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

Abstract

In this study, we examined the effects of greater than ad libitum rates of fluid intake on 2-h running performances. Eight male distance runners performed three runs on a treadmill at 65% of peak oxygen uptake (VO2peak) 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 x 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 x 70 kg(-1) body mass (approximately 130 or 300 mL) every 15-20 min. Higher (approximately 0.9 vs 0.4 L x h(-1)) rates of fluid intake in the 350 mL x 70 kg(-1) trial than in the other trials had minimal effects on the subjects' urine production (approximately 0.1 L x h(-1)), sweat rates (approximately 1.2 L x h(-1)), declines in plasma volume (approximately 8%), and rises in serum osmolality (approximately 5 mosmol x L(-1)) and Na+ concentrations (approximately 7 mEq x L(-1)). A greater (approximately 1.0 vs 0.5 g x min(-1)) rate of CHO ingestion in the 350 mL x 70 kg(-1) trial than in the other trials also did not affect plasma concentrations of glucose (> or = 5 mmol x L(-1)) and lactate (approximately 3 mmol x L(-1)) during the performance runs. In all three performance runs, increases in running speeds from approximately 14 to 15-16 km x h(-1) and rises in exercise intensities from approximately 65% to 75% of VO2peak elevated plasma lactate concentrations from approximately 1.5 to 3 mmol x L(-1) and accelerated CHO oxidation from approximately 13 to 15 mmol x min(-1). The only effect of the additional intake of approximately 1.0 L of fluid in the 350 mL x 70 kg(-1) trial was to produce such severe gastrointestinal discomfort that two of the eight subjects failed to complete their performance runs. 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.