Body composition refers to the relative percentage of muscle, fat, bone, and other tissue of the body. However, body composition is mostly talked about in terms of fat percentage. There are several ways to measure body fatness, and the methods of measurement can be classified as direct or indirect. Direct measures of body fat are the best measures because they directly measure body fatness. Indirect measures are estimates of body fatness.
Two examples of direct measures are DEXA (dual-energy X-ray absorptiometry) and underwater weighing. DEXA is the most accurate and advanced test available for body fatness, though it was developed for measuring bone mineral density. Because DEXA can distinguish between fat, bone, and muscle (a three-compartment model), it provides the most precise measure of body fatness.
Before DEXA, underwater weighing was considered the best measure of body fatness. Underwater weighing requires participants to sit on a seat suspended by a large scale while in water. Participants then exhale all the air in their lungs and hold their breath as they sink. Because fat is less dense than water, fat floats, and the resulting weight that is measured by the scale is the person’s underwater weight. The underwater weight is subtracted from the weight of the person on land, and the difference is considered the weight of the fat. This weight is then divided into the total body weight to determine the percentage of the person’s weight that is fat, or percent fat. See the calculations below for an example using a 150-pound (68 kg) male:
Weight on land: 150 pounds (about 68 kg)
Weight in water: 125 pounds (about 56.7 kg)
Difference between land and water weight (weight of fat):
150 pounds - 125 pounds = 25 pounds of fat
(in metric, 68 kg - 56.7 kg = 11.3 kg of fat).
Percentage of body weight that is fat (% fat):
25 pounds ÷ 150 pounds x 100 = 16.6% body fat
(in metric, 11.3 kg ÷ 68 kg x 100 = 16.6%).
While DEXA is a three-compartment model (fat, muscle, and bone), underwater weighing is a two-compartment model (fat and everything else). Underwater weighing employs Archimedes’ principle of buoyancy. Surfers use this principle, too.
DEXA and underwater weighing are expensive and time-consuming methods of assessing body fatness compared to skinfold measurements. The skinfold method is an indirect measure of body fatness. Skinfold measures are taken with an instrument called calipers. Skinfold calipers squeeze a fold of skin with constant pressure and measure the thickness of the fold.
Skinfolds actually measure the thickness of subcutaneous (under the skin) fat from selected sites on a person’s body, and then estimate body fatness from these measures. It’s important to remember that the measure is just an estimation. When estimating a person’s percentage of fat, skinfold measures must make assumptions about body fatness, such as these:
- Subcutaneous fat (fat under the skin) is representative of whole-body fatness.
- Fat is uniformly distributed throughout the body.
Another indirect measure of body fatness is bioelectrical impedance. Don’t place too much emphasis on the exact number that you get when measuring percent fat using indirect measures. Rather, take repeated measurements under similar conditions (for example, the same day of the week, the same time of day, or the same time after a meal), and look at the changes in percent fat to monitor your progress.
After all this talk of body fat, you might be wondering what percentage of body fat is considered healthy.
