Bioelectrical Impedance Analysis Does Not Detect an Increase in Total Body Water Following Isotonic Fluid Consumption

To determine if single-frequency foot-to-foot bioelectrical impedance analysis (BIA) can detect acute changes in total body water (TBW) following consumption of isotonic saline. All participants ate a sodium-free meal at 4 h prior to the data collection visit and had euhydration confirmed using urine specific gravity at the beginning of the experimental visit. Subjects drank 466 mL of isotonic saline (Na 140 mmol·L) following baseline measures. Blood sampling and BIA were performed at baseline and every 30 min for 3 h after saline consumption. Ten healthy participants completed this study. Plasma volume (5%Δ, < 0.001) and serum sodium concentration (1%Δ, < 0.001) increased by 60 min and 90 min, respectively. Body mass ( < 0.001) displayed a biphasic response increasing to a peak at 30 min (+0.38Δkg) and then decreasing to its minimum at 180 min (-0.35Δkg). BIA impedance ( = 0.678) was unaffected by the saline administration. BIA-derived TBW ( = 0.039) decreased from baseline starting at 150 min (0.21Δkg). Athletes and coaches wishing to achieve hyperhydration can do so through the consumption of isotonic fluid. 50 kHz foot-to-foot BIA-derived TBW is inadequate for measuring hyperhydration. Future studies should examine the physiological and performance effects of such a hyperhydration protocol.