Table 2.

Intervention Studies on Hydration and Neurological Function ¹.

Citation	Objective	Population	Design	Intervention/Control	Summary/Conclusion
Neurological Function in Children
Edmonds et al., 2017 (Children) [38]	Examine dose–response effects of water on cognition in children	60 children (58% F) ages 7–10 years.	Acute RCT	Children consumed the assigned water 20 min prior to cognition tasks: 0 mL water (n = 20, 10/10 M/F) 25 mL water (n = 20, 10/10 M/F) 300 mL water (n = 20, 5/15 M/F)	No significant time × volume interaction for visual attention (Letter cancellation task) and working memory (digit span task). Significant increase in pre- and post-water scores for visual attention for 25 mL and 300 mL with t-test analysis. No significant results on memory task.
Trinies et al., 2016 [39]	Assess the role of hydration on cognition in children living in hot, low income environments	279 children in grades 3–6 in schools across Eastern Zambia.	Parallel RCT	Students in area where water was not nearby were provided: No water bottles (n = 143, 68/65 M/F; Control group) Refillable water bottle (water group, n = 149, 61/82 M/F) Ambient temperature was 26.6–35 °C, and humidity ranged from 7–28%. Cognition was tested in the morning and after lunch on the day of intervention.	Afternoon hypohydration, assessed by mean urine specific gravity, was lower in children provided water (9.8%) compared to the control children (67.2%). No significant difference between groups in visual memory (indirect image difference), short-term memory (forward digit recall, and reverse digit recall), or visuomotor skills (line trace). Two visual attention tests were included, with a significant difference in one visual attention test (direct image difference; p = 0.05), but not the other (letter cancellation). When grouped by urine specific gravity (≥1.020 as cut-off for hypohydration) no significant difference was observed.
Perry et al., 2015 [40]	Assessed whether the benefit of drinking water on working memory and attention depends upon children’s hydration status and renal response.	52 children (50% F) age 9–12 y.	Acute crossover	All subjects performed a baseline cognition test after standard breakfast (included 25 mL water and 250 mL skim milk). During the water intervention, subjects consumed 250 mL water, followed by cognition test battery (short-term effect), and another 500 mL water over the next 3 h, followed by another cognition test batter (long-term effect). During the control intervention, no additional fluid beyond breakfast was provided.	Based on osmolality, 65% of the population was dehydrated prior to breakfast intake, with 35% remaining dehydrated at the end of the control test period compared to 3.8% at the end of the active (water) period. Children who exhibited smaller decreases in urine osmolality following water intake (i.e., had underlying hypohydration) performed significantly better on the water day compared to the control day on the digit-span task (verbal memory) and pair-cancellation task (sustained attention). Children who exhibited larger decreases in urine osmolality following water intake performed better on the control day compared to the water day. No significant effects on working memory (tested using delayed match-to-sample task).
Booth et al., 2012 [41]	Investigate the effects of water supplementation on visual attention and motor performance in schoolchildren.	15 students (age 8–9 y)	Acute RCT	Children were provided no water or 250 mL bottle of water and instructed to drink as much or as little 20 min prior to cognition test and mood ratings.	When offered water, children drank an average of 168 mL water. Children performed better on tasks testing visual attention and fine motor skills (Letter Cancellation Task and Wii Ravin Rabbids game) after water consumption compared to no water consumption. There were no differences for tasks testing gross motor skills (Ball Catching and Step Ups) and happiness rating.
Fadda et al., 2012 [42]	Assessed the effect of drinking water on cognitive performance, fatigue, and vigor in school children.	168 children age 9–11 y living in a hot climate (Southern Italy).	Parallel RCT	Control group (n = 75, 35/40 M/F) and a supplemental water group (n = 93, 47/46 M/F). The water group received 1 L of additional water for the day; the control group did not receive additional water.	Based on urine osmolality measurement, 84% of children were dehydrated (morning Uosm >800) at the start of the school day. Drinking water benefited short-term memory (auditory number span) and verbal reasoning (verbal analogies) but not selective attention (Deux de Barrage). No significant differences were found in POMS fatigue or confusion scores. Significant beneficial relationship between hydration and vigor was noted.
