The Effect of Fluid Intake Following Dehydration on Subsequent Athletic and Cognitive Performance: a Systematic Review and Meta-analysis
McCartney et al. Sports Medicine - Open
The Effect of Fluid Intake Following Dehydration on Subsequent Athletic and Cognitive Performance: a Systematic Review and Meta-analysis
Danielle McCartney 0
Ben Desbrow 0
Christopher Irwin 0
0 School of Allied Health Sciences and Menzies Health Institute Queensland, Griffith University , Gold Coast , Australia
Background: The deleterious effects of dehydration on athletic and cognitive performance have been well documented. As such, dehydrated individuals are advised to consume fluid in volumes equivalent to 1.25 to 1.5 L kg−1 body mass (BM) lost to restore body water content. However, individuals undertaking subsequent activity may have limited time to consume fluid. Within this context, the impact of fluid intake practices is unclear. This systematic review investigated the effect of fluid consumption following a period of dehydration on subsequent athletic and cognitive performance. Methods: PubMed (MEDLINE), Web of Science (via Thomas Reuters) and Scopus databases were searched for articles reporting on athletic (categorized as: continuous, intermittent, resistance, sport-specific and balance exercise) or cognitive performance following dehydration of participants under control (no fluid) and intervention (fluid intake) conditions. Meta-analytic procedures determined intervention efficacy for continuous exercise performance. Results: Sixty-four trials (n = 643 participants) derived from 42 publications were reviewed. Dehydration decreased BM by 1.3-4.2%, and fluid intake was equivalent to 0.4-1.55 L kg−1 BM lost. Fluid intake significantly improved continuous exercise performance (22 trials), Hedges' g = 0.46, 95% CI 0.32, 0.61. Improvement was greatest when exercise was performed in hotter environments and over longer durations. The volume or timing of fluid consumption did not influence the magnitude of this effect. Evidence indicating a benefit of fluid intake on intermittent (10 trials), resistance (9 trials), sport-specific (6 trials) and balance (2 trials) exercise and on cognitive performance (15 trials) was less apparent and requires further elucidation. Conclusions: Fluid consumption following dehydration may improve continuous exercise performance under heat stress conditions, even when the body water deficit is modest and fluid intake is inadequate for complete rehydration.
Athletic; Cognitive; Performance; Mood; Dehydration; Fluid intake
2. The magnitude of improvement was greater when
the continuous exercise was performed at elevated
environmental temperatures and over longer
3. Whilst the volume of fluid consumed (relative to
BM lost) did not appear to influence the size of the
treatment effect, fluid intake at levels that comply
with current recommendations for restoring body
water content (1.25–1.50 L kg BM lost−1) are yet to
be thoroughly investigated.
The deleterious effects of dehydration (fluid loss) on
athletic and cognitive performance have been extensively
researched. Recent meta-analyses detected meaningful
decrements in aerobic  and anaerobic  exercise
performance and muscular strength and endurance 
when subjects commenced activity in an already
dehydrated state. Experimental investigations have also
demonstrated motor-skill impairments on sport-specific
exercise tests (e.g. cricket , basketball [4, 5], golf ,
field hockey  and surfing ) following fluid loss.
Whilst evidence indicating a detrimental effect of
dehydration on cognitive function is less consistent ,
decline in memory, perceptual discrimination and mood
state has been observed in some studies .
Dehydration is commonly observed amongst athletes [11–14]
and manual workers (e.g. military, fire fighters and
labourers) , who rely upon physical and mental
proficiencies to compete or train at elite levels and remain
productive in the workforce. This evidence has provided
the rationale for fluid replacement recommendations.
The American College of Sports Medicine (ACSM)
Guidelines on Exercise and Fluid Replacement  and
the Position of the Academy of Nutrition and Dietetics
on Nutrition and Athletic Performance  recommend
dehydrated individuals consume 1.25 to 1.50 L of fluid
per kilogram of body mass (BM) lost to replenish body
water content, if the fluid deficit is large and recovery
time is limited (i.e. <12 h). Whilst the importance of
returning to euhydration over a period of a day(s) is not
in dispute, many individuals are required to undertake
repeated bouts of activity, where limited time between
tasks exists or the demands of a subsequent activity (i.e.
type, duration and intensity) and/or the environment
(e.g. conflict zone) may influence the appropriateness of
the aforementioned guidelines. Within this context,
consuming fluid has the potential to enhance or inhibit
performance. Thus, determining rehydration strategies
that counteract the detrimental effects of fluid loss,
whilst optimizing performance on subsequent tasks, is
Ingesting large volumes of fluid may cause
gastrointestinal (GI) discomfort, impeding performance.
