In a Nutshell
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A pound of fat takes up nearly twice as much space as a pound of muscle because muscle is considerably more dense than fat. Which is where the saying “muscle weighs more than fat” comes from. Due to this fact it is possible for someone to have a low percentage of body fat and still be considered “overweight” by BMI standards (we will cover this shortly), which is not uncommon among athletes. In this case a better measure of leanness is to determine body composition or fat to lean mass ratio. There are various ways to calculate body composition such as skin calipers, underwater weighing, or electrical impedance to name a few.
BMI or body mass index uses an equation that operates from a few assumptions. It assumes that the person that the BMI is being applied to is not an athlete and non-weight lifter. It also assumes that the population it is applied to will develop predictive chronic diseases directly related to the BMI categories. Additionally part of why it is used is because it is fast, cheap, and much easier to apply than taking an accurate body composition.
BMI for a normal weight person 20 years of age or older is between 18.5 to 24.9 and is considered a low chronic disease risk (i.e. cancer, heart disease, diabetes etc). One is considered overweight with a BMI between 25 to 29.9, which is thought to correlate to being 25-30 pounds overweight as well as having an increased risk of chronic disease. One is considered obese if he/she has a BMI of 30 or higher, which is thought to correlate to being 30 pounds or more overweight as well as being considered at high, very high, or extremely high risk of chronic disease (risk increases as the higher the BMI climbs).
So the long and short is, if your doctor told you, that you’re overweight, or stated that you need to lose weight, most likely they determined this by using BMI which also identified you as having an increased risk for chronic diseases. Unless you exercise regularly, are an athlete or live an active lifestyle, most likely the BMI is a relatively accurate measurement. If you do not know your BMI, you can quickly configure it by using one of the two equations listed below:
1. BMI= weight in kilograms/ height in meters (squared) or it can be configured
2. BMI= weight in pounds/height in inches (squared)
Now as an example to illustrate BMI let’s say you’re a woman who is 5’5 and 155 lbs. So as not to have to convert this into metric, we used the second equation. Which would look like:
155 65 inches2x 703=BMI
155 4225 x 703= BMI
0.03668639 x 703 = 25.79 or rounded up BMI= 26
As you can see in this case, this woman would be considered overweight by BMI standards. However, if this woman lifted weights, was an athlete, or lives an active lifestyle BMI may not be the best predictor of chronic disease risk or assessment of leanness. In which case she might consider getting her body composition calculated.
To illustrate the difference between body fat and BMI let’s look at Arnold Schwarzenegger when he was competing for Mr. Olympia. Here we would find his BMI to be 30.1 (indicating he is obese by BMI standards). However, when his body fat was measured we found his body fat to be around 7% placing him in the athlete category (listed below). If you're sedentary or primarily sedentary BMI may be a good indicator, but if you're active you'd likely get a more accurate assessment by having your body composition calculated.
The ranges for body composition are broken down by age and gender. They are expressed as a percentage of body fat and are as follows:
Clark, M., Sutton B.G., & Lucett, S., NASM Essentials of Personal Fitness Training. Burlington, MA: Jones & Bartlett Learning, 2014. Print
Hatfield, H.C., Fitness: The Complete Guide. International Sports Sciences Association, 2013. Print