Kempton et al., 2011 [43]	Investigate the effects of dehydration on brain function	10 healthy adolescents (50% females), average age 16.8 y	Acute RCT	Subjects consumed 500 mL of water the evening prior to test day. On test day, subjects consumed a further 500 mL of water 1 h before a 90-min thermal exercise dehydration protocol (with thick and multilayered clothing) or a 90-min non-thermal control exercise protocol.	Subjects lost an average 1.65% body mass during the thermal dehydration exercise compared to 0.53% during the non-thermal control exercise. Hypohydration resulted in increased fronto-parietal brain activations during a task of executive function, lateral ventricular volume, and mental and physical sedation, but did not affect results of the executive function task.
Edmonds and Jeffes, 2009 [44]	Assess the effect of water consumption on cognition in children.	23 children (61% female) age 6–7 y from one classroom.	Acute parallel	Children were separated into water group (n = 11, 4/7 M/F) or no water (n = 12, 5/7 M/F). Children were tested for baseline function as a group, then the no water group left the room and those remaining were provided 500 mL water to drink ad libitum. The post-consumption test occurred 45 min later.	Significant positive changes were reported for children consuming water on the thirst and happiness ratings, as well as the visual attention and visual search tests. Visual memory and motor performances were not significantly different. Although there was an effect on mood, the authors caution making strong conclusions based on this finding due to the lack of significance on follow-up testing.
Edmonds and Burford, 2009 [45]	Assessed the effect of a drink of water on children’s cognitive function.	58 children, age 7–9 y.	Acute RCT	Control (no water) group (n = 30, 15/15 M/F) and a water group (n = 28, 11/17 M/F). Children were tested at baseline and 20 min later. Children in the water group were provided with 250 mL of water between tests.	Children who drank additional water rated themselves as significantly less thirsty and performed better on letter cancellation task (visual attention) and spot the difference task (visual attention and memory). There were no differences between groups for story memory and visuomotor tracking tasks.
Benton and Burgess, 2009 [46]	Examined the influence of giving additional water to school children on measures of memory and attention.	40 children (45% female) in a school in South Wales, average age 8 y.	Acute RCT	Children were tested in the afternoon after receiving 300 mL of water or no water at the beginning of the mid-afternoon break, with testing occurring 20–35 min after consumption.	Immediate memory (recall of objects) was significantly better from children after consumption of water. The ability to sustain attention (paradigm of Shakow) was not significantly influenced by water consumption.
Neurological Function in Adults
Stachenfeld et al., 2018 [47]	Investigate whether mild dehydration would adversely impact executive function tasks, with no effects on simple tasks, and that these changes in cognitive performance are independent of changes in emotion	12 healthy women (age 18–34 y)	Crossover RCT	Subjects performed cognitive tasks and rated mood under three difference hydration conditions: Control: Typical water intake (mean of 2.3 L) Dehydration: Water deprivation (mean of 0.7 L) Euhydration: Water intake conformed to EFSA and IOM recommendation (mean of 2.4 L) Control condition was always performed first, and order of dehydration and euhydration was randomized. Tests were performed during early follicular phase of the menstrual cycle, but a week apart for those on hormonal contraception.	Water deprivation increased plasma osmolality from ~283 to 287 Uosm/kg H2O. Water deprivation increased errors for tests for visual memory or working memory (Continuous Paired Associate Learning) and executive function and spatial problem solving (Groton Maze Learning Test) when compared to control and euhydration conditions. No hydration effect on simple reaction time, choice reaction time, visual attention, motor speed, visual motor function, visual learning, working memory assessed with One and Two Back Tasks, and cognitive flexibility. There were no changes in mood outcomes.