Particularly if the amount of time available to consume fluid is
limited or fluids with higher calorie loads (e.g.
milkbased beverages) and hence slower rates of gastric
emptying are ingested [18, 19]. The nature of the
subsequent activity, e.g. the mechanical ‘bouncing’ action
caused by high intensity running, may also impact GI
symptomology . Conversely, drinking large fluid
volumes promotes rapid initial gastric emptying ,
facilitating fluid absorption, and may convey greater
benefit than drinking smaller volumes. To date, the
majority of investigations examining the effect of ingested
fluid volume on subsequent performance have employed
a prolonged (i.e. overnight) rehydration period [21–25],
reducing the probability of GI disturbance influencing
subsequent performance. Thus, the importance of
ingested fluid volume and its impact on subsequent
exercise performance outcomes remains unclear. The
aim of the present systematic review and meta-analysis
was to examine the impact of consuming fluid following
a period of dehydrating sweat loss on subsequent
athletic and cognitive performance. Understanding how
to maximize the benefits of fluid intake under these
circumstances will inform the development of future
fluid replacement guidelines.
The following research protocol was devised in
accordance with specifications outlined in the Preferred
Reporting Items for Systematic Reviews and Meta-Analysis
Protocols PRISMA-P 2015 Statement . The
methodology of this review is registered at the International
Prospective Register for Systematic Reviews,
identification code CRD42016036560.
Potential research studies were identified by searching
the online databases PubMed (MEDLINE), Web of
Science (via Thomas Reuters) and Scopus from
inception until April 2016 using the terms exercise, athletic,
performance, mood and cognit* (the symbol was used to
capture all words beginning with cognit, e.g. cognitive,
cognition), each in combination with “fluid replacement”
(the enclosed quotation marks were used to search for
an exact phrase), “fluid ingestion”, “fluid intake”, “fluid
consumption”, “fluid administration”, rehydrat* and
euhydrat*. Records that contained irrelevant terms
(patient, rat, mouse, aged care, reaction, disease, illness,
bacteria, children and elderly) were excluded from the
literature search using the Boolean search operator
‘NOT’. Two investigators (D.M. and C.I.) independently
screened potential research studies to identify relevant
texts. Full details of the screening process are presented
in Fig. 1. Initially, all irrelevant titles were discarded. The
remaining studies were systematically screened for
eligibility by abstract and full text, respectively. The final
decision to include or discard research studies was made
between two investigators (D.M. and C.I.), with any
disagreement resolved in consultation with a third
investigator (B.D.). The reference lists of all included studies
were then hand searched for missing publications.
Inclusion and Exclusion Criteria
Research studies containing a control-arm and one or
more intervention-arms fulfilling the following criteria
were eligible for inclusion:
1) Repeated measures experimental design.
2) Human studies on adult (≥18 years of age) male
or female participants with no known medical
conditions or co morbidities.
3) An athletic or cognitive performance outcome
(see “Primary and Secondary Research Outcomes”
section below for full description) was measured
under control and intervention conditions. The
control condition was dehydration with no fluid or
negligible fluid intake, where ‘negligible’ fluid intake
was accepted as ≤200 mL. This threshold was
intended to broaden the inclusion criteria, allowing
greater data capture and increased statistical power,
since this was the first review to examine the effect
of fluid intake on subsequent athletic and cognitive
performance. The intervention condition was
defined as dehydration with concurrent and/or
subsequent fluid intake >200 mL.
4) The mode of dehydration was standardized, i.e. all
participants were subjected to the same dehydration
protocol, with or without fluid intake, on
intervention and control trials.
5) Hydration status was manipulated before the
performance task commenced, i.e. dehydration and
fluid ingestion occurred before, not during, the
performance assessment. A schematic representation
of the experimental protocol is displayed in Fig. 2.
6) There is ‘limited’ time to consume fluid, defined as:
≤4 h between completing the dehydration protocol
and commencing the subsequent performance
test, unless performance followed an overnight
7) An objective measurement of hydration status (e.g.
body weight, urine specific gravity, plasma or urine
osmolality or plasma volume) was used to indicate
the level of dehydration attained.
8) Accessible full text articles written in English.
Studies were excluded from the review if: (1)
dehydration involved restriction of food intake; (2) fluids were
not administered orally (e.g. intravenous infusions) or
(3) were co-administered with another experimental
treatment (e.g. glycerol, L-alanyl-L-glutamine or external
cooling); (4) subjects ingested >200 mL of fluid or an
unspecified volume of fluid on control trials (e.g. Bardis
et al.  and Baker et al. ); (5) macronutrient intake
was not matched on experimental trials or (6)
performance data was not adequately reported, i.e. values
were not quantified, or descriptive terms were not
For the purpose of this systematic review, research
studies containing multiple intervention-arms that were
eligible for inclusion (each paired against a suitable ‘no
fluid’ control group) (e.g. McConell et al.  tested
athletic performance under two different fluid conditions
and Hillman et al.  tested athletic performance under
different environmental conditions) were treated as
separate experimental studies termed ‘trials’. Separate
trials derived from a single research study are denoted
by additional letters (i.e. a–d) in the citation.
Methodological Quality Assessment
All eligible studies were examined for publication bias
using the Rosendal Scale . Excellent methodological
quality is indicated by a Rosendal Score ≥60% .
