Why BMI Is Misleading

Body-mass index (or BMI for short) is the most common way of measuring whether or not a person is obese. This figure is obtained by dividing a person’s body weight from their height (squared). It’s built on the idea that a person of a certain height should have a certain weight in order to be considered healthy.

More and more, people are beginning to realize that this is not the best way to measure obesity, or the lack thereof. There is some good research to back up this conclusion. In the study linked above, it researchers found that BMI was not a good predictor of cardiovascular or metabolic health. In this article, we will examine the reasons behind this new perspective.

Why Is BMI Misleading?

The use of BMI as an indicator of obesity is misleading for more than one reason. If you think about it, they are only looking at two factors: Height and overall weight. Although the height-weight ratio is important, it is not all-important. Let’s look at some of the things that aren’t taken into account when using BMI.

Muscle Mass Vs. Fat Mass

For BMI numbers to be truly accurate, we would have to assume that everyone has the same proportion of muscle to fat. Obviously, this is not the case. Let’s put it another way: Let’s say a person weighs 200 pounds. If the person is strong and muscular, that muscle might make up a large percentage of their total mass. If the person has a lot of body fat and has very little muscle, then the majority of their excess mass will indeed be fat. A BMI measurement would measure both of these individuals as overweight or even obese.

It is commonly known that muscle weighs more than fat. However, some people have attempted to “debunk” this old saying with an argument that is entirely nonsensical. They say that a pound of muscle and a pound of fat weigh the same. Because the weight of a pound does not change from one substance to another, this statement is technically true, but it shows a lack of understanding. Muscle is more dense than fat, and as a result, a pound of muscle is much smaller than a pound of fat. Muscle is, in fact, heavier than fat.

Bone Density

When we talk about fat masses and muscle masses, we forget about one big thing: Your skeleton. For most individuals, the skeleton makes up 12-15% of their body weight. For some, this number might be higher. For instance, it is a fact that some races have a higher bone density (on average) than others. BMI takes none of this into account.

Body Proportions

Once again, BMI only considers height and weight. But there are many different body types in the world, and BMI only considers the average build. A prominent British professor named Nick Trefethen wrote an interesting letter to a magazine called “The Economist,” in which he outlined his objections to the use of BMI as the obesity standard.

Mr. Trefethen was not a professor of health or medicine, but a professor of mathematics. He says that the mathematical properties of BMI measurement result in two particular miscalculations. He said that it divides the weight by too much when a person is shorter than average and that it divides the weight by too little when they are taller than average. In other words, the system stops working as soon as you introduce a non-average body type.

Does BMI Correlate with Other Health Markers?

In some cases, BMI results do correlate with other health markers (such as actual body fat percentage). In some other cases, they do not. Therefore, this question has a mixed answer. For instance, this study took five girls from five different ethnicities and attempted to determine how accurate BMI measurements were in predicting body fat percentage. Overall, they found BMI to be an accurate predictor of excess fatness.

However, there is one big problem with this study. It is based on “self-reported” BMI values, meaning that the researchers did not verify the measurements. This seems like very sloppy research, and we were able to find another study that demonstrates and explains the flaws in this method. They did a similar study on five test subjects and found that reported BMI was usually lower than actual BMI. In other words, the participants were lying or incorrect.

Instead of trying to cover every health marker known to man, let’s focus on the most important one of all: Overall risk of death. This study found that BMI was not a good predictor of early mortality. They considered all causes of death as well as differences in build between men and women. The researchers found that the ones with the highest risk of death were those who had a low BMI and a high body-fat percentage. That means people with a lot of fat and very little muscle.

Why Do People Use BMI As The Standard For Obesity?

As far as we can tell, this is mainly used for the sake of convenience. It’s a lot easier to punch a couple of numbers into a calculator than it is to accurately measure the amount of fat in a person’s body.

To be fair, we have already seen that BMI measurements do correlate with some health markers. However, they don’t correlate with some others, including the most important one, which is the overall risk of early death. Still, the degree of correlation that has been observed is probably also responsible for the continued use of this method.


There are many other methods of measuring overall body fat. While BMI numbers might be useful and accurate in some cases, it’s better to measure your body-fat percentage if you want a better picture of your overall health. There are several ways to do this, so pick the method that you prefer. We also hope that you will prefer to follow us on Facebook since we have shared all of this useful knowledge with you!

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1 comment
  • I’m a healthy adult male that’s approximately 6’0” and 108 lbs or so, and my BMI is 14.9… that doesn’t seem right. BMI truly is bogus

    Noah Ater on

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