Understanding the BMI Classification

Happy Monday Friends,

Today’s post is going to be pretty brief, but I believe it is an important one. I’m going to address the topic of BMI which stands for Body Mass Index as I feel individuals often misunderstand the value given to them at their doctors office. The BMI is used as the universal measure of weight classification. This is the number that is used to determine if you are under, normal, or overweight. Overall, the BMI allows for us to compare weight changes across different time periods and track trends in populations. However, as I will get into below, there are some major flaws when using the value on an individual basis to assess healthy (or unhealthy) weight status. Here is a chart outlining the current BMI normative values:



So, as you can see, a BMI of 18.5 or less is underweight, 18.5-24.9 is normal and above 25 is overweight. The number is calculated using the following equation:

BMI= body weight in kg/height in meters squared.

As an example, here is my BMI calculation:

Height- 6 feet, 1 inch Weight- 191 lbs

BMI= 86.63 kg/3.44 meters squared= 25.18 (Overweight)

Now, I realize that not everyone reading this post knows me personally, but I can assure you that this is not an accurate representation of my body size. This brings me to the issues associated with BMI:

1) Lean body mass is not taken into account. Essentially, the BMI calculation assumes we are all just balls of material with identical proportions when in reality some of us carry more muscle and less fat than others.

2) What classifies a healthy body weight? There is not a lot of research that can show with certainty what weight is “most” healthy. While it is well-supported that being extremely underweight or overweight is associated with adverse health effects, there is a ton of variation in between.

3) Weight is gender dependent. Men and women have different body structures, this is a fact. BMI fails to account for this, leading to women often being classified as overweight or obese when it may not be the case.

The Main Point

This post is not meant to destroy BMI. Like I mentioned before, BMI is a very useful tool in comparing populations and is really helpful in research. I suppose my main point would be this- if you go to the doctor and are told you are overweight, ask them to provide you with how much lean mass you carry vs. fat mass. This can be estimated using skinfold techniques or more advanced metrics. Now, if you are classified as obese (unless you are an extremely muscular specimen) or underweight, that still should serve as a sign to manage your weight to a more healthy number.