Items 7, 8 and 9 of the scale, pertaining to the use of
blinding procedures, were omitted from the evaluation
as oral fluid ingestion cannot be blinded. Scoring was
determined by dividing the number of ‘yes’ responses by
the total number of applicable items and reported for all
included studies. Studies were excluded from
metaanalyses if they received a Rosendal score <50%.
Data Extraction and Synthesis
Data were extracted from relevant publications following
the Cochrane Handbook for Systematic Reviews of
Interventions Checklist of Items to Consider in Data
Collection or Data Extraction  and entered into a
Microsoft Excel spread sheet.
Primary and Secondary Research Outcomes
The first primary research outcome was (1) objective
indicators of athletic performance; subjective
measurements of performance (e.g. ratings of perceived exertion)
were not examined in this review. The types of athletic
performances studied were broadly classified as follows:
(a) continuous exercise; (b) intermittent exercise; (c)
resistance exercise; (d) sport-specific exercise and (e)
balance tasks. Performances that incorporated a
coordinated motor-movement resembling some skill involved
in a particular sporting event were categorized as
‘sportspecific’ exercises, whereas non-specific sporting
activities (e.g. sprint running) were categorized into one of
the remaining groups (where possible). Where more
than one type of athletic performance was measured
within a single experimental trial (e.g. Walsh et al. 
examined performance on continuous and resistance
exercise tasks), the performances were presented in their
respective categories and treated as separate trials. The
second primary research outcome was (2) objective
indicators of cognitive function, including subjective
measurements of mood state. The decision to include
mood as a primary research outcome was based on
previous suggestions that mood and symptom
questionnaires may be more sensitive to subtle changes in hydration
status than tests of cognitive ability . Subjective ratings
of GI discomfort and thirst following fluid ingestion were
intended as the secondary research outcomes. However,
very few investigations evaluated GI symptomology  or
thirst [35–37]. Thus, insufficient data were available to
complete secondary analyses.
Other Relevant Data
Other information extracted from relevant research
1) Participant characteristics: description, age,
euhydrated body mass (BM) and maximal oxygen
consumption (VO2 max)
2) The dehydration protocol: mode of dehydration,
ambient temperature and relative humidity,
protocol duration, level of dehydration and time
from finishing the dehydration protocol to
3) The rehydration protocol: fluid type, volume of fluid
consumed, drink time and time from finishing fluid
to commencing performance task
4) Performance task: task description and performance
outcomes, ambient temperature, relative humidity,
rate of airflow, intensity and duration, where
Percentage of BM loss was used to indicate the level of
dehydration attained. If the percentage of BM loss was not
directly reported, values were calculated from euhydrated
BM (kg) and BM mass loss (kg) using the following
BM loss ðkgÞ
% BM lost ¼ Euhydrated BM ðkgÞ
The volume of fluid consumed was expressed as a
proportion of BM loss, i.e. relative fluid intake (L kg BM
lost−1). If the quantity of fluid consumed was not
expressed as a proportion of BM loss, values were
calculated from fluid intake (L) and percentage of BM loss
using the following formula:
Fluid intake ðLÞ
L⋅kg BM lost‐1 ¼ ð% BM loss 0:01Þ euhydrated BM ðkgÞ
If the volume of fluid consumed was unknown, the
BM deficit post-rehydration has been reported.
Time from completing the dehydration protocol to
commencing the subsequent performance task (recovery
time) and time from commencing fluid ingestion to
commencing the subsequent performance task (fluid
assimilation time) were approximated from the
experimental protocol, where adequate information was
If necessary information was not available from the
published article and it was published within the
previous 10 years, authors were contacted via email with a
request to provide missing data.
