Separate and combined effects of dehydration and thirst sensation on exercise performance in the heat

Using intravenous infusion, we separated the physiologic consequences of 3% body mass dehydration from the conscious awareness of fluid replacement on time trial (TT) performance in the heat. Eleven trained cyclists performed 90min of steady-state (50% ) cycling followed by a self-paced 20-km TT in a hot-dry (35 degrees C, 10% relative humidity, wind speed 3.0m/s) environment while euhydrated-not thirsty (EU-NT), euhydrated-thirsty (EU-T), dehydrated-not thirsty (DH-NT), or dehydrated-thirsty (DH-T). Thirst was manipulated by providing (NT) or withholding (T) ad libitum 35 degrees C water oral rinse. Distinct hydration states existed, with 0.4 +/- 0.5% dehydration following the 20-km TT (EU) compared with 3.2 +/- 0.6% in DH (P<0.001). Greater perceived thirst existed in T (7 +/- 2 on a 1-9 scale) than NT (4 +/- 2, P<0.001) after the TT. No significant differences in power output existed during the TT between hydration (EU 202.9 +/- 36.5W vs DH 207.0 +/- 35.9W, P=0.362) and thirst conditions (NT 203.3 +/- 35.6W vs T 206.6 +/- 36.8W, P=0.548), nor were there differences in completion time (P=0.832) or pacing profile (P=0.690). Within the range of up to 3% body mass loss, neither the physiologic effects from lowered hydration status nor the perception of thirst, separately or combined, affected sustained submaximal exercise performance in the heat for a healthy and fit population.