Tuesday, March 22, 2016

BMI Definitely Lacks TMI

I figured I would write an article as a follow up to the Washington Post article I was quoted in last week to further elaborate on what we can take away from the use of BMI.  But first, a little background information on BMI. For those who aren't familiar with the Body Mass Index (BMI) it is simply a ratio of your height and weight. It is often used to classify one's bodyweight as healthy, or overweight, or obese.

However, the problem with the use of this index to classify an individual's body weight is that it doesn't tell us anything about the COMPOSITION of that weight.  For example, I am someone whose BMI has always been around or above 30 kg/m2 which means that I would fall in the category "Obesity class 1" according to the table on the right.  However, when I take into consideration my actual body composition my body fat percentage usually falls within the range of 10-15% depending on my level of training and focus on nutrition at any given time. When classifying my body fat percentage I typically fall within the "lean" or "moderately lean" categories (see table below).  So what is body composition actually measuring? Well, like the name implies, it is telling us what the composition or make-up of an individual's bodyweight actually consists of.  Depending on what type of technology you have available to you, you can get very precise with the measurement and get an idea of fat mass, lean muscle, bone mass etc. 

The most common measurements used outside of a laboratory setting uses a 2 compartment model that is used to assess body composition. From this, you typically get an individual's body fat percentage, as I described above, that can then be used to calculate the amount of weight someone has that consists of fat mass and fat-free mass. To some extent, we need a certain amount of body fat for survival and this can be described as essential body fat and then we have non-essential body fat that accumulates if one constantly consumes an excess amount of calories and may compromise your health.

So back to the Washington Post article...  The article focused on the idea that even though your BMI may be "normal" you may be a high risk for developing certain medical conditions, increasing your risk of dying prematurely. So how can this be? Well, with a "low" and even "normal" BMI you may actually have an excess of non-essential body fat. Because, as was mentioned above, BMI tells us nothing about someone's body composition. So just because someone is of normal weight according to their height they could be (what I like to refer to as) "skinny fat" meaning they do not have an optimal ratio of fat-free mass to fat-mass per their overall body weight.  Or, a low BMI could even indicate that you have too low of a body fat percentage and could potentially be malnourished.  Regardless of the scenario, it is difficult to extract any kind of beneficial information from the BMI and therefore I always recommend body composition testing if you are trying to classify yourself as "healthy" or having excess body fat in order to assess your risk of developing cardiovascular disease, metabolic disorders etc.  Just make sure you are using an accurate test for determination of your body fat percent...stay tuned for more on that!
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