Sufficient data were available to perform a meta-analysis
examining the impact of fluid consumption following a
period of dehydration on subsequent continuous
exercise performance. Meta-analyses were not performed on
other types of athletic performance or cognitive function
because: (1) intermittent and sport-specific exercise
performance trials were methodologically
heterogeneous, particularly in regards to the exercise protocol
and outcomes used to determine a treatment effect; (2)
few authors responded to an email request for raw data
regarding resistance exercise performance, preventing
computation of the correlation coefficient and (3)
cognitive performance data was rarely quantified (descriptive
Meta-analysis on Continuous Exercise Performance
All statistical procedures were performed using IBM
SPSS Statistical Software, Version 22.0 and
Comprehensive Meta-Analysis, Version 3.0. Repeated measures
intervention effect sizes were calculated as Hedges’ g
, where the mean difference between each
intervention and control performance score was standardized
against the SD of the performance change and corrected
for bias due to small sample size. The magnitude of
effect was defined in accordance with Cohen : ES ≤0.2
= small; ≥0.5 ES >0.2 = medium and ≥0.8 = large, where
a positive Hedges’ g value indicates a beneficial effect of
fluid intake on continuous exercise performance. Where
the SD of the performance change was not reported, the
missing value was imputed using a correlation
coefficient  calculated with the following formula:
SDΔ ¼ SD2No Fluid þ SD2Fluid 2 R SDNo Fluid SDFluid
Where SDΔ is the missing standard deviation of
change and R is the correlation coefficient. R was
approximated as the mean correlation coefficient (R = 0.84)
calculated using raw performance data from nine
continuous exercise trials (derived from four separate
publications). Sensitivity analysis was performed using R =
0.50, 0.74 and 0.94 to test the robustness of the imputed
correlation coefficient. The weighted mean treatment
effect was calculated using random-effect models, where
trials were weighted by the inverse variance for the
standardized performance change. Statistical significance
was attained if the 95% CI did not include zero. Data are
described as mean ± SD, unless otherwise indicated;
articles that reported SEM had their values multiplied by
the square root of the sample size to convert to SD. All
research studies measuring performance on a
continuous exercise task used a single objective measurement to
demonstrate the presence or absence of a treatment
effect (e.g. time to exhaustion or power output). Hence,
no additional precautions were taken to limit data
Heterogeneity and Sensitivity Analyses
Heterogeneity was assessed using Cochran’s Q and the
I2 index. Low, moderate and high heterogeneity was
indicated by an I2 value of 25, 50 and 75%, respectively
. A p value <0.10 for Cochran’s Q was used to
indicate significant heterogeneity . Sensitivity
analyses were performed by removing individual trials and
examining the effect of each study on the results of the
weighted mean treatment effect.
A priori, we identified the volume of fluid ingested (L kg−1
BM lost) and fluid assimilation time as variables that
might moderate the effect of fluid intake on athletic
performance. However, prior research indicates that
environmental temperature, exercise duration and the
ecological validity of the exercise protocol employed
may influence the effect of dehydration on athletic
performance [29, 41–44], as might level of fluid loss
(% BM loss) incurred. Therefore, we explored the
relationship between these variables and the magnitude
of the treatment effect (Hedges’ g) using a restricted
maximum likelihood multiple meta-regression (random
effects) model that controlled for potential confounders.
Restricted maximum likelihood simple meta-regression
was also performed to explore the influence of
environmental temperature on Hedges’ g values. The ecological
validity of each continuous exercise protocol was defined
in accordance with Goulet , where fixed-power time
to exhaustion (TTE) exercise protocols were considered
non-ecologically valid and time-trial type exercise
protocols (including protocols measuring work completed
within a set timeframe) were classified as ecologically
valid. Exercise duration was taken as the mean total
exercise time (min) for control and intervention trials.
One study did not report total exercise time , therefore
exercise duration was approximated as per Stewart et al.
, who performed a comparable performance test.
As per Savoie et al. , regression analyses were
examined for influential cases and outliers (studentized
residuals and cook’s distance). Tests for normality of
residuals (Shapiro-Wilk test), multicollinearity (variance
inflation factor, VIF), autocorrelation (Durbin-Watson
statistic), homoscedasticity and linearity of the
relationship between dependent and independent variables (plot
of residuals versus predicted values) ensured that
analyses did not violate assumptions of meta-regression.
Statistical significance was accepted as p < 0.05.
All athletic and cognitive performances are presented in
the systematic review investigating the effect of
dehydration and fluid intake on subsequent athletic and cognitive
performance. Whilst it was our intention to calculate
within-subject intervention effect sizes for all athletic
performance outcomes, the vast majority of the
publications included in this review did not provide the necessary
data to complete a paired analysis. Further, the types of
performances investigated varied widely amongst studies,
such that the missing SD of change could not be estimated
from a known correlation coefficient. To enable
comparison of effects across studies, ES were approximated as
Hedges’ g for independent groups. The mean difference
between each intervention and control performance score
was standardized against a pooled SD and corrected for
bias due to small sample size using the supplementary
spreadsheet by Lakens . This approach will likely
underestimate the magnitude of the true effect. Cognitive
performance outcomes are presented in descriptive terms
only, since few publications quantified the effect of fluid
intake numerically. Statistical significance was accepted as
p < 0.05 in all studies.
Overview of Studies and Study Quality
Sixty-four repeated measures trials (n = 643 healthy
participants, 93% male, excluding Del Coso et al.  where
gender was not specified, NS) derived from 42 original
publications were included in the present systematic
review. Methodological quality assessment yielded a
median Rosendal Score of 58%. Two trials received a
Rosendal Score <50% [47, 48]. Whilst these studies are
presented in the systematic review, they were not deemed
eligible for inclusion in subsequent meta-analysis. The
highest Rosendal Score of 83% was calculated for
Rodrigues et al. . Complete results of the quality
assessment are displayed in Additional file 1: Table S1.
Characteristics of included studies are summarized in
Tables 1, 2, 3, 4, 5 and 6. (Full details are presented in
Additional file 1: Table S2, S3, S4, S5, S6 and S7).