Edmonds et al., 2017 (Adults) [38]	Evaluate the dose–response effect of water on cognitive performance and mood in adults	96 adults, average age 21 y.	Acute RCT	Acute consumption of 300 mL water (n = 32, 10/22 M/F), 25 mL water (n = 32, 7/25 M/F), and 0 mL water (n = 32, 11/21 M/F) 20 min before cognition tasks.	Significant time × volume interaction for visual attention (letter cancellation task), whereby scores increased from baseline in a dose-dependent manner, with 0 mL having the lowest increase and 300 mL having the highest increase. Significant time × volume interaction for working memory (digit span task). Only the increase in the 300 mL group was significant for the memory test.
Benton et al., 2016 [48]	Assess whether a loss of 1% of body mass due to hypohydration adversely influenced cognition, and examined the possible underlying mechanisms	101 healthy adults. Water group aged 18–30 y; control group age 18–31 y.	Acute RCT	Water consumption group (n = 51, 26/25 M/F) and no water consumption group (n = 50, 26/24 M/F). Subjects were exposed to 30 °C for 4 h, during which they either did or did not drink 300 mL pure water.	Subjects in the no water group had greater body mass loss (−0.22% vs. +0.05%) and increase in osmolality (−117.24, no water vs. water). At 90 and 180 min, water consumption resulted in better episodic memory (word list recall task) and focused attention (arrow flanker test). Energy and depression ratings were unaffected by water consumption. Anxiety rating decreased with water consumption at 90 min, but not 180 min.
Pross et al., 2014 [49]	Evaluate effects of changing water intake on mood and sensation in habitual high- and low-water consumers.	52 subjects (79% F) average age 25 y were selected based on daily fluid consumption: Low <1.2 L/d (average 1.0 L/d) High ≥2 L/d (average 2.5 L/d)	Open label 2-d intervention	Intervention conducted in controlled setting (inpatient facility) with meals (details not provided) and sleep/wake cycles standardized. Baseline data were collected during days 1–2, and intervention conducted days 3–5. Defined drinking programs were: Low consumers increased water intake to 2.5 L/d (11/19 M/F) High consumers reduced water intake to 1L/d (0/22 M/F)	Increasing water intake decreased urine osmolality from mean of 841 to 392 mOsm/kg. Decreasing water intake increased urine osmolality from mean of 222 to 720 mOsm/kg. At baseline, POMS ratings were comparable except for thirst and some depression scores. Restricting water intake in high-consumers resulted in a significant increase in thirst and decrease in contentedness, calmness, positive emotions, and vigor/activity scores. Increasing water intake in low consumers significantly decreased fatigue/inertia, confusion/bewilderment, and thirst scores, with a non-significant decrease in sleepiness.
Edmonds et al., 2013 [50]	Explore the effects of water and knowledge of aims of study on cognitive performance	44 adults age 18–57 y.	Acute RCT	No water, no expectancy condition (n = 9, 3/6 M/F) No water, expectancy condition (n = 11, 6/5 M/F) Water, no expectancy condition (n = 14, 5/9 M/F) Water and expectancy condition (n = 10, 1/9 M/F) Subjects in the water groups were provided 200 mL water prior to mood and cognitive testing. Subjects in the expectancy groups were told that water consumption is believed to aid cognitive performance. Subjects in the no water groups were not provided water. Subjects in the no expectancy groups were not informed of the connection between water and test outcomes.	There were no differences in the amount of water consumed (out of 200 mL) between the water + expectancy vs. the water + no expectancy groups. Visual attention (letter cancellation task) improved with water consumption, regardless of expectancy condition. Working memory (backwards digit span task) was better in the no water groups vs. water groups. There was no effect of expectancy condition. Water and expectancy condition did not affect scores for simple reaction time or mood assessed using VAS. Water consumption effects on visual attention are due to the physiological effects of water, rather than expectancies about the effects of drinking water.
