Dehydration Impairs Cycling Performance, Independently of Thirst: A Blinded Study

Purpose The aim of the present study was to examine the effect of dehydration on exercise performance independently of thirst with subjects blinded of their hydration status. Methods Seven male cyclists (weight, 72 9 kg; body fat, 14% 6%; peak oxygen uptake, 59.4 +/- 6 mLkg(-1)min(-1)) exercised for 2 h on a cycle ergometer at 55% peak oxygen uptake(,) in a hot-dry environment (35 degrees C, 30% relative humidity), with a nasogastric tube under euhydrated-non-thirst (EUH-NT) and dehydrated-non-thirst (DEH-NT) conditions. In both trials, thirst was matched by drinking 25 mL of water every 5 min (300 mLh(-1)). In the EUH-NT trial, sweat losses were fully replaced by water via the nasogastric tube (calculated from the familiarization trial). After the 2 h of steady state, the subjects completed a 5-km cycling time trial at 4% grade. Results Body mass loss for the EUH-NT and DEH-NT after the 2 h was -0.2% +/- 0.6% and -2.2% +/- 0.4%, whereas after the 5-km time trial, it was -0.7% +/- 0.5% and 2.9% +/- 0.4%, respectively. Thirst (35 +/- 30 vs 42 +/- 31 mm) and stomach fullness (46 +/- 21 vs 35 +/- 20 mm) did not differ at the end of the 2 h of steady state between EUH-NT and DEH-NT trials (P > 0.05). Subjects cycled faster during the 5-km time trial in the EUH-NT trial compared with the DEH-NT trial (23.2 +/- 1.5 vs 22.3 +/- 1.8 kmh(-1), P < 0.05), by producing higher-power output (295 +/- 29 vs 276 +/- 29 W, P < 0.05). During the 5-km time trial, core temperature was higher in the DEH-NT trial (39.2 degrees C +/- 0.7 degrees C) compared with the EUH-NT trial (38.8 degrees C +/- 0.2 degrees C; P > 0.05). Conclusions These data indicated that hypohydration decreased cycling performance and impaired thermoregulation independently of thirst, while the subjects were unaware of their hydration status.