Dehydration and rehydration protocols were
heterogeneous amongst included trials. In 57 out of the 61 trials
reviewed, dehydration was accomplished via passive heat
exposure (n = 11) [42, 47, 50–56] or physical activity
(n = 47), conducted in a thermoneutral laboratory
≤25 °C (n = 16) [24, 25, 29, 35, 48, 55, 57–61], heated
environmental chamber (n = 28) [22, 29, 33, 36, 37,
41, 46, 49, 62–71] or environmental conditions not
specified (n = 3) [72, 73]. The remaining trials reduced
body water content through warm water immersion
(n = 2) [74, 75] or dietary fluid restriction in
combination with 2-h moderate intensity exercise 24 h prior
to testing performance (n = 2) [76, 77]. In 46 trials
(74%) [22, 24, 29, 36, 37, 41, 42, 46, 47, 49–53, 55–
59, 61–63, 66, 67, 71, 73–77], dehydration yielded
BM losses ≥2%. Of these, 27 trials (43%) dehydrated
participants by ≥3.0% of initial BM [22, 24, 29, 37,
41, 42, 46, 50, 52, 53, 56, 61, 62, 66, 67, 74–77].
Mean BM losses ranged from 1.3  to 4.2% .
Ingested fluids were predominantly water or saline
solution (0.05–0.50% NaCl). Studies administering
carbohydrate-containing fluids were often excluded
due to unequal provision of macronutrients on
control and intervention trials. Two included studies
failed to specify the type of fluid consumed (this was
presumed to be water) [42, 56], and one study did
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not report dietary standardization procedures or
specify whether subjects consumed food during the
prolonged (>12 h, overnight) period of intervention .
The volume of fluid administered ranged from 0.40
 to 1.55 L kg−1 BM lost . In 20 trials (33%),
participants ingested a volume of fluid to replace
<100% of sweat losses [24, 25, 33, 35, 47, 57–59, 62,
68, 70–72, 76, 77]. Only 2 trials [22, 65] provided a
volume of fluid that complied with current
recommendations for restoring fluid loss (1.25–1.50 L kg
BM lost−1) [16, 17]. In 14 trials, dehydrated control
subjects ingested a small volume of non-nutritive
fluid (≤200 mL) [42, 55, 56, 62, 63, 66, 71] or mouth
Forty-nine trials (n = 461, 95% male, excluding Del Coso
et al.  where gender was NS) examined the effect of
fluid intake on athletic performance tasks. Findings from
research studies that evaluated athletic performance are
summarized in Tables 1, 2, 3, 4 and 5. (Full details are
presented in Additional file 1: Table S2, S3, S4, S5 and S6).
Continuous Exercise Performance
Twenty-two trials (n = 170 subjects, 98% male) measured
the effect of fluid intake on continuous exercise
performance (Table 1). The majority of testing was
completed on well-trained individuals (mean VO2 max 57.5–
68.4 mL kg−1 min−1) [24, 25, 29, 33, 36, 47, 50, 62, 63,
76, 77] (n = 13 trials). In n = 11 trials, exercise was
performed in a warm or hot environment (30–40 °C), by
acclimated (n = 2) [63, 77] and unacclimated (n = 5) [29,
36, 47, 62, 76] subjects, where environmental adaptation
was specified. The remaining trials were completed
under thermoneutral (18–25 °C) [24, 25, 29, 37, 41, 42]
(n = 7) or cold (2–10 °C) [41, 42] (n = 2) conditions,
where ambient temperature was specified. Fluid intake
significantly improved continuous exercise performance
in 13 out of the 22 trials reviewed.
Meta-analyses and Meta-regression Analyses
Eighteen trials (n = 139 subjects, 97% male) were
included in the meta-analysis examining the effect of
fluid consumption on continuous exercise performance.
Four continuous exercise trials included in the
systematic review were omitted from the meta-analysis on the
basis of: (1) duration of the TTE performance test was
capped (n = 2) [36, 62]; (2) Rosendal score <50% (n = 1)
 and (3) extreme outlier, exceeding the mean effect
estimate by >3 SD with a studentized residual of 2.82
(n = 1) , with the results possibly confounded by
fatigue. In this study , untrained participants
completed 1-h dehydrating exercise at 32 °C before
commencing a TTE test at 80% VO2 max, without any recovery. All
other investigations completed on participants with a
VO2 max less than 50.0 mL kg−1 min−1, i.e. untrained or
physically active, employed passive methods of dehydration
or allocated ~2 h recovery post-active dehydration [41, 42].
Excluding this trial did not influence the results of the
meta-analysis (Hedges’ g = 0.48, 95% CI 0.33, 0.63), thus it
The weighted mean treatment effect summary
indicates fluid intake following a period of dehydration
significantly improved continuous exercise performance
(g = 0.46, 95% CI 0.32, 0.61) (Fig. 3). Data were normally
distributed (Shapiro-Wilk Test, p > 0.05). High
heterogeneity was evident between trials (I2 = 80.5, p < 0.01).