Lindseth et al., 2013 [51]	Examine the effect of fluid intake and possible dehydration on cognitive flight performance of pilots	40 healthy pilots (average age 20.3 y) enrolled during the third term of the commercial phase of their collegiate aviation program at a Midwestern university	Crossover RCT	High- or low-fluid controlled diets (≥80 oz/d vs. ≤40 oz/d or ≥2.4 L/d vs. ≤1.2 L/d) for 2 weeks, with 2 week washout.	No difference between high and low fluid diets for flight performance (General Aviation Trainer full-motion flight simulator), spatial cognition (Vandenberg Mental Rotation Test), and memory (Sternberg Item Recognition Test). Scores for flight performance and spatial cognition were poorer for pilots who were dehydrated (1–3% body mass loss). Hypohydration did not affect memory.
Edmonds et al., 2013 [52]	Investigate the effect of water supplementation on cognitive performance and mood in adults, and whether subjective thirst moderates the relation between water supplementation and cognitive performance and mood	34 healthy adults (74% F), age 20–53 y.	Acute RCT	Water group consumed 0.5–1 L water prior to cognitive and mood testing. The no water group was not provided water prior to testing.	Water supplementation had a positive effect on simple reaction time, whereby those who were thirsty and did not have water performed more poorly compared to those who were not thirsty. However, the poorer performance of thirsty subjects was “normalized” when they were provided water. No significant results for visual memory, visual learning, immediate and delayed memory, comprehension, learning, acquisition and reversal, sustained attention, forced choice recognition, and choice reaction time. Participants rated themselves as more tired and tense if they were thirsty, and consumption of water did not affect mood ratings.
Ely et al., 2013 [53]	Determine the impact of acute exposure to a range of ambient temperatures (10–40 °C) in euhydration and hypohydration states on cognition, mood and dynamic balance	32 men (average age 22 y)	Acute RCT	4 groups (n = 8/group) matched for aerobic fitness. Each group went through euhydration and hypohydration conditions in a crossover fashion, separated by 1 week. All subjects went through a dehydration exercise regimen. For the euhydration condition, subjects were provided water to restore body weight to their pre-dehydration body weight. For the hypohydration condition, water was only provided to ensure that body mass loss did not exceed −4.5%. Following the exercise + water consumption (if needed), subjects rested in preassigned temperatures and performed cognition tasks.	Sustained attention, choice reaction time, short-term spatial memory, and grammar-based logical reasoning were unaffected by hypohydration (4.0–4.2%body mass loss) or ambient temperature during cognition testing. Hypohydration (4.0–4.2%body mass loss) led to increased total mood disturbance, with increased ratings of anger/hostility, confusion/bewilderment, depression/dejection, and fatigue, without affecting vigor/activity and tension/anxiety. Temperature did not affect mood.
Pross et al., 2013 [54]	Assess no drink allowed for 23–24 h	20 healthy women, average age 20 y	Crossover RCT	Subjects completed the following: 24-h fluid deprivation: no beverages from 6–7 p.m. until 6 p.m. next day. Fully hydrated control: water intake at fixed periods. Standardized meals containing (50 g/d water) were provided. 10–30 d washout.	Urine specific gravity significantly increased and color significantly darkened at 9 h and remained so throughout 24 h, but plasma osmolality was unchanged. Generally higher sleepiness and lower alertness scores throughout, but significant at 14–16 h with no fluids. Significantly greater fatigue and lower vigor ratings with no fluids. No significant differences on sleep parameters.
