PLASMA-VOLUME EXPANSION WITH ORAL FLUIDS IN HYPOHYDRATED MEN AT REST AND DURING EXERCISE

Background: The purpose for this study was to evaluate various carbohy drate (CHO)-electrolyte fluid formulations for consumption by astronau ts to maintain or restore their plasma volume (PV! and total body wate r (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 c ontent would be more important than osmotic (Osm) content for increasi ng or maintaining PV during the RE and EE. Methods: In the RE, 5 healt hy men (23-44 yr), previously dehydrated for 24 h, drank 6 fluid formu lations (Water, 19.6 Na, 157 Na, 19.6 Na + glucose, and the prepared d rinks Performance(R) and Power)-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% p eak (V) over dot O-2). Results: Rest: Subjects who consumed formulatio ns With total Osm concentrations nearer the normal range (157 Na - 270 mOsm.kg(-1), Performance(R) with 19.6 mEq.L-1 Na - 380 mOsm, and to s ome extent Power with 23.5 mEq.L-1 Na - 390 mOsm) had the greater incr eases in PV; intake of drink 157 Na, with the largest Na content, indu ced the greatest hypervolemia of 7.6% (p < 0.05). The various addition al ions, in addition to 19.6 Na, probably contributed to the 4.6% (p < 0.05) hypervolemia with Performance(R). Water was not effective. Exer cise: Stabilization of PV between 15-70 min was not related to drink t otal CHO, Na or Osm content. Performance(R) and 157 Na were no more ef fective than 19.6 Na or 19.6 Na + glu for PV stabilization. Water was the least effective. Regulatory mechanisms controlling PV during exerc ise appear to be independent of oral fluid formulation Osm-electrolyte content. Conclusions: Drink cation (sodium) content is more important that its total osmotic content for increasing plasma volume at rest. Fluid formulations with greater hypervolemic action in resting subject s may not be as effective during exercise; therefore different formula tions for use during exercise appear to be necessary.