Table 1.
Summary of research studies evaluating effects hydration levels on cognitive performance
| Citation | Participants | Protocol | Hydration loss levels (% Δ body mass |
Fluid Type | Key Findings | Limitations | ||
| Physiological and subjective measures |
Cognitive domains assessed |
Hyperthermia and Hypohydration effects on cognition |
||||||
| Goodman et al. (29) |
N=15 12M, 3 Gender not revealed Military Defence Force 21–34yrs USA |
Crossover designed, 90min self- paced military march in standardised military attire in the heat with a 20kg backpack Fluid restriction and or prescribed fluid intake throughout the exercise. Cognitive test battery Envir. Conditions: 39.5–41.8°C, 28–42% RH |
2.28 (HYP trial- no fluid) 0.53 (EUY trial – Ad libitum fluid) |
Ad libitum water |
HYP: ↑Core body temp. ↑HR, ↑RPE, ↑Thirst. EUY: No significant difference in core body temp, perceived thirst |
Information processing, memory, impulsivity, attention, and concentration, response time domains |
HYP: ↓working memory ↓response times ↓attention task ↓depression ↓accuracy No significant effect on immediate or delayed memory, accuracy, and response speed. |
Participants not blinded to hydration status |
| Wittbrodt et al. (30) |
N=13 7M, 6F 19–28yrs Healthy recreationally active adults USA |
2-week; Counterbalanced 150min trial (intermittent exercise protocol): Three experimental; no exercise heat stress (CON), exercise heat seat stress with fluid replacement (EUY), exercise heat stress with dehydration (DEHY), exercise heat stress without fluid replacement (HYP) Visuomotor Pacing Task (VMPT) Envir. Conditions: EUY, HYD, HYP : 45°C, 15% RH CON: 22 °C, 30% RH |
3.1 DEHY, HYP (3.3 men 3.1 women) 0.2 EUY 0.0. CON |
EUY, DEHY: water equivalent to sweat HYP: No fluid, only mouth rinse once per hour |
↑HR, ↑Rectal temp., RPE, Thirst |
Visuomotor functioning, Accuracy, reaction time |
EUY, DEHY: Visuomotor performance impaired A significant effect on processing accuracy, and reaction time |
Participants not blinded to hydration status |
| MacLeod et al. (1) |
N=8F 19–22yrs Healthy unacclimatized elite hockey players UK |
4 experimental sessions: 50min Hockey intermittent treadmill protocol with prescribed fluid intake to replace sweat loss; ad libitum water intake, or no fluid Cognitive testing after treadmill protocol Envir. conditions: Hot; 33.2–33.4°C, 58–60% RH Moderate; 13–19°C, 51–55% RH |
HYP: ∼2 no fluid EUY: ∼ 0.0 No difference in ad water intake on moderate temp. |
Ad libitum water |
↑RPE ↑Thirst (HYP) prior to treadmill protocol No significant effect on HR and Temp (body core) |
Process speed, working memory, perceptive discrimination, visual scanning/ processing speed |
HYP: ↑Psychomotor function, visual scanning/ process speed EUY; ↑ working memory |
Participants not blinded to hydration status |
| Piil et al. (13) | N=139 139M 30–32yrs Recreationally active Cyprus, Denmark, Greece, Spain (Compiled in Greece) |
Laboratory experiment: (EUY, DEHY) Occupational study (urine sampling), 8M for laboratory experiment in an environmental chamber with fluid replacement Motor-cognitive test battery pre- & post- Envir conditions: Manufacturing; 29°C, 25% H Agriculture; 29 °C, 55% RH Police officers; 27 °C, 50% RH Tourism; 30 °C, 55% RH Construction; 26 °C, 54% RH Environmental chamber; 40 °C, 25% RH |
∼ 2.0 (no fluid) 0.0 (fluid replacement) |
Water | ↑RPE, ↑Core body temp., ↑Thermal comfort, ↑Thirst ↑HR |
Process speed, working visual scanning/ processing speed |
No significant effect on cognitive domains |
Participants unaware of the researcher's hypothesis and naive to the purpose of the studies |
| Van den Heuvel et al. (31) |
N=17 17M 25yrs Healthy, non-smoking Australia |
Three Passive thermal-hydration protocol (water immersion) with states and then clamped using controlled, isotonic fluid administration. Unique immersion protocol establishment in the first trial and replicated in subsequent trials averaging 185min (137–242min) Envir. Conditions: Temperate; 34–35 °C, Warm water; 40–41 °C |
