The effect of volume ingested on rehydration and gastric emptying following exercise-induced dehydration

The purpose of this study was to examine the effects of different drink volumes on rehydration, gastric emptying, and markers of fluid balance following exercise-induced dehydration. Nine male subjects (27.3 ± 5.47 yr of age, 77.8 ± 7.9 kg) exercised for 90 min (or until 2.5% of initial body weight was lost) on a cycle ergometer in a hot environment (30°C with 60% RH). Following exercise, subjects were moved to a neutral environment (23°C 50% RH) and rested for 30 min prior to beginning a 3-h rehydration period. During rehydration, subjects were serially fed with an electrolyte solution (14.98 mmol-l-1 Na+, 13.51 mmol·l-1 Cl, and 7.95 mmol·l-1 K+) every 30 min with either 100% or 150% of the fluid lost during exercise. Gastric contents were determined every 15 min using double sampling. Blood samples, urine samples, and body weights were taken before and after exercise and at 1-h intervals throughout rehydration. Blood samples were analyzed for percent change in plasma volume, electrolyte concentration, aldosterone levels, and renin activity. Urine electrolyte concentrations were also measured. The final percent rehydration was 48.11 and 67.90 for the 100% and 150% conditions, respectively. During rehydration, the subjects emptied 98.9 and 86.0% of the fluid ingested, and the % emptied and used for weight gain at the end of rehydration was 55.1 and 54.6 for the 100% and 150% trials, respectively. Urine production was significantly higher in the 150 compared with the 100% condition while renin and aldosterone levels did not differ significantly. These results suggest gastric emptying did not represent a limiting factor in rehydration. Furthermore, although there was a greater % rehydration observed in the 150% condition, this was attained via relatively large gastric fluid volumes, with concomitantly large urine volumes; thus, forced fluid intake with a dilute, low sodium rehydration solution was not an effective method of rapid rehydration and may be detrimental to subsequent performance. Additionally, the large urine productions observed in the last hour of rehydration for both trials indicate that fluid retention hormones were not maintained at an adequate level to stimulate rapid rehydration in a short amount of time following dehydration.