Eur J Appl Physiol (1995) 71:153-160 © Springer-Verlag 1995 Tracy A. Robinson • John A. Hawley • Garry S. Palmer Gary R. Wilson • David A. Gray • Timothy D. Noakes Steven C. Dennis Water ingestion does not improve 1-h cycling performance in moderate ambient temperatures Accepted: 8 February 1995 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 approxim- ate 1.71 sweat loss measured in a previous 1-h famil- iarisation performance ride at approximately 85% of peak oxygen uptake with artificially sweetened, col- oured 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 avail- able 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-l(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 1.7 1" h - 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 1.5 1 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 -~ sweat losses during high-intensity, short duration exercise in a moderate T.A. Robinson - J.A. Hawley (N~). G.S. Palmer - G.R. Wilson D.A. Gray" T.D. Noakes • S.C. Dennis Bioenergetics of Exercise Research Unit of The Medical Research Council and University of Cape Town, Department of Physiology, University of Cape Town Medical School, Observatory 7925, South Africa environment would not appear to induce beneficial physiological effects, and may impair exercise perfor- mance. Key words Ambient temperature • Drinking • High-intensity exercise • Thermoregulation Introduction Since the early investigations which have shown that exercise-induced dehydration decreases endurance ca- pacity in subjects walking at less than 40% of maximal oxygen uptake (ITO2max) for many hours in the heat (Adolph 1947; Bean and Eichna 1943; Eichna et al. 1945; Pitts et al. 1944), little attention has been paid to the influence of fluid ingestion on high-intensity perfor- mance in athletes. Recent studies of fluid replacement during exercise have largely focused on the reduction in endurance associated with the large (equal to or greater than 4% of body mass) sweat losses incurred during prolonged (equal to or greater than 2 h), moderate intensity (equal to or less than 65% of 1202max exercise at high (equal to greater than 30°C) ambient temper- atures (Barr et al. 1991; Montain and Coyle 1992a,b). To our knowledge, only two studies have examined the effects of drinking during high-intensity exercise (greater than 80% of l?O2max in warm environments (equal to or greater than 30°C). Walsh et al. (1994) have found that when subjects drank sufficient fluid (a 20 mmol. 1 NaC1-1 solution) to replace their approx- imate sweat losses of 1.11 during 1-h ride at 70% of ~]~O2max in the heat, they were able to cycle for 34% longer during a subsequent exercise period at 90% of 1202m,x. Below and Coyle (1993) have reported that with the ingestion of 1.3 1 of water during a 50-rain ride at 80% of VO2m,x subjects were able to complete a sub- sequent (approximate 10 min) exercise period 6.5% fas- ter than without fluid ingestion. Hence, attempts to