Greenleaf JE, Jackson CGR, Geelen G, Keil LC,
Hinghofer-Szalkay H, Whittam JH

Plasma volume expansion with oral fluids in hypohydrated men at rest and during exercise

Aviat Space Environ Med. 1998; 69:837-44

Background: The purpose for this study was to evaluate various carbohydrate (CHO)-electrolyte fluid formulations for consumption by astronauts to maintain or restore their plasma volume (PV) and total body water (TBW) during and after extravehicular activity (exercise experiment, EE) and for a few hours before reentry and immediately after landing (rest experiment RE).

Hypothesis: That fluid formulation electrolyte content would be more important than osmotic (Osm) content for increasing or maintaining PV during the RE and EE.

Methods: In the RE, 5 healthy men (23–44 yr), previously dehydrated for 24 h, drank 8 fluid formulations (Water, 19.6 Na, 157 Na, 19.6 Na + glucose, and the prepared drinks Performance® with 19.6 mEq/L Na– 380 mOsm, and Power with 23.5 mEq/L Na – 390 mOsm) - one each at weekly intervals, and then sat for 70 min. In the EE, four healthy 24-h dehydrated men (30-46 yr) exercised for 70 min supine on a cycle ergometer (load = 71±1% peak Vo2).

Results: During rest, the greatest hypervolemia (+7.6%) occurred with the near isotonic sodium chloride - sodium citrate formulation with the largest Na content, followed by Performance®. Water was not effective. Exercise: Stabilization of PV between 15-70 min was not related to drink total CHO, Na or Osm content. Performance® and 157 Na were no more effective than 19.6 Na or 19.6 Na + glu for PV stabilization. Water was the least effective. During exercise, stabilization and attenuation of the hypovolemia was not related to drink total carbohydrate sodium, or osmotic contents.

Conclusion: Regulatory mechanisms controlling PV during exercise appear to be independent of oral fluid formulation Osm-electrolyte content. Drink cation (sodium) content is more important that its total osmotic content for increasing plasma volume al rest.

Practical considerations: Rest (sitting) that may be applicable for astronauts prior to and following reentry from spaceflight, and moderately heavy (71% peak Vo2) exercise in the supine position simulating astronauts undergoing extravehicular activity. Fluid formulations with greater hypervolemic action in resting subjects may not be as effective during exercise; therefore different formulations for use during exercise appear to be necessary.