site to track your body composition

You can purchase (for roughly fifty bucks) a scale that measures body weight and calculates body fat percentage. Go to amazon.com and search under the name "tanita". The body fat measurement is taken in the following way. While you stand on the scale, your feet are touching four electrodes. A tiny (imperceptible) electric current is sent through your body. Since lean tissue conducts electricity better than fatty tissue, the resulting impedance measurement reflects fat percentage. Having said all this the quality of the mathematical model is probably geared toward the general population, and will be less accurate for very athletic people. I say this because on my home machine my body fat is 14%. But on a hand held version at the gym I register at 6%. (And I'm not bottom-heavy, if that's what you're thinking). Thus either of these devices is better used for measuring trends, rather than the absolute amount; for instance, this could be useful if you're trying to lose weight and want to monitor lean tissue loss.

The Tanita scale I have has a "sports" setting, along with a "general" setting. It is about a 9% difference in the reading (which matches what exrunner saw). The company reports, in general, what can affect the body fat reading, but I wish they included more details. I can see large shifts depending on the exercise I've done that day (running or tae bo, versus swimming). The impression I have, both from using it and from reading other people's accounts, is that you have to take the absolute value with a grain of salt. But it is good for monitoring changes (are you getting fatter or leaner).