MANAGING YOUR WEIGHT: MEASURES OF BODY FAT

Hydrostatic Weighing TechniquesFrom a clinical perspective, the most accurate method of measuring body fat is through hydrostatic weighing techniques. This method measures the amount of water a person displaces when completely submerged. Because fat tissue has a lower density than muscle or bone tissue, a relatively accurate indication of actual body fat can be computed by comparing a person’s underwater and out-of-water weights. Although this method may be subject to errors, it is one of the most sophisticated techniques currently available.
Pinch and Skinfold MeasuresPerhaps the most commonly used method of determining body fat is the pinch test.Numerous studies have determined that the triceps area (located in the back of the upper arm) is one of the most reliable areas of the body for assessing the amount of fat in the subcutaneous (just under the surface) layer of the skin. In making this assessment, a person pinches a fold of skin just behind the triceps with the thumb and index finger. It is important to pinch only the fat layer and not the triceps muscle. After selecting a spot for measure, the person assesses the distance between the thumb and index finger. If the size of the pinch appears to be thicker than 1 inch, the person is generally considered over-fat.Another technique, the skinfold caliper test, resembles the pinch test but is much more accurate. In this procedure, a person pinches folds of skin at various points on the body with the thumb and index finger. This technique uses a specially calibrated instrument called a skinfold caliper to take a precise measurement of the fat layer. Besides the triceps area, the points most often used in these measurements are the biceps area (front of the arm), the subscapular area (upper back), and the iliac crest (hip). Once these data points are assessed, special formulas are employed to arrive at a combined prediction of total body fat.In the hands of trained technicians, the skinfold caliper test can be fairly accurate. If the person doing the test is inconsistent about the exact locations of the pinch or if there is difficulty in determining the difference between fat and muscle, the results may be inaccurate. In addition, the heavier a person is, the more prone this technique is to error. For obese people, difficulties in assessment are magnified because of problems in distinguishing between flaccid muscles and fat. Also, most currently available calipers do not expand far enough to obtain accurate measurements on the moderately obese (20 to 40 percent overweight) or the morbidly obese (more than 50 percent overweight). Additional errors in skinfold assessments may occur as a result of failure to account for certain age, sex, and ethnic differences in calibrations.
Girth and Circumference MeasuresAnother common method of body fat assessment is the use of girth and circumference measures. Diagnosticians use a measuring tape to take girth, or circumference, measurements at various body sites. These measurements are then converted into constants, and a formula is used to determine relative percentages of body fat. Although this technique is inexpensive, easy to use, and commonly performed, it is not as accurate as many of the other techniques listed here.
Soft-Tissue RoentgenogramA relatively new technique for determining body fat, the soft-tissue roentgenogram, involves injecting a radioactive substance into body and allowing this substance to penetrate muscle (lean) tissue so that distinctions between fat and lean tissue can be made by means of imaging.
Bioelectrical Impedance Analysis Another method of determining body fat levels, bioelectrical impedance analysis (BIA), involves sending a small electric current through the subject’s body. The amount of resistance to the current, along with the person’s age, sex, and other physical characteristics, is then fed into a computer that uses special formulas to determine the total amount of lean and fat tissue.
Total Body Electrical ConductivityOne of the newest (and most expensive) assessment techniques is total body electrical conductivity (TOBEC), which uses an electromagnetic force field to assess relative body fat. Although based on the same principle as impedance, this assessment requires much more elaborate, expensive equipment, and therefore is not practical for most people.Although all of these methods can be t they can also be inaccurate and even harmful unless the testers are skillful and well trained. Before agreeing to undergo any procedure, be sure you are aware of the expense, potential for accuracy, risks, and training of the tester.*7/277/5*