3 and 5 (HYP) 0.0 (EUY trial) |
sodium chloride NaCI+ |
↑HR, Thermal state, Core body temp., in HYP at 3% and 5% |
Visual perception, working memory |
↑Decision process modified ↓Depression ↓Discriminative ability (hyperthermia) No significant effect visual and working memory following 3–5% dehydration |
Participants not blinded to hydration status |
| Gamage et al. (32) |
N=30 30M 22yrs, elite cricketers UK |
Fluid restriction(4ml/kg/h) or fluid provision (12–15 ml/kg/h) during 2h of standardised cricket training Envir Conditions: Outdoors: 27.2–32.8 -°C89, R66H, ∼2mph wind speed |
3.7 fluid restriction trial 0.9 fluid provision trial |
Not reported | Not reported | Process speed, working memory, perceptive discrimination, visual scanning/ processing speed |
Not reported | Participants not blinded to hydration status Fluid type unknown No validity or reliability testing of sport (cricket) skill |
| Wittbrodt et al. (33) |
N=12 12M Recreational active USA |
Vigorous exercise intensity for 50mins Fluid assimilation time >50min Envir Conditions: Ambient temp 32°C, 65 RH |
1.5 | Water | ↑ HR ↑altered skin temp. ↑ Thirst, ↑ fatigue |
Process speed, working memory, perceptive discrimination, visual scanning/ processing speed |
No effect | Participants not blinded to hydration status Exercise intensity not mentioned |
| Wilson et al. (34) | N=8 8M Licensed jockeys UK |
Exercise for 45 minutes Fluid assimilation time ∼35min |
1.8 | Water | Not reported | Response inhibition |
No effect | Participants not blinded to hydration status |
| Owen et al. (35) | N=13 13M 22 yr olds, soccer semi- professional players UK |
LIST protocol (90mims) with prescribed fluid intake to replace 89 sweat loss; ad libitum, water intake, or no fluid LSST and LLSPT performed after LIST protocol Envir Conditions: 19.4°C, 59.4 RH |
0.3 (water intake) 1.1 (ad libitum water) 2.5 (no fluid) |
Ad libitum water |
↑ RPE (no fluid than water intake) ↑HR (no fluid than water intake and ad libitum water) |
Process speed, working memory, perceptive discrimination, visual scanning/processing speed |
No effect | Participants were not blinded to hydration status |
| MacLeod et al. (3) |
N=8 8F 22yr olds, Elite field hockey players UK |
2-day experiment Day 1: Baseline hockey skill measurement Passive heat stress (39.9 °C, 73 RH) → controlled fluid intake to induce HYPO or EUH Day 2: 60 min-hockey imitated and designed intermittent treadmill protocol Hockey skills test in a gymnasium Envir Conditions: Treadmill protocol; 33.3 °C, 59 RH Gym 16.3 °C 22.2 °C, |
∼ 2 (HYP trial) Day 2: ∼ 0 (EUY trial) No difference in fluid intake Replacement fluid loss (88 vs 80) % |
Ad libitum water |
↑RPE and ↑Thirst (HYPO) prior to treadmill protocol No significant effect on HR and Temp (body core) |
Process speed, working memory, perceptive discrimination, visual scanning/ processing speed |
↓ decision making time (skills test) ∼7 slower (HYP vs EUY) prior to treadmill protocol No significant effect on decision making time post treadmill protocol |
Protocol, not field sportspecific but intermittent treadmill protocol Use of Day 1 passive heat stress for Day 2 trials may be invalid Participants not blinded to hydration status |
| Hoffman et al. (36) |
N=10 10F 21 yr division 1 college Basketball player |
USA 40 min live scrimmage exercise Quick board lower body reaction agility, Dynavision D2- visual reaction time – all performed prior and post live scrimmage Envir Conditions: Indoors 22.6°C, 50.9 RH |
2.3 no fluid) Not availed (water intake) |
Water | No significant effect on HR and player load |
Psychomotor function/process speed, visual scanning/processing speed |
No significant effect on visual reaction time |
