Figuring Body Mass Index

Recently the Health and Human Services announced that an obesity rating for every American must be included as part of a stimulus bill mandated electronic health records provision. This article lead my fertile brain to wonder what this BMI stuff was all about.

In particular, I wanted a way of figuring body mass index so I could understand better what it really means. Second, why was the government mandating that everyone have a BMI on their electronic records as part of a stimulus bill? I'm not even going to bother figuring out the second questions. That belongs under politics so let's tackle the first, figuring body mass index. Turns out it is a fairly simple calculation.

In the US, it is the product of your weight in pounds and 703 divided by height in inches squared. (Using metric units is even easier as their is no scale factor.)

From there you take the resulting number to determine your 'fitness' The scale works something like this:

Not hard at all to figure out how to calculate Body Mass Index but what does it mean?

What BMI Means - Body mass index is a numerical assessment of health. If you are in a certain range, you are deemed healthy. If you are outside of that range you may be considered overweight, underweight, obese etc. In a sense it is an approximate estimate of health in a body.

The theory goes if your BMI is within a specific range, the amount of fat on your body for your height is considered healthy. This data is a based on a study published in the early 70's by by Ancel Keys. It was designed as a proxy for estimating body fat for epidemiological studies. (The study of disease in populations.) The idea caught on and became very widely used by those in the medical and health fields.

The problem is the BMI was not meant as an actual assessment of health. Keys even stated as much as part of his research on BMI. It was intended to serve as a quick and easy statistical calculation used to simplify the study populations as it relates to health and weight. Over time it has been co-opted to assess an individuals health.

Why BMI Should Not be Used on Individuals - Turns out each of us is different! Men a different than women and the young are different than the older etc. Go figure. BMI does not take into account forms lean body mass such as muscle. Therefore a person who is more athletic and has more muscle may be classified as overweight or obese when in fact they are quite healthy and anything but fat.

Here's an example of gold medal athletes considered overweight using the BMI Scale. These people are all highly trained athletes in peak physical condition.

Now not all of us are Olympic athletes but there are many who train or work hard and carry more muscle. Just as men tend to carry more muscle than women and the young tend to have more muscle than the old, it seems quite illogical to do a simple height/weight calculation and stuff someone in a box.

This is compounded by the fact that children and infants have their own scales. Boys and girls have their own scales. Furthermore, some suggest this oversimplified measure of height/weight can lead to all kinds of social issues related to body image etc. When you look at the use of the BMI as a measure of health, it actually it seems intellectually lazy and as some have suggested probably scientifically nonsensical.

Figuring out the Figuring on Body Mass Index - Seems to me that the calculation of body mass index is fairly straight forward. I could see how it may be used as a training tool or rule of thumb. I can also see how someone studying a bunch of data would want a way to simplify the height/weight data to come up with a single value to normalized a person's body type. These all make sense.

What is not as apparent is why the government is getting involved. This seems fodder for another hub in the politics section. So for those of you who like me wanted to know how to calculate your BMI, here it is!

Sept 2011 - Michigan has started tracking kids with BMI.