Subsequent analyses (see below) determined that 82% of
variation between trials is due to differences in the ambient
environmental temperature at which the exercise was
performed. Thus, sensitivity analysis was completed with
trials sub-grouped by environmental temperature, where
cold-thermoneutral = ≤25 °C and warm-hot = >25 °C. The
magnitude and statistical significance of the treatment
effect was stable during sensitivity analysis where trials were
sequentially removed, with Hedges’ g ranging between
0.29–0.35 and 0.70–0.81 for cold-thermoneutral and
warm-hot ambient temperature subgroups,
respectively (all ps <0.01). Findings are comparable across
different levels of correlation (R = 0.50, 0.74, 0.84 and
0.94), therefore the meta-analysis (and subsequent
meta-regression analyses) are robust to the imputed
R = 0.84 (full details of sensitivity analyses are
presented in Additional file 1: Table S8 and S9).
One continuous exercise trial  was excluded from
the simple meta-regression analysis to determine the
relationship between changes in ambient temperature and
the magnitude of the weighted mean effect after failing
to report environmental temperature at the time of
exercise performance. Analyses of the remaining 17 trials (n
= 129 subjects, 97% male) detected a strong significant
correlation (R2 = 0.82, p < 0.01) between these
parameters (Fig. 4). Therefore, fluid intake may enhance
continuous exercise performance to a greater extent at
increasing environmental temperatures.
The influence of environmental temperature, exercise
duration, ecological validity of the exercise protocol and
the level of dehydration were controlled in the modelling
of the relationship between the volume of fluid
consumed (L kg BM lost−1) and the magnitude of the
weighted mean effect (Fig. 5). The volume of fluid
administered ranged between 0.50–1.15 L kg BM lost−1.
No correlation was observed between these parameters
(p = 0.625) (Table 7). Mean exercise duration ranged
between 4 and 30 min, since no trials involving an exercise
task lasting >30 min were eligible for inclusion (as
outlined above). There was a trend for fluid intake to improve
performance to a greater extent with increasing exercise
duration (p = 0.071). The majority of trials (n = 12)
measured continuous exercise performance on an ecologically
valid exercise protocol, e.g. total work or power output
completed within a predefine timeframe (n = 10) [25, 29,
41, 42] or time to complete a set distance (n = 2) [37, 63].
The remaining trials (n = 5) employed a fixed-power TTE
exercise protocol with low ecological validity [24, 33, 76,
77]. No significant correlation was observed between the
ecological validity of exercise protocol employed and the
magnitude of the weighted mean effect (p = 0.188), or the
level of dehydration and the magnitude of the weighted
mean effect (p = 0.845).
One trial  failed to report time from
commencing fluid ingestion to beginning the subsequent
performance task and was excluded from the multiple
regression analysis of fluid assimilation time vs.
Hedges’ g. Modelling of this relationship corrected for
the influence of environmental temperature and type
of exercise protocol. Exercise duration was omitted
from the model due to collinearity with fluid
assimilation time (VIF = 3.18, where all other analyses
yielded values ≤1.7). Fluid assimilation time ranged
between 45 and 390 min. Analyses of the 16 eligible
trials (n = 121 subjects, 94% male) did not detect a
significant influence of fluid assimilation time on the
weighted mean effect (p = 0.11).
Ten trials (n = 95 male subjects) evaluated intermittent
exercise performance (Table 2). Exercise was undertaken
in hot (32–36 °C) [22, 65], thermoneutral (19–22 °C)
[51, 59] and cold (16 °C)  environmental conditions,
where ambient temperature was specified. The majority
of testing was done using team sport participants (i.e.
individuals who are accustomed to intermittent exercise)
[22, 58, 59, 71]. Participants in the remaining trials were
untrained , physically active  or endurance
cyclists , where the participant population was
defined. Fluid intake (0.8–1.55 L kg BM lost−1)
significantly improved intermittent exercise performance on 4 out
of the 10 trials [22, 58, 65, 71]. The magnitude of
improvement ranged from small to large (Hedges g = 0.19–0.97).
Nine trials (n = 83 subjects, 100% male, excluding Del Coso
et al.  where gender was NS) evaluated resistance
exercise performance (Table 3). Across the 8 trials reviewed, 22
separate performance tests were identified. The majority
were knee extension or elbow flexion exercise tasks, at
variable intensities (n = 18 tasks) [46, 49, 52, 53, 57, 66],
although 2 trials measured performance via repetition lifts
[54, 74]. Individuals who were accustomed to performing
resistance exercise were rarely studied [54, 74]. Fluid intake
(1.0–1.10 L kg BM lost−1) significantly improved
performance on 7 of 22 resistance exercise tasks completed across 5
trials (Hedges’ g = 0.22–5.57) [49, 53, 54, 60, 66, 75], and
significantly decreased performance on 1 task .
