Body mass index is a common tool used to classify levels of obesity. Although it does not measure body fat directly, it is nevertheless a quick-and-easy method that generally correlates well with measures of body fat in the general populace. However, it is far less reliable when attempting to gauge the health of athletes and bodybuilders. BMI is good for setting goals and assessing health risks, so it should be used as a tool and not as a final arbiter.
Calculations
BMI is calculated by dividing weight in pounds by height in inches squared. This number is then multiplied by 703. For example, a 150-lb. person who is 65 inches tall would use this calculation: 150/65 squared x 703. Remember to square 65 first and then divide. In this example, 65 x 65 is equal to 4,225. Then you divide 150/4,225 and multiply by 703. The final calculation would equal 24.96. For adults of at least 20 years of age, BMI is measured on this scale: Below 18.5 is underweight, between 18.5 and 24.9 is normal, between 25 and 29.9 is overweight, and a BMI of at least 20 is considered obese.
Significance
BMI assumes that body weight scales precisely with levels of body fat. However, because it cannot gauge body fat percentage, some people may have a high BMI but very little fat. For example, highly trained athletes and bodybuilders have a high BMI because of increased musculature but generally low levels of body fatness. Bone minerals, body water and other fat-free mass also determine weight levels.
Tissue Density
Muscle density is approximately 1.06 g per cm cubed, determined by a study published in the journal "Metabolism." It is slightly more than the density of adipose tissue, which is about 0.9 g per cm cubed. The difference can be accounted for by the fact that muscle proteins and fibers are packed tightly together. Lipids, such as fat, tend to be light and flabby. Muscles, therefore, will actually raise your BMI more than fat will.
Body Fat Percentage
Body fat percentage is the more precise measurement of fat levels no matter who you are, but it is also more difficult to gauge. It can only be analyzed with a specific measurement designed to isolate fat from the surrounding tissue. These measurements can include: a bio-electrical impedance analysis, which uses the concept that lean tissue and water conduct electricity better than fat; skin-fold measurements with calipers; near-infrared reactance, which uses a fiber optic probe to emit an infrared light beam that is absorbed and reflected differently by muscle and fat; and hydrostatic densitometry, or underwater weighing, which is considered the best measurement of body fat percentage.
Considerations
BMI is not a completely irrelevant measurement. Most people with BMI in the obese range, including bodybuilders, will tend to have increased levels of body fat. But muscular individuals can probably take at least five points off the BMI or simply shift the scale upward; anyone with a BMI around 25, therefore, would be considered normal. It's also possible to use a new measurement known as body volume index. However, it requires a 3-D scanner to calculate body shape and type. It's inaccessible for everyday use.
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