Reliability of fat-free mass estimates derived from total-body electrical conductivity measurements as influenced by changes in extracellular fluid volume.

Our study addresses the concern that the relationship between total-body electrical conductivity (TOBEC) measurements and the fat-free mass (FFM) or total-body water (TBW) of an individual is altered if significant fluid and electrolyte changes occur. Body composition and TOBEC measurements were obtained from 11 healthy miniature piglets before and after an intraperitoneal injection of physiological saline. The procedure expanded the extracellular fluid (ECF) volume by 11.8-34.1%, which represented an average increase in TBW of 7.9%. The linear regressions that related the preinjection and postinjection estimates of TBW or FFM to the corresponding transformed TOBEC signals were the same. Thus, the prediction equations derived for the untreated piglets accurately predicted both TBW and FFM in the piglets whose volume was expanded. These data suggest that prediction equations derived from healthy subjects should be equally valid for subjects with altered fluid status.