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 (O2 peak) for 90 min and then ran "as far as possible" in 30 min in an air temperature of 25°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)-electrolyte solution either ad libitum or in set volumes of 150 or 350 mL·70 kg-1 body mass (130 or 300 mL) every 15-20 min. Results: Higher (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 (0.1 L·h-1), sweat rates (1.2 L·h-1), declines in plasma volume (8%), and rises in serum osmolality (5 mosmol.L-1) and Na+ concentrations (7 mEq·L-1). A greater (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 (5 mmol·L-1) and lactate (3 mmol·L-1) during the performance runs. In all three performance runs, increases in running speeds from 14 to 15-16 km·h-1 and rises in exercise intensities from 65% to 75% of O2 peak elevated plasma lactate concentrations from 1.5 to 3 mmol·L-1 and accelerated CHO oxidation from 13 to 15 mmol·min-1. The only effect of the additional intake of 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°C environment.
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