Measuring Body Fat

Continued...

Pros and cons of various methods

Body fat content is measured indirectly through a variety of techniques, each with a built-in range of error. The sheer number of measurement techniques is an indication that no single method is best.

Underwater weighing is the most accurate method, and the one most often used in research studies on body composition. Because it must be performed in a lab, however, it is relatively costly (about $50 in the Los Angeles area, for example) and not as widely available as other methods.

Based on the principle that fat has greater buoyancy than other tissue, and thus weighs comparatively less under water; testing involves sitting in a chair submerged in water and suspended from a scale. After fully exhaling, you bend forward until you are completely submerged. An observer records your under-water weight while you remain motionless for five to 15 seconds.

Scientists repeat the procedure 10 to 20 times and use an average of the three highest weights to calculate your percentage of body fat. Even in the hands of the best researchers, underwater weighing has an error range of plus or minus 2 to 3.5 percent.

Skin-fold measurement, like underwater weighing, has been used for more than 20 years. The method is based on the hypothesis that the amount of subcutaneous fat is related to total body fat.

Skin and subcutaneous fat are pinched between calipers and the thickness is measured. Measurements are taken three times at each of five sites (waist, thigh, triceps, abdomen and upper back). Equations use the average of these measurements to estimate total bodyfat percentage.

Equations for this method are more accurate for some populations than for others, based on age, sex, body type and fitness level. The accuracy of this technique is also very dependent on the experience and skill of the measurer.

What else causes errors in body fat measurements?

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