Body electrical impedance measurements: body fat or body fluid?

An electrical impedance measuring scale (Tanita Model TBF-551) was used to make repeated estimations of the body fat of an adult male subject undergoing dietary changes that resulted in a 4.7 kg weight increase. The results showed that the body fat decreased approximately 3 kg with the weight gain. Over a period of five days of normal diet and activity the body fat change returned to the control condition. It appeared the weight gain was caused mainly by fluid changes not a body fat increase. The results suggest serious errors can occur in body fat estimates based on body impedance measurements, but use as a fluid change indicator may be possible.