REHYDRATION FOLLOWING EXERCISE - EFFECTS OF ADMINISTRATION OF WATER VERSUS AN ISOTONIC ORAL REHYDRATION SOLUTION (ORS)

The effects of administering (1) 6L isotonic oral rehydration solution (ORS), similar in composition to plasma (except for an elevated potas sium concentration) and with an osmotic skeleton and (2) GL water (no osmotic skeleton),were evaluated in five thoroughbred horses following exercise-induced dehydration. The horses were exercised on a treadmil l for 10min at walk (1.7m.s(-1); similar to 15% VO2max), 40min at trot (3.7m.s(-1); similar to 25% VO2max) and 10min at walk (1.7m.s(-1); si milar to 15% VO2max). Exercise was undertaken on a 3 degrees incline a t 30 degrees C/ 80% RH. Solutions of water or ORS at 20 degrees C were administered by nasogastric tube over 60 s 5 min following exercise. Mean weight loss following exercise was 9.2+/-1.7kg (2.0+/-0.4% body w eight; mean+/-SEM) with water and 9.2+/-1.1kg (2.0+/-0.2% body weight) with ORS and was not different between treatments (P>0.05). Water tre atment resulted in a fall in plasma [Na+] (similar to 3mmol.L-1) and C l- (1-2mmol.L-1) concentrations by 30min after administration and the effect persisted until the end of the study (300min post fluids). Ther e was little change in plasma total protein (TP) from that at the end of exercise, suggesting a failure of water to restore or maintain PV. In contrast, ORS administration resulted in a small increase in plasma [Na+] (1-2mmol.L-l) and [Cl-] (2-3 mmol.L-1) with a corresponding dec rease in plasma TP. By 120min post ORS, plasma TP and PV were no longe r significantly different from rest or pre-exercise (P>0.05), whilst w ith water, TP was elevated (similar to 3-4g.L-1) and PV reduced (simil ar to 4-5 mL.kg(-1)). Total urine output was not significantly differe nt between water (1096+/-135mL) and ORS (750+/-215mL, P>0.05). Estimat es of expected plasma volume and electrolyte concentration changes as a result of either treatment compared well with measured changes of TP and PV. On the basis of calculated or measured changes, it was estima ted that only 1L of water contributed to rehydration following exercis e compared to 4L isotonic, plasma-like ORS. The administration of 6L O RS restored the PV deficit induced by exercise with minimal or no dist urbance of plasma electrolyte concentrations. In contrast, water alone resulted in minimal improvement in PV. When fluid intake after period s of fluid loss, such as induced by exercise or transport, is not acco mpanied by food intake, the present study has clearly demonstrated tha t water alone is ineffective in promoting rehydration compared with an isotonic, plasma-like ORS.