Ask The Experts: How Should I Measure My Body Fat Percentage?

Q.

Dear Experts,

I really enjoyed Matt’s article on optimal racing body weight.  As a multisport athlete, I’m very concerned about getting an accurate picture of my body fat percentage. I currently have a body fat scale (with hand-held attachment) that measures the percentage via bioelectrical impedance. However, I suspect it is inaccurate by more than a few percentage points. I’ve heard of body fat scales calibrated specially for athletes—what is the technology behind this, and is it worth the investment? Should I switch to calipers instead (and what is the correct method to using them)? Other than hydrostatic body fat testing, what is the next best method you suggest I use?

Max

A.

Dear Max,

Body fat testing is an inexact science. The only way to directly measure body fat is through autopsy, which I don’t recommend. Even the so-called “gold standard” method of measuring body fat, hydrostatic weighing, is very indirect. In this case, what is actually being measured is the relationship between body volume and body density. A formula based on the known difference in density between feet tissue and lean body tissue is used to convert this measurement into an estimate of body fat percentage. As you might imagine, this formula contains a bit of guesswork. One consequence of this is that the relationship between the body volume/body mass ratio and body fat percentage differs by ethnicity. So the best formula for Caucasians is not the same as the best formula for African-Americans, and neither is 100 percent accurate for its designated ethnic group.

Bioelectrical impedance is really no more indirect a way to measure body fat percentage than hydrostatic weighing. In this case, a weak electrical current is sent through the body and the degree to which the body tissues resist, or impede, that current is measured. A formula based on the known difference between the electrical impedance of body fat tissue and lean body tissue is used to convert this measurement to an estimated body fat percentage. Studies comparing bioelectrical impedance and hydrostatic weighing have generally found a fairly high degree of agreement between them. Since the bioelectrical impedance method is much cheaper and easier to use, I consider it a good alternative to hydrostatic weighing. Although not perfectly accurate, it is consistent within individuals, making it suitable for tracking individual fluctuations in body fat percentage, if not for body fat contests with your friends.

The “athlete mode” available on some body fat scales that use bioelectrical impedance does not involve a different form of measurement. Rather, it involves a different conversion formula much like the alternative formulas used for different ethnicities in hydrostatic weighing. All highly active individuals should purchase units with an athlete mode.

I don’t recommend calipers because they are a pain in the butt to use and require training to use properly. While they are accurate when used correctly, doing so is not that easy, and even when they are used correctly they are no more accurate than the much simpler bioelectrical impedance method.

If you’re really concerned about accuracy, dual x-ray absorptiometry (or DEXA scanning) is the way to go. DEXA scanning offers the closest thing to a direct body fat measurement, as it entails looking inside the body in a way that clearly distinguishes fat, bone, and muscle tissue. DEXA scanning is best known as the tool physicians use to measure bone density, but it is in fact also now recognized as the most accurate way to measure body composition. Be forewarned, though: body fat percentage measurements obtained through DEXA scanning are almost always greater than those arrived at through other methods.

Most DEXA scanners are in hospitals. Many physicians provide DEXA scanning services as part of a routine physical or bone health check-up. But a growing number of facilities are offering DEXA scanning services specifically for individuals interested in tracking their body composition.

Matt

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