Participants not blinded to hydration status No trial report for Δ body mass during water intake Cognitive tests not validated prior |
| Brandenburg & Gaetz (37) |
N=12 12F 24yr Basketball Elite players Canada |
A descriptive study covering 2 international indoor matches Envir Conditions: 22.5 – 23.5 °C 44–50 RH |
1st match -2.1 to +5 2nd match -2 to +0.1 |
Diluted ad libitum and water according to individual taste |
↑ HR | Process speed, working memory, perceptive discrimination, visual scanning/processing speed |
No significant effect on field goal percentage Adverse relation (goal vs body mass loss in the 2nd match |
Carbohydrate has the confounding potential effect on Goal percentage No controlled trial (EUY) |
| Ely et al. (38) | N=32 32M Healthy and non-heat |
3-week experiment EUY and HYP trials, 3h work-rest cycle, |
4 | Sodium chloride (NaCI) + water |
HYP (no fluid replacement) |
Psychomotor function/process |
No significant effect on mood and cognition |
Carbohydrate ingestion may have confounding |
| Carvalho et al. (39) |
N=12 12M 14–15yr Basketball national team players Portugal |
90 min training session HYP trials Basketball drills before and after training Envir Conditions: Indoors; 21.9–26.0 -°C 5,4 4.18 .3 RH |
2.5 (no fluid) 1.1 fluid intake) |
Ad libitum water |
HYP trial: ↑RPE in |
Process speed, working memory, perceptive discrimination, visual scanning/processing speed |
Not availed | Participants not blinded to hydration status EUY (control) trial not available Basketball drill not validated prior |
| Ali et al. (40) | N=10 10F Soccer Premier division players New Zealand |
90min LIST protocol with fluid intake (15ml/kg) or without LSPT performed before, during, and after LIST Envir Conditions Not availed |
2.2 (HYP) 1.0 (EUY) |
Water | HYP trial: ↑RPE, core temperature, HR, blood lactate |
Processing speed, perceptive discrimination, visual scanning |
No significant effect; perceived activation and (dis- pleasure) |
Participants not blinded to hydration status EUY (control) trial not available |
| Giano et al. (17) | N=24 24M Physically fit USA |
3-day laboratory experiment. DEHY + Diuretic DEHY + Placebo EUY + Placebo Envir Conditions: 26.1–27.9 °C, 54 Wind speed |
1.59 | Water | HYP trial: ↑RPE, core temp, HR |
Process speed, working memory, perceptive discrimination, visual scanning/ processing speed |
↑Processing speed and working memory ↓Fatigue |
Participants were not blinded to hydration status |
| Bandelow et al. (21) |
N=20 20M University soccer players UK |
Cognitive battery tests: Sternberg test Corsi block test, Finger tapping test Visual sensitivity test Trials before, at half-time, after the match HYP Envir Conditions: 34°C, 62 -65 RH |
2.5 | Ad libitum water Sports drink |
Not reported | Process speed, working memory, perceptive discrimination, visual scanning/ processing speed |
↓working memory (HYP) No significant in fine motor speed, working memory, reaction time |
EUY trial not available (control) No sport-specific cognitive tests |
| D'Anci et al. (16) a |
N=31 16M; 15F University lacrosse and rowing athletes USA |
Study 1: HYP trial, EUY trial Coach-run, hard natural practice Cognitive test battery post-practice Envir Conditions; RH not stated Assimilating time 60–70min |
2.0 (HYP) 0.1 (EUY) |
Water | HYP trial: ↑Thirst, ↑POMS: tension, anger, fatigue, depression ↓ vigor |
Vigilance attention, shortterm memory, simple and choice reaction, map planning, visual perception, mathematical addition, mood |
HYP: ↑Processing speed ↓Vigilance, depression (3–4%) No effect on spatial memory, reaction time, map planning, mathematical addition |
Participants were not blinded to hydration status |
| D'Anci et al. (16) b |
N=24 12M; 12F University lacrosse, rowing, and American football athletes USA |
Study 2: HYP trial, EUY trial Coach-run, hard natural practice Cognitive test battery postpractice Envir Conditions RH not stated Assimilating time 60–75min |
