter than average. Chances are that the average person is, well, average, and BMI will work well for them.
  • By its very definition, BMI doesn’t account for anyone’s activity level. What the problem with this is, I still can’t figure out. It’s clear that this particular criticism is aimed towards saying that even if a person is classified as overweight or obese, BMI is wrong because that person may lead an active lifestyle and they’re healthy. Again, BMI works well for populations, and the farther an individual strays from being average in terms of weight or height, the less reliable BMI becomes. However, this activity level issue shouldn’t be a problem unless you are as active as an athlete, and you have the extra muscle mass to show it.
  • Why would BMI tell you about problem areas? Is there anything in its definition that even considers them? What is a problem area to begin with? See, this criticism seems to come from the same school of thought that claims that spot reduction is a thing. For those who may not know, spot reduction is the idea that we can target specific places in our body and only lose the fat located in them. The reality is that fat is put on and lost by our body in a manner that is beyond our control beyond what we put into our mouths. In other words, this criticism is rather nonsense.
  • BMI cannot get muscle mass or body fat wrong because BMI doesn’t even account for them. Take a look at BMI’s definition, do you see a variable that specificly accounts for muscle mass or body fat? On the contrary, both are being implicitly accounted for in a person’s weight. As stated previously, if you’re an athlete then BMI won’t even be a thing you take into account, but if you’re smack down average, then BMI will work well.
  • I don’t even know what lying by scientific authority means. Do we use Einstein’s field equations because Einstein came up with them? No, we use them because they work, just like we use quantum mechanics where applicable and Newt0n’s Law of Gravitation where it gives the right results. That’s where science’s authority comes from. Perhaps what this criticism is trying to say is that BMI is flawed but it’s been given an air of authority, which would be an appropriate criticism, especially considering athletes, tall and short people. If the meaning of this criticism is a different one, then I’ve no idea what it is.
  • I would be surprised if BMI wasn’t taken into account when setting insurance premiums. However, going from there to the conspiratorial side is a little extreme, since it’s not insurance companies that have established the different classifications due to BMI.
  • This last one is a perfect criticism because it’s true. There are indeed better tools than BMI, but the question then is: better at what? If you want a quick and cheap way to classify populations based on height and weigh, BMI works fine. If you want to take a more careful look into an individuals body fat levels or muscle mass then yes, BMI won’t give you any information about that. The task determines the tool, and for the average person BMI is good enough.

Closing thoughts

BMI works well for populations and most people within them. If you are an outlier due to height or because you’re an athlete, then BMI will give you the wrong results, but if you’re the average person, then BMI will give you results that are appropriate.

Hope you enjoyed this post, if you would like to see more content like it, check our Rebuttals to Fatlogic section.

See you next week!