THE INFLUENCE OF VOLUME ON GASTRIC-EMPTYING AND FLUID BALANCE DURING PROLONGED EXERCISE

The purpose of this study was to determine the effect of ingesting approximately 800, 1200, or 1600 ml.h-1 of a carbohydrate (CHO) solution on gastric emptying (GE), ratings of stomach fullness (RF), plasma volume changes (PVC), electrolytes, fluid loss, and blood glucose (BG) during 2 h of cycling at 70% of VO2max. Eight male cyclists completed three rides during which they ingested either 11.5 (LV), 17.1 (MV), or 23 ml.kg-1.h-1 (HV) of a 7.5% CHO solution every 15 min. Blood samples were taken at 0, 60, 90, and 120 min and 15 min after the ride for the determination of PVC, sodium (Na+), potassium (K+), and BG. Body weight was recorded before and after the rides to determine dehydration. Rates of GE were determined via stomach aspiration after the rides. A significantly (P < 0.05) greater volume of fluid was emptied in the HV trial (2268 +/- 70.8 ml) compared with the MV and LV trials (1860 +/- 72 and 1452 +/- 24 ml, respectively). The rate of GE in the MV trial was also significantly greater than in the LV trial. RF were significantly higher in the MV and HV trials compared with the LV trial throughout the rides. There were no significant differences in PVC (0.01 +/- 2.2%, 1.05 +/- 1.62%, and 0.97 +/- 1.28% for LV, MV, and HV, respectively), Na+, K+, or BG between the three trials. There was a significantly greater decrease in body weight in the LV (-0.77 +/- 0.14 kg) compared with the MV and HV trials (-0.38 +/- 0.13 and -0.14 +/- 0.14 kg). These data suggest that during prolonged exercise the higher volumes enhanced the rate of GE but had little effect on PVC, electrolytes, and BG compared with the lower volume. In addition, large feedings may be most effective in preventing dehydration but can lead to large gastric residues which cause discomfort that may impair performance.