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.


Introduction
Mega sporting events will continue to take place in diverse hot geographical environment across the globe as they have been in the past and in the present. As always, hypohydration can be expected in these events. The events include the Olympic Games: Beijing 2008; Rio 2016, Tokyo 2021; World Athletics Championships Doha 2019, and Federation International Football Association World Championships Qatar 2022 1,2 . With such exposure to hot environments, athletes mainly in prolonged vigorous exercise, racket and intermittent team sports experience significant and exceeding >2% body fluid loss due to thermoregulation 2,3 . Inadequate and/or no fluid loss replacement can cause endurance capacity impairment associatd with physiological and cognitive function alterations 4,5 . Indeed excessive dehydration impacts are major cause of concern to athletic trainers and sports medical staff.
Dehydration and hypohydration deleterious effects on athletic performance and cognition have been widely researched 1,3,5 . It is well known that a 2% body mass loss can impair endurance performance in humid/hot environments 6,7 . There has been limited research on the impact of hypohydration on athlete's cognitive performance and mood during individual and intermittent team sport 8,9 . Literature has supported that dehydration may impair cognitive performance [10][11][12] and functional task 13 . However, it is known that rehydration may cause minimal or no effect on athletic, cognition and immunological performance if the outcome to be assessed is insensitive to the modest (up to 2% of body weight) fluid losses 1,5,8 . Severe dehydration may disturb, aggravated fatigue, dizziness, confusion and often severe cases lead However, inconsistent conclusions exist in the current literature 4,5,8,12 . Some studies have demonstrated discrepancies in literature may be due to task complexity, test duration, magnitude of heat stress, test combined 11,13 . Prolonged exercising in hot, humid environments with inadequate fluid replenishment may increase core body temperature (hyperthermia) to ~4 0 C provoking athlete's mental status that worsen in moderately and untrained athletes 4,22,23 . Despite that elite acclimated athletes may physiologically negotiate hyperthermic conditions, athletic trainers, sports scientists and sports medical staffs tirelessly work to uncover cooling techniques to curb core body temperature, delayed onset peripheral and central fatigue 4,[23][24][25][26] . Thus, researchers have investigated dehydration, hypohydration and fluid ingestion aspects and their subsequent athletic performance effects 2,8 remains unclear. To date, no papers have reviewed and collectively discussed these aspects to equip professionals better understand impact on individual and team sport performances. Therefore, the aim of this systematic review was to summarise to summarise the literature assessing impact on hypohydration and fluid balance in relation to cognitive function in semi-professional to elite athletes exercising in humid, hot environments.

Methods
The study protocol was devised following the specifications outlined in the Preferred Reporting Items for Systematic Reviews and Meta-analysis (PRISMA) Statement 27 .

Literature Search strategy
Relevant research studies on dehydration and hypohydration effects on cognitive function when training in hot, humid environments identified on electronic database from 2005 until May 2020. Available literature before 2005 focused mainly on athletic performance of elite athletes. For the purpose of the current review focus was on cognitive function and mood of semi-professional compared to elite athletes. The database include: PubMed/Medline, Sports Discuss, and Ebsco. Keywords and terms search were hydration, athletic, exercise, mood, attention, vigilance, decision making, reaction time, sweat loss, individual/team sport, ad libitum, water, fluid (eg. administration, consumption, ingestion, intake, replacement, replenishment), hydration (de-, eu-, hypo-, re-), each combined with cognitive/cognition (aspects, function), brain were explored.
Searches were restricted to full-length peer-reviewed published articles in the English language. Unpublished experimental observations, published abstracts records that contain irrelevant terms (children, elderly, patient, disease, rat/mouse) were excluded. Twenty four original research studies involving fluid balance (water, flavoured water/ ad libitum water/ sodium chloride solution/ sports drink) not controlled by investigators, and sweat rate in athletes were included. These studies measured impact of hypohydration on cognitive performance. Little attention was paid to effects of physical/ athletic performance considering that the data was available and up to date.

Inclusion
All research studies with fulfilling the following criteria were eligible for inclusion; a. All relevant South Africa and international studies. b. Studies with standardized dehydration protocol. c. Focussed on male or female humans with no underlying medical conditions (≥ 13 years of age). d. Fluid consumption was done in limited time ≤ 4 hours between dehydration protocol and subsequent performance test. e. A cognitive function and athletic performance outcome measured.
Exclusion a. Unpublished experimental observations, published abstracts records that contain irrelevant terms (elderly, patient, disease, and rat/mouse) were excluded. b. Experimental designs without cognitive performance outcome measured.

Data Extraction
All published scientific research peer-reviewed articles meeting the inclusion criteria were extracted and considered for the following characteristics; participant, exercise and hydration protocol, change in body mass, study findings, and study limitations. Research studies that contained more than one intervention and eligible for inclusion tested cognitive performance under two different conditions were treated as separate trials 16 . These trials derived from one study are denoted and cited by letters (a-b). Where necessary information was not given, the author considered it as unavailable.

