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FAT CHANCE


Brian Rowley, MS

Do you live and die by your bodyfat percentage? Here's what you need to know about the flaws in the methods used to test it.












You're dieting and exercising, but the numbers on the scale aren't budging. And forget those height/weight tables based on body mass index, or BMI--they don't distinguish muscle from fat, and are therefore useless for us athletic types. So how can you tell whether your program is working? The key, experts say, is keeping track of how much bodyfat you've lost instead of relying on the bathroom scale. Yet so many ways to estimate bodyfat exist that even some scientists are confused. We weigh in with a few tips to make the most of skinfold-caliper testing, and explain the limitations of digital scales and handheld devices that provide a digital readout of your bodyfat percentage. We also give you the skinny on such increasingly popular high-tech options as the Bod Pod, DEXA and other methods available at hospitals and universities. Knowing the pluses and minuses of each approach can help you avoid the scams and find out what you're really made of.

Dual Energy X-Ray Absorptiometry
At present, the most precise technique--combined with minimum risk, non-invasiveness and relatively small cost--appears to be dual energy X-ray absorptiometry (DEXA). In fact, Alan Martin, PhD, professor of human kinetics at the University of British Columbia in Vancouver, British Columbia, Canada, recommends it for the general consumer, as a DEXA screen can cost as little as $60-$70 and, unlike densitometry, takes into account your bone mass. DEXA is available at many hospitals and health centers. It gives you a chemical breakdown of your body's contents--lipid (fat), bone mineral and so on, and uses these data to estimate how much fat vs. muscle you have.

Underwater Weighing Before DEXA and magnetic resonance imaging (MRI), the gold standard for body-composition testing was underwater weighing. The problem is, this doesn't measure bodyfat, only body density, which is why it's called densitometry. While the density of your body is usually a good measure of how much fat mass you have relative to lean body mass--fat floats, lean mass sinks--it isn't 100% reliable. Fat is just fat, but lean body mass is made up of both muscle and bone, with bone being denser than muscle.

That's fine if you're a non-elderly Caucasian person with a predictable amount of muscle relative to bone. But if you're older and have less bone weighing you down than expected due to osteoporosis, densitometry will overestimate your bodyfat percentage - you sink less than expected. The same happens to women who work out, though for a different reason. A study reported in the American Journal of Clinical Nutrition found it to be the method best able to detect gains in muscle due to bodybuilding training in a group of 20 women ages 50-70 years. Because some women athletes have a surprisingly large amount of muscle relative to bone, their "lean" isn't as dense as expected, and percentage bodyfat is exaggerated.

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