Measuring Body Fat

Continued...

Errors are caused by different states of hydration of the subject, using the wrong equations for the type of calipers, and the difficulty of finding and measuring exactly the same site on the same person at different times. Even in the best hands (someone who has tested more than 200 people) the error range is still plus or minus 2 to 5 percent.

In an attempt to reduce human error; new measuring devices have been developed and are being aggressively marketed.

Bioclectrical impedance analysis (BIA) uses an electrical current to estimate body fat. Four electrodes are placed on the skin and a current is passed between them. The impedance (opposition) to this current flow is measured; a built-in computer calculates body-fat percentage from this measurement. BIA is based on the principle that fat provides more impedance than other tissue, because it contains less water (water being a conductor of electricity).

Conditions must be tightly controlled to assure consistent results. Differences in skin temperature, blood flow, hydration and placement of electrodes will yield inconsistent results. if, for example, one day you're tested in a warm room after you've been exercising, and the next day you're tested in a cool room after you've been resting, the results will be considerably different.

The reliability of your test will also depend on the type of machine (there are currently three on the market) and which of two equations is used to calculate a percentage. The combination that's right for you is based on your body configuration, age and sex. The overall error range for BIA testing is plus or minus 3 to 3.5 percent.

Near-infrared spectrophotometry and ultrasound use infrared light or ultrasound rays to estimate bodyfat percentage. The device that emits the rays is placed on a body part, usually the arm.

Ultrasound measurements vary greatly with the amount of pressure exerted on the skin. Infrared measurements eliminate this source of error; but both methods use equations with built-in assumptions that may be inaccurate for your particular body.

How important is all this?

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Foreword: Billie Jean King

Comments by Barb Harris
Editor in Chief,
Shape Magazine

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