Dilution of body fluid electrolytes affects bioelectrical impedance measurements

Abstract

This study was designed to assess how acute perturbation in electrolyte concentration of body fluids affects the bioelectrical impedance analysis (BIA) measurement of resistance (R) and derived total body water (TBW) in humans. After an initial measurement of R, subjects drank either water (group W: n = 8, 1211 ± 119 ml) or a hypotonic rehydration beverage (group H: n = 8, 1211 ± 118 ml, 18 mEq/L) or an isotonic solution composed of the rehydration beverage supplemented with 209 mEq/L sodium chloride (group I: n = 8, 1196 ± 118 ml, 308 mEq/L) to produce a 3% increase in TBW. R was measured approximately 3 to 5 minutes following fluid intake, and three additional times at 30‐minute intervals. In all groups, R increased significantly immediately following fluid intake (p ≤0.05). However, the subsequent changes in R were dependent on beverage electrolyte concentration. Ninety minutes after fluid ingestion, the mean R for groups W and H remained elevated, whereas R for group I had decreased significantly (p <0.05). Although the volume of fluid intake was similar among groups (approximately 1200 ml), the BIA‐estimated TBW decreased by ‐375 ± 65 ml and ‐88 ± 122 ml for groups W and H, respectively, whereas the TBW increased 348 ± 123 ml for group I; all were significantly different (p = 0.0001) from the expected gain of 1200 ml. Furthermore, the change in estimated TBW was strongly correlated (r = 0.833) with electrolyte content in the drink. These findings indicate that acute perturbations in the electrolyte concentration of body fluids produced by ingesting drinks of varying electrolyte content adversely affects the BIA estimation of TBW. Fluid intake producing a 3% gain in TBW resulted in a 2 to 4% underestimation of TBW with the BIA technique.