Armstrong et al., 2012 [55]	Investigate if mild dehydration would primarily affect mood and symptoms of dehydration and have modest effects on cognitive function.	25 women, average age 23 y.	Acute RCT	Arm 1: Exercise-induced dehydration + placebo capsule containing no diuretic Arm 2: Exercise-induced dehydration plus a diuretic capsule Arm 3: Euhydration plus placebo capsule. 28 d washout between arms. During the euhydration arm, subjects consumed water equivalent to their body weight loss during and after the exercise bouts.	While in the dehydration arms, subjects lost ≥1% body mass (mean loss of 1.36%) Overall, sustained attention, choice reaction time, working memory, short-term memory, and logical reasoning were unaffected by dehydration. Subjects reported increased anger-hostility, increased fatigue-inertia, and decreased vigor-activity when dehydrated. Total mood disturbance score was worse with dehydration. Subjects also reported greater perceived task difficulty, lower concentration, and increased headache with dehydration.
Ganio et al., 2011 [56]	Assess the effects of mild dehydration on cognitive performance and mood of young males	26 men, average age 20 y	Acute RCT	Arm 1: Exercise-induced dehydration plus a diuretic Arm 2: Exercise-induced dehydration plus placebo containing no diuretic Arm 3: exercise while maintaining euhydration plus placebo. Washout was 4 d.	While in the dehydration arms, subjects lost ≥1% body mass (mean loss 1.59% body mass). Dehydration resulted in lower scores for attention (scanning visual vigilance task) and working memory (matching to sample task) and increased tension/anxiety and fatigue/inertia. No significant results for visual reaction time, choice reaction time, short-term memory, and logical reasoning.
Kempton et al., 2009 [57]	Investigate whether acute dehydration would lead to a reduction in brain volume and subtle regional changes in brain morphology such as ventricular expansion	7 healthy men (mean age 23.8 y)	Single arm	Subjects went through a thermal-exercise dehydration protocol to decrease body mass by 2–3%. Subjects received brain MRI scan before and after the dehydration protocol.	Average body mass loss due to dehydration protocol was 2%. Dehydration led to expansion of the ventricular system with the largest change occurring in the left lateral ventricle, without changes in total brain volume.
Petri et al., 2006 [58]	Measure the deterioration in mental and physical performance and dynamics of its onset during voluntary 24-h fluid intake deprivation.	10 healthy men, age 21–30 y.	Open label 24 h	Testing occurred over 2 d, every 3 h throughout the days for a total of 7 time points a day. During the first 24 h, subjects were allowed to drink freely. From 25–48 h, subjects were not allowed to drink. Daily water intake in food was 500 g. The environmental conditions and diet were controlled.	Speed, accuracy, and mental endurance decreased after 3 h of fluid deprivation. Stability decreased after 9 h of fluid deprivation Energy decreased after 15 h of fluid deprivation. No other effect on mood.
Szinnai et al., 2005 [59]	Determine the effect of slowly progressive dehydration on mental performance	8 healthy women (age 21–34 y) and 8 healthy men (age 20–34 y)	Crossover RCT	During the dehydration arm, subjects abstained from fluid intake for 28 h. During the control arm, subjects were allowed fluid ad libitum.	Urine osmolality increased during dehydration (2.45% body mass loss). Stroop test word naming (verbal response time) revealed significant dehydration-gender interactions, with slower response time in females, but faster response time in males with dehydration. No significant effect of dehydration or sex on visual attention, cognitive-motor speed, sustained attention, and divided attention). No difference with auditory event-related potentials P300. Subjective rating was greater for tiredness and lower for alertness with dehydration.
Shirreffs et al., 2004 [60]	Investigate the physiological responses and subjective feelings resulting from fluid restriction over 37 h compared to euhydration	15 subjects (40% F) average age 30 y	Crossover RCT	Subjects randomized to different hydration conditions for up to 37 h: Fluid Restriction: restrain from all fluid intake and ingest only foods that have low water content. Euhydration: consume normal diet and beverage intake.	Fluid restriction: water from food, 487 ± 335 mL; urinary loss, 1.37 ± 0.39 L; body mass decrease, 2.7 ± 0.6% at 37 h. Subjects reported decreased ability to concentrate, and decreased alertness, and more headaches. Euhydration: water intake, 3168 ± 1167 mL; urinary loss, 2.76 ± 1.11 L.
