Archive for October, 2010

BMI and Body Fat Percentage – Not Created Equal

How to Measure Body Fat Using a Tape Measure

Many of us are familiar with BMI (Body Mass Index) charts which tell us, supposedly, if our body mass is healthy, underweight or overweight.    

According to the Centers for Disease Control and Prevention (CDC), “BMI is a number calculated from a person’s weight and height and is a fairly reliable indicator of body fatness for most people. While BMI does not measure body fat directly, research has shown that BMI correlates to direct measures of body fat, such as underwater weighing and dual energy x-ray absorptiometry (DXA).” BMI is an easy-to-calculate method of screening for weight categories that may lead to health problems. The Internet is rife with BMI calculators. Here is a link to the CDC’s calculator for adults and children/teens: http://www.cdc.gov/healthyweight/assessing/bmi/

The chart, below, lists the standard weight status categories associated with BMI ranges for adults:

 

BMI Weight Status
Below  18.5 Underweight
18.6 – 24.9 Normal
25.0 – 29.0 Overweight
30.0 and Above Obese

 

The problem with relying only on BMI is that it is a very one-dimensional picture of a person’s body composition. For example, there are people who may have a BMI in the Normal range, but are actually fat as determined by body fat testing. Conversely, there are people who are deemed obese or overweight, who, in fact, are very lean with minimal body fat. This is because BMI does not take into account how much body weight is muscle and how much is fat. 

This is why it is important to consider body fat percentage in addition to BMI. Body fat percentage is calculated by dividing fat mass by body weight. This calculation can lead to results that not only don’t match the BMI categories, but in some cases, if not made known to the affected individual, could lead to undiagnosede health problems.  

For example, two women are both 5 feet 4 inches tall with a BMI of 24. But if woman #1 has 42 pounds of body fat and 98 pounds of lean mass, she has 30% body fat. This is what is often refered to as normal weight obesity. If woman #2 has only 28 pounds of body fat and 112 pounds of lean mass, her body fat is only 20%. Same weight + same height does not always equal the same composition of fat to lean mass.   

On the other side of the coin, you have people who are incorrectly labeled as overweight or obese according to a BMI chart, especially athletes. My 16-year old son, who works out regularly as a high school football player, is a prime example. According to a BMI chart, he is Overweight. But after measuring his body fat with calipers (which was not easy to do because the kid has almost no fat on him) he falls into the Athlete range on the Body Fat chart (see below). By the way, is there some reverse genetic engineering process that will allow him to pass this trait to his mother? Just asking.  

Body Fat Categories

Classification Women Men
Essential Fat 10-12% 2-4%
Athletes 14-20% 6-13%
Fitness 21-24% 14-17%
Acceptable 25-31% 18-25%
Obese 32%+ 25%+

While the ultimate way to test body fat is via underwater weighing or x-ray absorption as mentioned above, unless a friend or neighbor has x-ray vision or is an expert with his or her own water tank and highly sensitive equipment, and you don’t mind being stark naked – which is required for the water test, these means of measurement are not practical for the majority of people. However, you can measure your body fat, albeit somewhat less precisely, in the comfort of your own home by simply using a tape measure or body fat calipers (you can purchase basic calipers online for a few dollars – check out amazon.com). Body fat calipers require a bit of practice and unless you are a contortionist, someone to assist you with  measuring certain locations, like your lower back.  

Here’s a link on how to measure body fat using a tape measure: http://www.ehow.com/how_4869405_measure-fat-using-tape-measure.html  

If you want to use body fat calipers, you might want to visit this web site for information on how and where to take measurements: http://linear-software.com/online.html 

For many of us, BMI is a good indicator as to whether or not we are at a healthy weight, underweight or overweight. But, if you are more athletic or simply want a more precise indication of your ratio of lean tissue to fat, I recommend taking a few minutes to take your measurements. The results might prove comforting or might motivate you to make changes to improve your overall health and fitness.

October 2010
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