SODIUM-CHLORIDE CITRATE BEVERAGES ATTENUATE HYPOVOLEMIA IN MEN RESTING 12 H AT 2800 M ALTITUDE

Background: The mechanism for reduction and restoration of total body water and plasma volume (PV) during initial exposure to acute altitude (ALT) is not clear but may involve involuntary dehydration; i.e., del ayed voluntary fluid intake. Method: Ten men (24 +/- SD 3 yr, 180.8 +/ - 8.1 cm height, 78.8 +/- 12.8 kg weight, 1.99 +/- 0.19 m(2) surface a rea, and 12.2 +/- 4.0% body fat) were in a semi-reclining position for 12 h in a chamber at 2800 m (539 mmHg) ALT or at 321 m (732 mmHg; gro und). They ate a controlled breakfast (450 kcal + 3 ml . kg(-1) H2O) o n the ground, and lunch and dinner at ALT (or on the ground) for a tot al daily intake of 2850 kcal (14% PRO, 67% CHO, 16% fat, 2.6g NaCl). A t hour 10 they consumed fluid-electrolyte beverages or water (72 ml . kg(-1), 948 ml . d(-1)) in 4 sessions at weekly intervals. Beverage co mpositions were: a) 185 mEq . L-1 Na+, 283 mOsm . kg(-1) b) 21.6 mEq . L-1 Na+, 365 mOsm . kg(-1); c) water at ALT; and d) water on the grou nd. Results: After 10 h at ALT % Delta PV (Hb-Hct] decreased (p < 0.05 ) by: a) 9.0 +/- SE 1.5%; b) 6.2 +/- 1.7%; c) 7.4 +/- 2.2%; and d) by 9.0 +/- 2.4%, respectively. After drinking hom 1000-1030 h, PV at 1200 h changed by: a) +8.3 +/- SE 2.0% (p < 0.05); b) +2.8 +/- 2.7% (NS); c) -0.9 +/- 1.5% (NS); and d) by +0.8 +/- 3.5% (NS), respectively. The similar ground-induced hypovolemia suggests a response to confinement rather than an ALT effect and involuntary dehydration does not appear to be implicated. Conclusion: The significant increase in PV after co nsuming the (a) NaCl-NaCitrate beverage indicates that drink ionic com position appears to be more important than its osmolality for restorin g PV in these conditions. Practical considerations: Because this hypov olemia was probably due to the confinement rather that reduced ambient pressure, appropriate countermeasures could be consumption of isotoni c beverages, elastic stockings, leg exercise, and leg elevation.