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

ABSTRACTPurposeThe 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.MethodsSeven male cyclists (weight: 72±9 kg, body fat: 14±6%, VO2peak: 59.4±6 ml[BULLET OPERATOR]kg-1·min-1) exercised for 2 hours on a cycle ergometer at 55% VO2peak, in a hot-dry environment (35°C, 30% rh), with a nasogastric (NG) 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 mL[BULLET OPERATOR]h-1). In the EUH-NT trial sweat losses were fully replaced by water via the NG tube (calculated from the familiarization trial). Following the 2-h of steady state, the subjects completed a 5-km cycling time trial at 4% grade.ResultsBody mass loss for the EUH-NT and DEH-NT after the 2-h was -0.2±0.6 and -2.2±0.4%, while after the 5-km time trial 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 to the DEH-NT trial (23.2±1.5 vs. 22.3±1.8 km·h-1, P0.05).ConclusionThese data indicated that hypohydration decreased cycling performance and impaired thermoregulation independently of thirst, while the subjects were unaware of their hydration status. 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%, VO2peak: 59.4±6 ml[BULLET OPERATOR]kg-1·min-1) exercised for 2 hours on a cycle ergometer at 55% VO2peak, in a hot-dry environment (35°C, 30% rh), with a nasogastric (NG) 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 mL[BULLET OPERATOR]h-1). In the EUH-NT trial sweat losses were fully replaced by water via the NG tube (calculated from the familiarization trial). Following 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%, while after the 5-km time trial 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 to the DEH-NT trial (23.2±1.5 vs. 22.3±1.8 km·h-1, P0.05). Conclusion These data indicated that hypohydration decreased cycling performance and impaired thermoregulation independently of thirst, while the subjects were unaware of their hydration status. Corresponding author: Stavros A. Kavouras, PhD, FACSM, Hydration Science Lab, University of Arkansas, HPER 308Q, Fayetteville, AR 72701, USA, E-mail: kavouras@uark.edu The results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation and do not constitute endorsement by ACSM. SAK was a scientific consultant for Quest Diagnostics and has active grants from Danone Research. ADS is a scientific consultant for Gatorade Sports Science Institute, the rest of the authors do not have anything to disclose. This study was not funded. Accepted for Publication: 22 February 2018 © 2018 American College of Sports Medicine

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