Half-marathon running performance is not improved by a rate of fluid intake above that dictated by thirst sensation in trained distance runners

It has been demonstrated that exercise-induced dehydration (EID) does not impair, and ad libitum drinking optimizes, cycling time-trial (TT) performance. However, the idea that EID a parts per thousand yen 2 % bodyweight (BW) impairs endurance performance is well ingrained. No study has tested the impact of EID upon running TT performance. We compared the effects of thirst-driven (TD) vs. programmed fluid intake (PFI) aimed at maintaining EID-associated BW loss < 2 % on half-marathon performance. Ten trained distance runners underwent, in a randomized, crossover fashion, two, 21.1 km running TTs on a treadmill (30 A degrees C, 42 % relative humidity) while facing a wind speed matching running speed and drinking water (1) according to thirst sensation (TD) or (2) to maintain BW loss < 2 % of their pre-exercise BW (PFI), as recommended by the American College of Sports Medicine. Despite that PFI significantly reduced EID from 3.1 +/- A 0.6 (TD) to 1.3 +/- A 0.7 % BW (PFI), mean rectal temperature from 39.4 +/- A 0.4 to 39.1 +/- A 0.3 A degrees C, mean body temperature from 38.1 +/- A 0.4 to 37.7 +/- A 0.2 A degrees C and mean heart rate from 175 +/- A 9 to 171 +/- A 8 bpm, neither half-marathon time (TD 89.8 +/- A 7.7; PFI 89.6 +/- A 7.7 min) nor running pace (TD 4.3 +/- A 0.4; PFI 4.2 +/- A 0.4 min/km) differed significantly between trials. Albeit providing trivial cardiovascular and thermoregulatory advantages, in trained distance runners, PFI (1,380 +/- A 320 mL/h) offers no performance benefits over TD fluid intake (384 +/- A 180 mL/h) during a half-marathon raced under warm conditions.