Six trials (n = 64 subjects, 84% male) evaluated athletic
performance on exercise tasks that were specific to
either cricket (n = 1) , soccer (n = 3) [35, 59], squash
(n = 1)  or racehorse riding (n = 1)  (Table 4). All
participants were experienced on the sporting activity
for which they were assessed. Fluid intake had no effect
on soccer players’ ball-skills (e.g. passing and shooting)
. However, squash-specific movements, cricket
bowling accuracy and racehorse riding demonstrated
moderate to large performance improvements with fluid
intake (0.4–1.0 L kg BM lost−1).
Two trials (n = 49 male subjects) examined balance
performance (Table 5). A significant positive effect of
fluid intake was documented for 1 out of 8
balancerelated tests completed across both trials.
Cognitive Performance and Mood State Outcomes
Fifteen trials (n = 182 subjects, 90% male) examined the
effect of fluid intake on cognitive performance and/or
mood. Major findings are summarized in Table 6. (Full
details are presented in Additional file 1: Table S7).
Across the 15 trials reviewed, 49 neuropsychological
tests were identified. Evidence indicating a beneficial
effect of fluid intake on cognitive performance was
observed on 5 cognitive tests completed across 5 trials
[55, 69, 73]. Cognitive domains affected were memory,
psychomotor function and processing speed. Four out of
the 6 trials evaluating the influence of fluid intake on
mood state observed significant positive effects [55, 69, 73],
as indicated by decreased subjective ratings of
fatigue, anger, depression, tension and confusion and
Individuals prone to dehydration (e.g. athletes and
manual workers) may have limited opportunity to
adequately rehydrate prior to performing physically or
cognitively demanding activities. The present systematic
review and meta-analysis examines evidence for the
effects of fluid intake on subsequent athletic and
cognitive performance following dehydrating sweat loss. A
beneficial effect for fluid intake was strongest when
athletic performance involved continuous exercise tasks.
Further, the magnitude of improvement appeared greater
when the continuous exercise was performed at elevated
environmental temperatures and over longer exercise
durations. Whilst the volume of fluid consumed (relative
to BM lost) did not appear to influence the size of the
Table 7 Summary of moderator variables for the meta-regression
analysis of the effect of fluid volume on the magnitude of the
weighted mean treatment effect
Level of dehydration
Coefficient (95% CI)
0.002 (−0.006, 0.009)
0.025 (0.015, 0.036)
0.011 (−0.001, 0.023)
0.218 (−0.124, 0.561)
0.013 (−0.126, 0.151)
treatment effect, fluid intake at levels complying with
current recommendations for completely replacing lost
fluid (1.25–1.50 L kg BM lost−1) [16, 17] are yet to be
thoroughly investigated. Evidence for a beneficial effect
of fluid intake on intermittent, resistance and
sportspecific exercise performance and cognitive function or
mood is less apparent and requires further elucidation.
The weighted mean effect suggests that fluid ingestion
following a period of dehydration significantly improves
continuous exercise performance, compared to control
conditions (no fluid or negligible fluid intake). Individual
estimates all indicated a beneficial effect from fluid
intake; however, the magnitude of the improvement was
heterogeneous (I2 = 80.5%) which may reflect differences
in the methodologies employed between studies. Simple
meta-regression determined that 82% of variation
between trials can be attributed to differences in the
ambient environmental temperature at which
subsequent exercise was performed, with fluid intake
demonstrating greater efficacy under heat stress conditions.
The decline in aerobic performance that occurs with
hypohydration has largely been attributed to circulatory
strain, whereby reductions in blood volume limit oxygen
transport to the exercising muscle [78, 79]. Under
elevated environmental temperatures, blood flow is also
redirected to the skin facilitating evaporative cooling,
augmenting circulatory strain and further impairing
exercise performance . These physiological
perturbations are typically characterized by increased heart rate
and core temperature [41, 78, 79]. Hence,
thermoregulatory parameters were monitored in many of the studies
reviewed (11/15) [24, 25, 29, 33, 36, 37, 41, 42, 47, 62,
63, 76, 77]. The majority of reviewed studies reported
that consumption of fluid was associated with significant
reductions in core or rectal temperature (7/11) [36, 41,
42, 47, 62, 63, 76] and heart rate (6/10) [36, 41, 42, 47,
62, 63, 76, 77] at various time points during continuous
exercise performance (i.e. for at least one fluid
intervention). Thus, fluid intake may offset circulatory strain
typically observed when exercise is undertaken in warm
environments. The multiple meta-regression analysis
also suggests that differences in the duration of the
continuous exercise performed may account for a
proportion of the heterogeneity observed between
experimental trials, with exercise performed over longer
durations yielding greater benefit from fluid intake than
short duration exercise. However, as the majority of
performance tests included in the analysis were relatively
short in duration (4–30 min), we cannot be certain that
this relationship would hold true over longer exercise
durations (i.e. 2–8 h).
Results of the meta-regression failed to indicate a
statistically significant relationship between the volume of
fluid consumed and continuous exercise performance
improvements. However, the majority of trials tested a
quantity of fluid that was within a narrow fluid intake
range (i.e. 1.0–1.05 L kg BM lost−1, n = 13 out of 18).
