What is the Body Mass Index (BMI)?

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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 began our Rebuttals of Fatlogic series by talking about the starvation response (starvation mode for those who don’t know) and metabolic adaptation (aka metabolic damage). We then did a sub–series focused on macronutrients: carbohydrates, proteins, lipids/fats, and alcohols, where we talked about their composition, structure, biological uses, and how they are digested. Along the way we’ve also talked about thermogenesis, the thermic effect of food, and the glycemic response. It seems that the time is right to go back to deal with a couple of fatlogic myths and concerns before we head into what micronutrients are, so today we’ll talk about the Body Mass Index (aka BMI).


The Body Mass Index

BMI is defined as a person’s weight in kilograms, divided by their height in meters squared, namely (and copied from Wikipedia because I can’t write math in WordPress):

Now, let’s try to interpret that definition. To begin with, BMI’s units are of mass over surface area, so it’s analogous to surface mass density. Secondly, this surface mass density is made up of a person’s weight is distributed uniformly in a square whose sides are equal to that person’s height.


Let’s refer to the square representation of a person’s height with the term “square-person”. Hopefully in this way, you can understand BMI as a measurement of a given square-person’s mass density. Intuitively, two square-people of the same height but different weights will have to spread those different weights throughout the same surface, and two people of the same weight but different heights will have to spread that same weight over different surfaces. Simple, right?


BMI categories

Let’s get something out of the way, BMI is useful when it comes to populations, as it provides a quick and easy way to check wether an individual is underweight, of normal weight, overweight, or obese within that population. This does not mean, however, that it is the be all and end all criterion for establishing someone’s weight-status, although it provides a quick reference for that. This is something which fatlogicians don’t seem particularly interested in understanding, and appear to be more focused in throwing away the concept of BMI altogether.

I won’t cite specific BMI values, but it is worth noting some of the categories they reference: underweight, normal weight, overweight, and obese. It is also worth mentioning that each of those categories encompasses a whole range of BMI values, and thus there is a whole spectrum for an individual’s weight-status within a given population. This is another thing fatlogicians seem to forget: although the cutoff BMI values between one category and another might appear harsh and arbitrary, the reality is that there are ranges of BMI values that are compatible with having a normal weight or being overweight or obese, so focusing on whether one is a decimal above or belove a given cutoff is missing the forest for the trees.


BMI’s applicability and limitations

It would be senseless to assume a definition for BMI like the one given above would suit everyone. Clearly, extremely tall or short people would have issues fitting into the “correct” weight category. Likewise, muscular people tend to be bumped into the overweight or obese categories. However, unless you are some sort of outlier, or can handily lift a small car, chances are that BMI and its weight-categories are well suited to you.

So, that was a primer on BMI, hope you enjoyed this post and see you next week, when we will talk about how this blog doesn’t hate fat people, and what the point of this project is.

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