J.D. Adams, Yasuki Sekuguchi, Adam Seal, Hyungyu Suh, Cameron Sprong, Lisa Jansen, Stavros A. Kavouras, FACSM; Hydration Science Lab, University of Arkansas, Fayetteville, AR.

Hypohydration >2% body mass loss impairs endurance exercise performance, but the majority of previous studies are confounded by the lack of subject blinding. Previous blinding hydration studies have used intravenous infusion methods which bypass the oropharyngeal receptors and gastrointestinal tract which have been previously shown to play an important role on thirst and performance. PURPOSE: The purpose of this study was to examine the effect of hypohydration on exercise performance in a blinded manner using intra-gastric water delivery to manipulate hydration status, while thirst was matched between trials. METHODS: Seven male cyclists (weight: 71±8 kg, body fat: 14±6%, VO2peak: 59.4±6 ml/kg/min) 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 (EUH) and hypohydrated (HYP) conditions. In both trials, thirst was matched by drinking 25 mL every 5 min (600 mL total). In the EUH sweat losses fully were replaced via the NG tube (calculated from the familiarization visit). Following the 2 hours of steady state, the cyclists completed a 5-kilometer cycling time trial at 4% grade. RESULTS: Following 2 hours of steady state cycling, post-exercise body mass loss for EUH trial was -0.1% compared to the HYP trial which was -2.2%. Thirst (28±11 vs. 42±12 mm) and stomach fullness (41±8 vs. 38±8 mm) were both similar between EUH and HYP trials (P>0.05). Cyclists completed the 5km time trial faster in the EUH trial compared to the HYP trial (777±47 vs. 822±55 sec, P0.05). CONCLUSION: These data indicated that hypohydration decreased cycling performance and impaired thermoregulation in the absence of thirst, while the subjects were unaware of their hydration status.

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