HYPERVOLEMIA IN MEN FROM FLUID INGESTION AT REST AND DURING EXERCISE

Background: Plasma osmolality (Osm) is important for controlling and m aintaining plasma volume (PV) and body water. The effect of oral rehyd ration fluids for ameliorating dehydration is well-established; but op timal compostion and Osm of fluids for hyperhydrating normally hy drat ed subjects is less clear. Methods: Six treatments were used without a nd with oral fluids of varying ionic and constituent concentrations fo r hyperhydrating six previously euhydrated men (30 +/- SD 8 yr, 76.84 +/- 16.19 kg, 73 +/- 12 ml.kg(-1) PV, 40 +/- 10 ml.min(-1).kg(-1) peak (V) over dot O-2) sitting at rest for 90 min ((V) over dot O-2 = 0.39 +/- SE 0.02 L.min(-1)) and during subsequent 70 min of submaximal exe rcise ((V) over dot O-2 = 2.08 +/- SE 0.33 L.min(-1), 70 +/- 7% peak ( V) over dot O-2). The hypothesis was that the fluid composition is mor e important than plasma Osm for increasing PV in euhydrated subjects a t rest and maintaining it during exercise. Drink formulation compositi ons, given at 10 ml.kg(-1) body wt, ((X) over bar = 768 ml), for the s itting period were: Performance 1 (P1; 55 mEq Na+, 365 mOsm.kg H2O-1), P2 (97 mEq Na+, 791 mOsm.kg(-1)), P2G (113 mEq Na+, 4% glycerol, 1382 mOsm.kg(-1)), AstroAde (AA; 164 mEq Na+, 253 mOsm.kg(-1)), and 01 and 02 (no drinking). The exercise drink (10 ml.kg(-1), 768 ml) was P1 fo r all treatments except 02 (no drinking); thus, drink designations wer e: P1/P1, P2/P1, P2G/P1, AA/P1, 0/P1, and 0/0. Results: PV at rest inc reased (p < 0.05) by 4.7% with P1 and by 7.9% with AA. Percent change in PV during exercise was +1% to +3% (NS) with AA/P1; -6% to 0% (NS) w ith P1/P1, P2/P1, P2G/P1, and 0/P1; and -8% to -5% (p < 0.05) with 0/0 . AA, with the lowest Osm of 253 mOsm.kg(-1), increased PV at rest (as did P1) and maintained it during exercise, whereas the other drinks w ith lower Na+ and higher Osm of 365-1382 mOsm.kg(-1) did not. Conclusi on: Drink composition appears to be more important than its Osm for in creasing PV at rest and for maintaining it during exercise in previous ly euhydrated subjects.