By Joe Cannon, MS, CSCS, NSCA-CPT
Today’s Dietitian
Vol. 13 No. 5 P. 66
“Get your body fat tested and get a free personal training session.” I recently saw this posting at my gym. Measuring body fat is a common fitness test at health clubs, which may offer this service to entice people to sign up for personal training. Gyms measure body fat in myriad ways, and this article will cover five of them.
• Hydrostatic weighing: Because it’s based on the Archimedes principle of fluid displacement, this method is often called underwater weighing. The American College of Sports Medicine (ACSM) has called it the gold standard because of its high accuracy. People are submerged under water and weighed. Two weights (water and land weight) are then entered into equations to generate body fat percentage. Many health clubs lack the equipment needed to conduct underwater weighing, making it unlikely that your clients will be measured in this way. Hydrostatic weighing may also be intimidating, since people need to be in a bathing suit, be completely submerged, and exhale as much air as possible.
• BOD POD: The BOD POD works on a similar principle as underwater weighing, except it measures air displacement. Several studies have noted that the BOD POD’s accuracy is similar to that of underwater weighing.1,2 Measurement with the BOD POD takes seconds. Perhaps the only downside to this technique is its high cost, which sets it beyond many health clubs’ budgets.
• Bioelectric impedance analysis: This may be the most popular method used in health clubs today. According to the ACSM, bioelectric impedance analysis (BIA) works by passing a low-intensity electric current through the body and measuring its resistance. The faster the current travels from one lead to another, the more muscle and less fat the person has. According to the ACSM, prior to the test, people should not eat or drink for at least 30 minutes, exercise for at least 12 hours, drink alcohol for at least 48 hours, or ingest any diuretics (including caffeine) unless prescribed by a physician. They should urinate 30 minutes before the test. People should follow these rules to generate the truest estimate.
While the department of kinesiology and health at Georgia State University notes that the accuracy of BIA can be ±3% from the “real” body fat, I find this statement to be overly complimentary of this method. I have witnessed gross overestimations of body fat in people who have used BIA in health clubs.
I would caution women of childbearing age to have the test done at the same time each month, as inconsistency may alter accuracy. Pregnant women and those with implantable pacemakers or defibrillators should avoid BIA.
• Skin fold analysis: In skin fold analysis, special calipers pinch different parts of the body. This method is made possible because a relationship exists between subcutaneous fat and the total amount of body fat.3 Thus, you can estimate total body fat by measuring the fat under the skin. While this method can yield good results when done properly, it has several drawbacks. For example, this method does not measure visceral fat. Also, different people pinch differently, which can alter accuracy. Having the same person do the test may minimize this error. It’s unlikely your clients will have their body fat measured in this way. Big-chain health clubs often do not use this method because people may feel uncomfortable being touched by strangers, and touching perceived as inappropriate can lead to lawsuits.
• Near-infrared interactance: In near-infrared interactance (NIR), a specialized probe is placed against an area of the body, emitting infrared light that passes through muscle and fat. The NIR machine then uses this information along with age and activity level to estimate body composition. While variations of this technique have been used in clinical settings since the 1960s, portable devices that are commercially available have been shown to be less accurate than other methods.4 Some research suggests that NIR might overestimate body fatness in lean people and underestimate it in people who are overweight.5 Most health clubs do not use NIR for this reason.
— Joe Cannon, MS, CSCS, NSCA-CPT, is a personal trainer, exercise physiologist, and health educator in the Philadelphia suburbs.
References
1. Maddalozzo GF, Cardinal BJ, Snow CA. Concurrent validity of the BOD POD and dual energy x-ray absorptiometry techniques for assessing body composition in young women. J Am Diet Assoc. 2002;102(11):1677-1679.
2. Utter AC, Goss FL, Swan PD, Harris GS, Robertson RJ, Trone GA. Evaluation of air displacement for assessing body composition of collegiate wrestlers. Med Sci Sports Exerc. 2003;35(3):500-505.
3. Heyward VH. Advanced Fitness Assessment & Exercise Prescription. 2nd ed. Champagne, Ill.: Human Kinetics; 1991.
4. McArdle WD, Katch FI, Katch VL. Essentials of Exercise Physiology. 3rd ed. Philadelphia: Lippincott Williams & Wilkins; 2006.
5. National Heart, Lung, and Blood Institute. The Practical Guide: Identification, Evaluation, and Treatment of Overweight and Obesity in Adults. Washington, D.C.: U.S. Department of Health and Human Services National Institutes of Health; 2000. NIH Publication No. 00-4084. Available at: http://www.nhlbi.nih.gov/guidelines/obesity/prctgd_c.pdf
