Sparkling Water Limits Cognitive Fatigue with Stable Pupil Diameter during Real World Esports Training

Abstract

Prolonged esports play induces cognitive fatigue with reduced executive function and pupil constriction as a neurobiomarker of prefrontal activity across expertise levels. Since this pupil-linked cognitive fatigue is mitigated by sparkling water intake in casual players, we hypothesized that sparkling water prevents cognitive fatigue and sustains esports performance in esports athletes under real training conditions that reflect competitive practice. To test this hypothesis, nineteen hardcore esports players participated in a randomized crossover trial. Each completed two 3-hour sessions of head-to-head virtual football matches while consuming either sparkling water or plain water. Subjective fatigue, enjoyment, and executive function (flanker task) were assessed, and pupil diameter, heart rate, and salivary cortisol were continuously monitored. In-game performance was also recorded. Accuracy declined and pupil constriction progressed under the plain water condition, whereas executive accuracy was preserved and pupil diameter was maintained over the three-hour session in the sparkling water condition. Subjective ratings of fatigue and enjoyment increased similarly in both conditions. Heart rate, including both mean and peak values, and salivary cortisol concentrations showed no significant differences between conditions. In terms of in-game performance, goals, passes, and interceptions did not differ between conditions, whereas the number of fouls was lower and the number of shots was higher in the sparkling water condition. These findings demonstrate that sparkling water mitigates cognitive fatigue and sustains esports performance in esports athletes under real training conditions. As a caffeine- and sugar-free beverage, sparkling water may serve as a practical, low-risk intervention for managing cognitive fatigue and supporting daily practice in modern esports settings.