Hypohydration impairs endurance performance: a blinded study

The general scientific consensus is that starting exercise with hypohydration >2% body mass impairs endurance performance/capacity, but most previous studies might be confounded by a lack of subject blinding. This study examined the effect of hypohydration in a single blind manner using combined oral and intragastric rehydration to manipulate hydration status. After familiarization, seven active males (mean ±  SD : age 25 ± 2 years, height 1.79 ± 0.07, body mass 78.6 ± 6.2, V O 2peak 48 ± 7 mL·kg·min −1 ) completed two randomized trials at 34°C. Trials involved an intermittent exercise preload (8 × 15 min exercise/5 min rest), followed by a 15‐min all‐out performance test on a cycle ergometer. During the preload, water was ingested orally every 10 min (0.2 mL·kg body mass −1 ). Additional water was infused into the stomach via a gastric feeding tube to replace sweat loss ( EU ) or induce hypohydration of ~2.5% body mass ( HYP ). Blood samples were drawn and thirst sensation rated before, during, and after exercise. Body mass loss during the preload was greater (2.4 ± 0.2% vs. 0.1 ± 0.1%; P <  0.001), while work completed during the performance test was lower (152 ± 24 kJ vs. 165 ± 22 kJ; P <  0.05) during HYP . At the end of the preload, heart rate, RPE , serum osmolality, and thirst were greater and plasma volume lower during HYP ( P <  0.05). These results provide novel data demonstrating that exercise performance in the heat is impaired by hypohydration, even when subjects are blinded to the intervention.