Blinded and unblinded hypohydration similarly impair cycling time trial performance in the heat in trained cyclists

Knowledge of hydration status may contribute to hypohydration-induced exercise performance decrements, therefore, this study compared blinded and unblinded hypohydration on cycling performance. Fourteen trained, non-heat acclimated cyclists (age 25 ± 5 y; V̇O2peak 63.3 ± 4.7 mL∙kg-1∙min-1; cycling experience 6 ± 3 y) were pair-matched to blinded (B) or unblinded (UB) groups. After familiarisation, subjects completed euhydrated (B-EUH; UB-EUH) and hypohydrated (B-HYP; UB-HYP) trials in the heat (31˚C); 120 min cycling preload (50% Wpeak) and a time trial (~15 min). During the preload of all trials, 0.2 mL water∙kg body mass-1 was ingested every 10 min, with additional water provided during EUH trials to match sweat losses. To blind the B group, a nasogastric tube was inserted in both trials and used to provide water in B-EUH. The preload induced similar ( P=0.895) changes in body mass between groups (B-EUH -0.6 ± 0.5%; B-HYP -3.0 ± 0.5%; UB-EUH -0.5 ± 0.3%; UB-HYP -3.0 ± 0.3%). All variables responded similarly between B and UB groups ( P≥0.558), except thirst ( P=0.004). Changes typical of hypohydration (increased heart rate, RPE, gastrointestinal temperature, serum osmolality and thirst, decreased plasma volume; P≤0.017) were apparent in HYP by 120 min. Time trial performance was similar between groups ( P=0.710) and slower ( P≤0.013) with HYP for B (B-EUH 903 ± 89 s; B-HYP 1008 ± 121 s; -11.4%) and UB (UB-EUH 874 ± 108 s; UB-HYP 967 ± 170 s; -10.1%). Hypohydration of ~3% body mass impairs time trial performance in the heat, regardless of knowledge of hydration status.