Fluid balance terminology
An athlete is responsible for maintaining normal hydration status (euhydration) for optimal body performance. Armstrong 28 suggests that change in body fluid balance is a resultant of baseline mass value compared to the individual body mass. Hydration status can be explained using terms such as; euhydration (normal baseline body water content), hypohydration (excessive body water deficit) and dehydration (progressive body water loss from normal baseline to hypohydration).
Search results 567 articles were potentially relevant. After the exclusion of duplicates, articles published before 2005, articles focusing on athletic performance, ageing, diseases and children below 13 years and review of full-text versions, 24 articles were selected for review as shown in Fig 1 and Table 1 below.

Discussion
This systematic review aimed to summarise literature assessing the impact of hypohydration and fluid balance on cognitive function in semi-professional to elite athletes exercising in humid, hot environments. The discussion considered the risk factors posed by an increase in sweat loss to ≥2% body mass loss. Major causes of hypohydration were discussed as environmental factors, exercise intensity, and/ or limited fluid replacement in relation to the brain and cognitive performance. Effects on cognitive performance and mood in the studies included in this review considered individual and team sports with training or competition duration of more than 1 hour 8,23,26 . Although hypohydration risk levels may vary in different sports, the review takes the notion that individual risk factors among athletes may be altered between low-and high-level categories depending on humidity, timing day/season and intensity level, hydrating behaviours, social and cultural considerations.
Fluid balance and the Brain The brain, a complex active part of the human body is known for its high metabolism. It accounts for ~15% of resting cardiac output and a relatively higher total body aerobic metabolism of ~20% 45,46 . To maintain its high metabolism, the brain depends solely on adequate circulation of oxygen, metabolic substrates, and metabolic by-products elimination 5 . Heat stress, hyperthermia, and dehydration are known physiological stressors to alter cerebral circulation and metabolism. Hypohydration was found to mediate brain function reduction by reducing cerebral blood flow and brain cell volume, hence increasing blood-brain permeability 8 . Exercise stimulus causes adjustments to Cerebral Blood Flow (CBF). A study by Kety and Schmidt47 showed that CBF could not be altered during the athletic rest-to-exercise transition. Recent temporal resolution methods showed a ~20% CBF rise due to endurance and moderate exercise intensities [46][47][48][49] . Indeed, CBF is subdued with high exercise intensities and significantly surpass rest levels due to exhaustion 50,51 .
Progressive dehydration during individual and/ or intermittent team sports without concomitant hyperthermia increases CBF 52 . However, when the athlete is resting, a 1.5 0 C increase in body core temperature causes a ~15% CBF reduction 53 . It should be noted that both dehydration and hyperthermia changes CBF mechanisms in different exercises 5 , intensities, and environments. Dehydration ≥3% body mass loss during endurance exercise in a hot, humid environment reduces CBF due to cerebrovascular instability and cardiovascular drift 54 -56 .
In contrast, CBF reduction is attenuated when there is equilibrium between body fluid lost through sweating and fluid replenishment during exercise 5, 54, 57 .
Heat-induced stress, hyperthermia, and dehydration effects on CBF are associated with prolonged aerobic exercise5. Previous studies reveal that CBF reduction is worsened during acute-intensity exercise in hot and humid environments 23 Prolonged exercise in hot environments without fluid replacement elevates core body temperature thereby creating a cognitive burden 21 . The symptoms of hypohydration including thirst and negative mood states have an equal effect on accomplishing cognitive tasks and consequently impair function 7,8 . Cognitive trials conducted in less than 5 minutes after dehydration protocol ended found that ≤ 2.8% body mass loss induced through fluid deprivation had no impact on cognitive-motor performance 13, 40, 42 -44 . Although many studies did not clearly show the time from the end of dehydration protocol to commencement of the cognitive tests, a significant raise in ratings of thirst, concentration, and ratings of perceived effort was found 13,16,42 .
In all the above trials, the long-lasting effects of physiological stressors employed may obstruct fluid intake influencing cognitive performance 8,60 . Fluid replenishment attenuates Total Mood Disturbance in 3 of the 5 trials where mood was measured 16,23,29,31 . Considering that mood effects and cognition were independent, it should be noted that the above three findings were objective compared to the subjective60. However, if not clearly stated, self-reported mood questionnaires are subjective and consider mood effects as dependent variables. It is certain that the influence of fluid replenishment on cognitive function and mood needs further research 23,60 .
It should be noted that rehydration may have no or little effect on cognitive function in cases where outcome measured is not receptive to the modest fluid loss effects 60 . The amount of fluid ingested, and the time when the fluid was administered has varied physiological responses. These may be confounded in response to dehydration protocol (control and intervention trials) which could have implications on cognitive and athletic performance.

Conclusion
Considering that, most of the studies measured up to 2.7% body mass loss, the impact of hypohydration and fluid balance on cognitive performance in individual and team sports remains equivocal. In all the studies involved, measures of cognitive function altered include processing speed, vigilance, and reaction time for working memory. It is important to note that visuomotor reaction, mental concentration, and visual scanning and/ perception were not significantly affected by fluid balance and hypohydration. This inconsistency should inform the need to consider objectivity, subjectivity, validity, reliability, and sensitivity of cognitive function assessment tools for the athletic population. The current review serves to draw attention to areas for future research.