Neave et al., 2001 [61]	Assess dehydration within normal physiological levels on mood and cognition.	24 generally healthy adults (50% F) average age 20.1 y	Crossover	Subjects did not eat or drink anything from midnight until testing the next morning. Compared 300 mL water vs. no intake (9–11 h no water intake).	Fasting was ~9 h with testing over 2 h. Sustained attention and working memory were not affected by water intake. No water intake negatively affected calmness and alertness, measured using Bond–Lader.
Rogers et al., 2001 [62]	Assessing no drinking vs. 120 mL or 330 mL of water intake	60 adults (50% F), average age 26 y	Parallel RCT	Subjects performed cognition task after acute consumption of the following: Nothing 120 mL water 330 mL water Background diets were customary with testing at 11 a.m. or 3 p.m.	Improved attention with acute fluid intake Increased alertness at 2 min, but not after 25 min. No effect on ratings for revitalization.
Gopinathan et al., 1988 [63]	Determine the effects of various degrees of dehydration on mental performance	11 healthy soldiers (age 20–25 y)	Crossover RCT	Subjects performed moderate work for 2 h under hot and humid conditions (30% relative humidity, 45 °C). Water was restricted during work to induce four different dehydration states: −1, −2, −3, and −4% body mass.	Short-term memory, arithmetic efficiency, and motor speed and attention deteriorated with increased dehydration, starting at 2% body mass loss.
Sharma et al., 1986 [64]	Investigate the effect of primary dehydration of various levels (1, 2 and 3% body-weight deficits) on mental functions in heat acclimatized subjects drawn from tropical regions of India	8 healthy men (age 21–24 y)	Crossover RCT	Subjects performed moderate work under hot and dry conditions (60% relative humidity, 45 °C) or hot and humid conditions (30% relative humidity, 45 °C) until they reached their target dehydration states: 0 (water replenished), −1, −2, and −3% body weight. Cognition tests were performed after 90 min rest in neutral conditions (27 °C, 50% relative humidity).	Symbol classification was not affected by dehydration. Concentration, memory, and perceptual motor coordination decreased at 2% and 3% body mass loss, compared to 0% dehydration.
Headache
Spigt et al., 2012 [65]	Investigate the effects of increased water intake on headache	102 adults who had at least two episodes of moderately intense headache or at least five mildly intense episodes per month and a total fluid intake of less than 2.5 L/day. Control group: average age 45 y; water group: average age 47 y.	Parallel RCT	Both groups received written instructions about stress reduction and sleep improvement strategies. Group 1: Also instructed to consume an additional 1.5 L water/d (n = 52, 16/36 M/F). Group 2: No additional intervention (n = 50, 13/37 M/F). 3 month intervention.	Subjects who drank more water reported better migraine specific quality of life. 47% in the intervention (water) group self-reported improvement against 25% in controls. Drinking more water did not result in relevant changes in objective effect parameters, such as days with at least moderate headache or days with medication use
Spigt et al., 2005 [66]	Explore whether there could be a positive effect of increased water intake in headache patients	15 adults who frequently (once a week or more) suffered from migraine or tension-type headache, mean age 44 y	Parallel RCT	Group 1: Instructed to consume an additional 1.5 L water/d (n = 8). Group 2: Received placebo tablet (n = 7). 3 month intervention.	Additional water consumption decreased total number of hours of headache and headache intensity, but effects were not statistically significant.

Abbreviations: C, Celsius; d, day; EFSA, European Food Safety Authority; F, female; g, grams; h, hours; IOM, Institute of Medicine; kg, kilogram; L, liter; M, male; min, minute; mL, milliliter; MRI, magnetic reasonance imaging; n, sample size; oz, ounces; POMS, Profile of Mood States; RCT, randomized clinical trial; Uosm, urine osmolality; VAS, Visual Analogue Scale; y, years. ¹ Intervention trials published since inception through April 2018.