Water ingestion does not improve 1-h cycling performance in moderate ambient temperatures

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Abstract
Eight endurance-trained cyclists rode as far as possible in 1 h on a stationary cycle simulator in a moderate environment (20°C, 60% relative humidity, 3 m·s−1 wind speed) while randomly receiving either no fluid (NF) or attempting to replace their approximate 1.71 sweat loss measured in a previous 1-h familiarisation performance ride at approximately 85% of peak oxygen uptake with artificially sweetened, coloured water (F). During F, the cyclists drank mean 1.49 (SEM 0.14)1 of which mean 0.27 (SEM 0.08)1 remained in the stomach at the end of exercise and mean 0.20 (SEM 0.05) 1 was urinated after the trial. Thus, only mean 1.02 (SEM 0.12)1 of the ingested fluid was available to replace sweat losses during the 1-h performance ride. That fluid decreased the mean average heart rate from 166 (SEM 3) to 157 (SEM 5) beats·min−1 (P < 0.0001) and reduced the final mean serum [Na−] and osmolalities from 143 (SEM 0.6) to 139 (SEM 0.6) matom·1−1(P < 0.005) and from 294 (SEM 1.7) to 290 (SEM 1.9) mosmol·1−1 (P = 0.05), respectively. Fluid ingestion did not significantly attenuate rises in plasma anti-diuretic hormone and angiotensin concentrations, or decrease the approximate-15% falls in estimated plasma volume in the F and NF trials. Nor did fluid ingestion significantly affect the approximate 1.71 · h−1 sweat rates, the rises in rectal temperature (from 36.6° to 38.3°C) or the ratings of perceived exertion in the two trials. Ingestion of approximately 1.51 of fluid produced an uncomfortable feeling of stomach fullness and reduced the mean distance covered in 1 h from 43.1 (SEM 0.7) to 42.3 (SEM 0.6) km (P < 0.05). Thus, trying to replace more than 1.01·h−1 sweat losses during high-intensity, short duration exercise in a moderate environment would not appear to induce beneficial physiological effects, and may impair exercise performance.