Use of bioelectrical impedance analysis to estimate body fluid compartments after acute variations of the body hydration level

Physiological measurements including body mass, plasma osmolality, natremia , plasma volume measured by Evans Blue dilution, and total body water (TBW) and extracellular water (ECW) volumes estimated by bioelectrical impedance analysis (BIA) were recorded in eight healthy young Caucasian subjects bef ore and after acute variations of their body hydration state on four separa te occasions: 1) euhydration or control trial (C); 2) heat-induced dehydrat ion of 2.8% body mass (D); 3) exercise-induced dehydration of 2.8% body mas s (E); and 4) glycerol-hyperhydration (H). Heart rate, rectal and mean skin temperatures were also recorded throughout the experiment. The main result of the study is that BIA only half predicted the body water loss after exe rcise, although conditions were standardized (electrode placement, side of the body, limb position, posture, and ambient temperature). Differences in body temperatures cannot explain such an unexpected result, nor did the stu dy of plasma osmolality and sodium concentration. If BIA appears to adequat ely predict changes in TBW after heat-induced dehydration and glycerol hype rhydration, further studies including measures of TBW and ECW by dilution t racer methods would be necessary to establish the validity of using the BIA method to measure such changes and to interpret ECW variations.