Thursday, March 24, 2011

The Mystery and Simplicity of Body Mass Index (BMI)

BMI is the common term given to Body Mass Index, a number calculated from your weight and height that roughly correlates to the percentage of your total weight that comes from fat, as opposed to muscle or bone. The higher your BMI, the higher the percentage of fat in your body. 
It comes down to this:
If your BMI is under 20, you might be underweight. 
Between 20 and 25, you are probably at a good healthy weight for your height. 
BMI over 25 is considered overweight, and over 35 is considered obese.

There is a simple tool to calculate a rough translations of true percentage of BMI, and a number of factors that might influence whether or not your BMI is a true reflection of your total body fat.

The calculation is pretty standard. Here is the simplified version from the Center for Disease Control:
(cut and paste into your browser)

Now REMEMBER muscle is denser than fat and takes up less space. Therefore, a heavily muscled person might weigh more than a same sized over-weight person, or two individuals with identical BMI might have widely different percent body fat. 
In this case, calculating your percent body-fat might require more sophisticated equipment, such as an immersion tank. (Under water. Yes, I did that. Not fun. Thought I was drowning.) Since fat is more buoyant than muscle, two same-weight individuals will not float at the same level if they have different percentages of body fat.

Women typically carry more fat (subcutaneous) than men do, particularly in the breast and hips, so their percent body fat may be higher without it necessarily being reflected in their BMI or having any adverse health effects. Very low body fat, which may or may not show up in a BMI, depending on the individual's musculature, might be unhealthy as well. Your body needs some stores of fat to draw upon for energy and if fat is absent, the body will begin to consume muscle mass to keep itself going.

Calculate YOUR BMI, and let's start there...

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