Purpose: To study hormonal factors that may account for the dissociation between beverage-induced plasma sodium p[Na+] and osmotic p[Osm] concentrations that appear to refute the high theoretical correlation between p[Na+] and p[Osm].
Methods: Ten men (24 +/- SD 3 yr of age) sat reclining (head up) for 12 h in a chamber (21-23 degrees C dry bulb, 25-33% relative humidity) at 2800 m (9184 ft, 539 mm Hg) altitude (ALT), and at 321 m (1053 ft, 732 mm Hg) on the ground (GND). During 1000-1030 hours they consumed 3 fluids (12 ml x kg(-1),X = 948 ml x d(-1)) with large differences in sodium and osmotic contents: AstroAde (AA) with 185 mEq x L(-1) Na+ and 283 mOsm x kg(-1), Performance 1 (Shaklee) (P1) with 22 mEq x L(-1) Na+ and 365 mOsm kg(-1), or H2O at ALT; and only H2O on the GND.
Results: After drinking: plasma volume (PV) increased at 1200 hours by 8.3% (p < 0.05) with AA but was not significantly (NS) changed in the other sessions (Xdelta = +0.9%, range -0.9 to 2.8%); p[Na+] and p[Osm] were unchanged. Urinary rates and free-water clearances were attenuated with AA and P1 vs. those with H2O. Correlations between and among p[Na+] and p[Osm] suggest that the pNa+ ion is more tightly controlled than pOsm; and that there was no clear hormonal response that could account for this dissociation from theoretical considerations.
Conclusions: There is significant dissociation between plasma sodium and osmotic concentrations after fluid intake. Induction and maintenance of hypervolemia requires increased (near isotonic) drink Na+ osmols rather than increased non-ionic osmols.