1.7 (HYP) +0.1 (EUY) |
Water | HYP trial: ↑Thirst, ↑POMS: tension, anger, fatigue, depression ↓ vigor |
Vigilance attention, short-term memory, simple and choice reaction, map planning, visual perception, mathematical addition, mood |
No effect on short-term and spatial memory, reaction time, map planning, mathematical addition |
No sport-specific cognitive tests administered |
| Adam et al. (20) | N=8 8M Active soldiers (6) USA |
Heat exposure for 300 min Envir Conditions: 20 °C, 50% RH Wind speed 1 to 2.2 m/s |
3.0 | No fluid | ↑Thirst, thermal discomfort ↑altered skin temp. ↑fatigue ↑HR |
Processing speed, working memory, perceptive discrimination, vigilance, visual scanning |
No significant effect on cognitive domains |
EUY trial not available (control) |
| Baker et al. (41) | N=11 11M 17–28yr male competitive basketball players USA |
Experimental: 3hr interval walking in heat chamber; HYP trials, EUY trials, 80 min stimulated match Attention variables test: baseline, post chamber, post-match Envir Conditions: 40 °C, 20% RH (heat chamber), room temp. (indoor match) |
HYP: 1%, 2%, 3%, 4% EUY: 0 |
No fluid Flavoured water |
HYP trial (1–4%): ↑lightheaded, overheat, fatigue No effect of core body temp. |
Attention variables, perceptive discrimination, vigilance, visual scanning |
HYP trial (1–4%): ↑ commission and omission errors ↓response time (6–8%) |
Participants were not blinded to hydration status Rationale of induced heat stress to attention variables test before a basketball match unrealistic |
| Edwards et al. (42) |
N=11 11M moderately active soccer players New Zealand |
90 min exercise: 45 min cycling, 45 min soccer match (80 fluid loss replacement) Post-match mental concentration test (number identification) Envir Conditions: 24- 25 -°C 5,5 4 R7H (cycling), 19- 21 -°C57, 4R6H (soccer match) |
0.7 (fluid intake) 2.1 (mouth rinse) 2.4 (no fluid) |
Water mouth rinse No fluid |
↑ HR ↑thermal discomfort ↑altered skin temp. ↑ Thirst, ↑ fatigue |
Processing speed, visual scanning |
No significant effect on mental concentration |
Participants were not blinded to hydration status The rationale of cycling before a match in soccer is doubtful |
| Serwah & Marino (43) |
N=8 8M 25yrs Healthy volunteers Australia |
90min discontinuous fixedintensity exercise: 3 experimental conditions (full fluid replacement, half fluid replacement, no fluid Own bicycle mounted on the electromagnetically braked cycle trainer Envir. Conditions: 31.3°C, 62.1–64.5% RH Wind speed 2m/s |
2.0 (full fluid) 1.0 (half fluid) 1.7 (no fluid) |
Water | No fluid: ↑HR ↑Skin temp., ↑Thirst No effect of core body temp. in full and half fluid conditions |
Processing speed, working memory, perceptive discrimination, vigilance, visual scanning |
No significant effect on cognitive domains |
Participants were not blinded to hydration status No sport can employ a discontinuous fixed-intensity nature of exercise protocol |
| Szinnai et al (44) | N=17 8M 7F 25–33yrs Health non-smoking volunteers Switzerland |
Experimental done in random order EXP; CON Female: Pre and post menstrual Men: Cognitive function test Envir. Conditions: Cognitive tests: 22°C |
1.75 CON 3.26 DEHY |
Mineral water | No fluid: ↑HR ↑Fatigue ↑Thirst ↓Alertness No significant effect in the control group |
Processing speed, working memory, perceptive discrimination, vigilance, visual scanning, reaction time |
No significant effect on cognitive domains in moderate dehydration |
Participants were not blinded to hydration status |
EUY- euhydration, HYD-Hydrated, DEHY- dehydration, HYP-hypohydration, CON- control, Envir. Conditions- environmental conditions, RH. relative humidity, HR- heart rate, N- Number, M- male, F-female, LIST- Loughborough intermittent shuttle test, LSPT-Loughborough soccer passing test, RPE-rate of perceived exertion, Temp- temperature, ↑- increase, ↓- decrease