The BMI Scale and It's Flaws

by Wray Watkins B.S.
(Lake Forest CA. USA)

I have been training clients for the past 10 years at MVP Sports Centers & Physical Therapy. Through these years I have had one issue that transcends every type of client. That issue is the concept of being overweight.

The official definition from the Center for Disease Control is as follows: “Overweight and obesity are both labels for ranges of weight that are greater than what is generally considered healthy for a given height. The terms also identify ranges of weight that have been shown to increase the likelihood of certain diseases and other health problems.1” According to the CDC, the way to find out if you’re overweight or obese is to use the BMI equation.

The actual equation for calculating your BMI is as follows: (weight (lb) / height (in) 2 x 703) . I invite the people who may read this to plug their information into this equation. Now after all the nasty mathematics, we can see where we fall in the BMI tables:

Below 18.5-Underweight: 18.5 – 24.9-Normal Weight: 25.0 – 29.9 Overweight: 30.0 and Above Obese.

Now, before we go and get all depressed and angry about the results, let’s look at the wonderful equation that got us here. I have several problems with this equation, but here are my main two:

1. The validity of the equation is questionable at best. The variables measured are height and weight. The end result is the ratio between the two. NO ACTUAL FAT MEASURED! If excess fat is the problem that leads to certain diseases and health problems - let’s get that measured then. When comparing the accuracy of the BMI vs. Body Fat Measurement, the Mayo Clinic College of Medicine did a recent study in 2008. Their conclusions are as follows. “The accuracy of BMI in diagnosing obesity is limited, particularly for individuals in the intermediate BMI ranges, in men and in the elderly. A BMI cutoff of > or =30 kg m (-2) has good specificity but misses more than half of people with excess fat. These results may help to explain the unexpected better survival in overweight/mild obese patients.2” With this being said, the BMI is misclassifying people and skewing the statistics.

2. Lastly, my favorite characteristic of the BMI is the age of the equation. A Belgian mathematician and sociologist named Adolphe Quetelet created the Body Mass Index between 1830 and 1850. He did this as a way to compare a person’s height with their weight.

This technique was originally meant to aid in social science education, and wasn’t intended to determine obesity levels. This equation may have been cutting edge at the time, but I don’t think anyone wants to be told they are at risk for health problems based on an equation that pre-dates the Civil War.

There have been a few technological advances since 1830 (airplanes, space shuttles, The Twilight Saga etc.), yet we still utilize this outdated technique. Yes, the BMI is easy to use and anyone with a calculator and a middle school IQ can figure it out, but anything worth anything takes some thought.

So in conclusion, when the Wii Fit uses this equation and your person on the screen looks chunky, resist the temptation to throw that controller throw the TV screen as I did. Take that energy and go to the gym.

Don’t get me wrong, I agree that Americans need more exercise, activity and nutritional advice, but I disagree on how we are assessing the issue. So I urge anyone who is starting their quest for better health and fitness, find a certified professional and get a true and valid assessment.


References:

1. http://www.cdc.gov/obesity/defining.html
2. Int J Obes (Lond). 2008 June; 32(6): 959–966.
Published online 2008 February 19. doi: 10.1038/ijo.2008.1