Effect of fluid intake volume on 2-h running performances in a 25°C environment

Purpose: In this study, we examined the effects of greater than ad libitum rates of fluid intake on 2-h running performances. Methods: Eight male distance runners performed three runs on a treadmill at 65% of peak oxygen uptake ((V) over dot O-2 peak) for 90 min and then ran as far as possible in 30 min in an air temperature of 25 degrees C, a relative humidity of 55% and a wind speed of 13-15 km.h(-1). During the runs, the subjects drank a 6.9% carbohydrate (CHO)-electrolyre solution either ad libitum or in set volumes of 150 or 350 mL.70 kg(-1) body mass (similar to 130 or 300 mL) every 15-20 min. Results: Higher (similar to 0.9 vs 0.4 L.h(-1)) rates of fluid intake in the 350 mL.70 kg(-1) trial than in the other trials had minimal effects on the subjects' urine production (similar to 0.1 L.h(-1)), sweat rates (similar to 1.2 L.h(-1)), declines in plasma volume (similar to 8%), and rises in serum osmolality (similar to 5 mosmol.L-1) and Na+ concentrations (similar to 7 mEq.L-1). A greater (similar to 1.0 vs 0.5 g.min(-1)) rate of CHO ingestion in the 350 mL.70 kg(-1) trial than in the other trials also did not affect plasma concentrations of glucose (similar to 5 mmol.L-1) and lactate (similar to 3 mmol.L-1) during the performance runs. In all three performance runs, increases in running speeds from similar to 14 to 15-16 km.h(-1) and rises in exercise intensities from similar to 65% to 75% of (V) over dot O-2 (peak) elevated plasma lactate concentrations from similar to 1.5 to 3 mmol.L-1 and accelerated CHO oxidation from similar to 13 to 15 mmol.min(-1). The only effect of the additional intake of similar to 1.0 L of fluid in the 350 mL.70 kg(-1) trial was to produce such severe gastrointestinal discomfort that two of the eight subjects failed to complete their performance runs. Conclusion: Greater rates of fluid ingestion had no measurable effects on plasma volume and osmolality and did not improve 2-h running performances in a 25 degrees C environment.