The difference between "percentage of body fat" and "body mass index"?

Is there a difference between “percentage of body fat” and “body mass index”, and if so, what is it? How do the two correlate?

There is a huge difference between body fat and body mass index (BMI). Body fat is a much better indicator of your health, fitness level, disease risk and ideal body weight than BMI. In fact, Shape up America, the anti obesity campaign started by Dr. C. Everett Koop, recently published a press release pointing out the shortcomings of BMI. According to the Shape up America website, BMI missclassifies one out of four people and should not be used by athletes.

To understand why BMI is not a good indicator of your health and fitness, you first need to understand what the BMI is. I pulled out my handy textbook, “Physiology of Sport and Exercise” By Wilmore and Costill to get the official definition. BMI was described in the glossary follows:

“BMI is a measurement of body overweight or obesity determined by dividing weight (in kilograms) by height (in meters) squared.”

The text went on to say that you’re considered overweight if you’re female and your BMI is 27.3 or greater or if you’re male and you’re BMI is 27.8 or higher. It also said that BMI correlates highly with body composition and is a better indicator of fitness than your weight alone.

If you’re sedentary, then I’ll buy the part about BMI being better than going just by your body weight alone, but the part about BMI correlating well to body composition is complete hogwash!

Let me show you an example of how far BMI falls short as a measure of body composition.

This morning I weighed in at 201 lbs. and I am 5′ 8 inches tall. Converted to metrics, 201 lbs. is 91.36 kilograms and 5′ 8″ is 172 cm or 1.72 meters. So now let’s plug my stats into the BMI formula and see what we come up with…

201 lbs. = 91.36 kilos.
1.72 meters squared = 2.96
91.36 kilos / 2.96 = 30.86

So, if we judge my physical condition according to my BMI of 30.86, then I’m “obese” and I need to lose some weight. As you can tell from my photos, that’s not the case. Even though I’m currently not in a pre-competition mode, my body fat is still in the single digits (9.3% last time I had it measured), which is considered very lean for men my age (35).

Why is BMI a poor measure of health and fitness? It’s simply because BMI does NOT take into account body fat vs. lean tissue. Body builders and other athletes carry more lean body mass than the average person and will therefore be classified as overweight if BMI is used as the criteria for measurement. Conversely, someone could have a “healthy” BMI of 19 to 22 and yet have a dangerously high level of body fat (a “skinny fat person”).

The solution of course, is to differentiate between your lean body weight and your fat weight. This can be done with body composition testing. There are many ways to test body fat, but the method I recommend is skinfold testing (the “pinch” test). You’ll get the most accurate reading if you have an experienced tester measure you at three or four skinfold sites, but if you don’t have access to an experienced professional, you can test your own body fat with a home testing kit called the Accu-Measure.

The Accu measure tests total body fat with a single skinfold pinch on your iliac crest (hip bone). I bought a set for myself (very inexpensive) and I found that the measurements were fairly close to the measurements I got from the $450.00 computerized “Skyndex” calipers we use at our health clubs. You can order the Accu Measure caliper online from many fitness websites.

In conclusion, body fat percentage is the only way to go. BMI is a lousy indicator of your health, fitness or ideal weight. Forget about BMI – and while you’re at it, forget about those Metropolitan height and weight tables too. According to the ideal weight tables, a man 5′ 8″ tall with a medium frame should have an ideal weight of 138 – 152. That means I need to lose at least 49 pounds!

In my ebook Burn the Fat, Feed the Muscle it has an entire chapter on how to test your own body fat or have it tested by someone else. There’s even a complete chapter about how to chart your progress based on your body fat and lean body mass. It even tells you exactly what to do in your training and nutrition programs based on the results of the body composition test each week. It’s a fat loss success formula that literally can’t fail.