Effects of hypohydration and fluid balance in athletes' cognitive performance: a systematic review

Background: The effects of progressive body fluid loss on athletic and cognitive performance are known to result from exposure to environmental heat stress, morphologic factors, and limited fluid replenishment. Athletes need to restore lost body water. However, athletes may fail to maintain euhydration during exercise. This systematic review investigated hypohydration and fluid balance effects on an athlete's cognitive function. Methods: The PubMed, Sports Discuss, and Ebsco databases were searched for studies reporting on hypohydration, fluid balance and heat on cognitive performance in sport. Multiple phrases including hydration, dehydration, fluid balance, mood, cognition, vigilance, decision making, and brain were explored. Participants in the studies did either receive fluid or did not receive fluid during exercise. Results: Twenty-four trials (n=493 participants) from 24 articles met the inclusion criteria. Significant hypohydration, >2% body mass loss was reported consistently in 16 publications. Five articles where hypohydration was associated with heat stress and limited fluid intake (3-5% body mass loss) impaired cognitive performance. Mood disturbance, fatigue, and ratings of perceived exertion constantly complemented hypohydration impairment on cognition. Conclusion: Findings show that hypohydration impairs cognitive performance and mood at higher levels of 3-5% body mass loss. However, sport-specific cognitive protocols of accessing hypohydration and fluid balance in individual and team sports remain equivocal.