Hence, the performance effects associated with ingesting
a comparably small volume of fluid (e.g. ≤0.75 L kg BM
lost−1) or an amount consistent with recommended
guidelines (e.g. 1.25–1.50 L kg BM lost−1) remains
uncertain. Three experimental investigations have
examined the dose-response effect of ingested fluid volume
on continuous exercise performance following a period
of dehydration with the results demonstrating
inconsistent findings [21, 24, 25]. Unfortunately, the investigation
with the greatest contrast in fluid volumes (i.e. 0.75 vs.
1.50 L kg BM lost−1 ) did not employ a ‘no fluid’
control and was unable to be included in the
metaanalysis. Findings from previous studies suggest that
fluid intake during exercise exceeding that dictated by
thirst may not provide additional performance benefits
. However, only three of the publications reviewed
measured subjective thirst within the investigation (and
these studies did not test different fluid volumes, i.e.
only one intervention vs. control). Therefore, it is not
clear whether the equivocal effect of fluid intake volume
can be attributed to thirst sensation. Based on current
evidence, prescribing fluid volumes required to optimize
performance on a subsequent continuous exercise task
If relatively small and large fluid intakes elicit
comparable treatment effects, individuals who have limited time
to rehydrate prior to performing aerobic activities may
opt to consume smaller fluid boluses, delaying complete
rehydration until circumstances permit (e.g. overnight).
This strategy may reduce the probability of the drinker
experiencing volume-induced GI discomfort during
subsequent activity, which may occur when larger fluid
volumes are ingested . Only one of the 42
publications reviewed monitored GI symptomology . In this
study, subjective ratings of GI discomfort following
different fluid intakes (~0.5 vs. 1.0 L kg BM lost−1) were
described as mild to moderate and moderate to high on
each trial, respectively. This suggests that larger fluid
volumes are likely to induce some degree of participant
discomfort which may compromise performance.
However, research examining continuous exercise
performance following two volumes of fluid intake (i.e. 0.75 vs.
1.50 L kg BM lost−1) demonstrated significantly faster
(~3.0%) running performance associated with the larger
bolus . Importantly, this study employed a prolonged
(i.e. overnight) rehydration period reducing the
probability of severe GI disturbance and allowing ingested fluid
to equilibrate throughout the body. Further research
examining exercise performance (and GI symptoms)
when large fluid volumes are ingested over short
rehydration periods is warranted.
The effect of fluid intake on intermittent, resistance,
sport-specific and balance exercise types remains
unclear. It appears that fluid ingestion following a period
of dehydration may improve performance on subsequent
intermittent, resistance and sport-specific exercise tasks.
However, methodological differences make comparison
of results across trials challenging.
In regards to intermittent exercise, 4 of 10 trials (n = 95
male subjects) observed a significant positive effect of fluid
intake on performance, whilst no trial reported a
significant performance decrement. Similarly to the results from
continuous exercise, beneficial effects of fluid intake are
apparent when intermittent exercise tasks have been
completed in warm environments [22, 65]. The impact of
task duration may also influence the likelihood of
observing performance effects, with longer duration tasks more
regularly demonstrating a performance enhancement
associated with fluid ingestion [22, 65]. For instance,
Maxwell et al.  observed that fluid intake only
benefited performance on a second repeated sprint bout
completed in the latter stages of testing.
Concerning resistance exercise, 6 of 9 trials (n = 83
subjects, 93% male) observed a significant positive effect
of fluid intake on performance. One trial reported that
fluid intake was detrimental to performance .
However, results from this study need to be interpreted with
caution as the strength performance task was performed
following an endurance task that varied in duration.
Evidence indicating a beneficial effect of fluid intake on
resistance exercise performance appears stronger when
tests of muscular endurance, rather than tests of
muscular strength, are employed . Findings from this
systematic review demonstrate significantly improved
performance on 3 out of the 4 submaximal intensity
resistance exercise tasks [53, 66, 74]. In contrast,
performance on only 4 out of 15 maximal intensity tests
demonstrated improvement with fluid intake [49, 54, 60].
Current evidence is inadequate to determine the influence
of other variables (e.g. participant population, mode of
dehydration) on the effect of fluid intake. Further research
examining the effects of fluid intake on resistance exercise
performance using standardized procedures is required.
The 6 trials (n = 64 subjects, 84% male) that evaluated
the effect of fluid intake on sport-specific exercise
performance exhibited considerable heterogeneity, with
tests of cricket , soccer [35, 58, 59], squash  and
racehorse riding  performance all being employed.
Whilst the majority of sports-specific research has
demonstrated no impact of fluid consumption on subsequent
performance, the paucity of data and lack of replication
studies makes it difficult to determine an overall effect
of fluid intake on sport-specific exercise performance.
The present systematic review identified 14 trials (n = 174
subjects, 90% male) examining the effect of fluid intake
following a period of dehydration on cognitive function and
mood state. Evidence indicating a beneficial effect of fluid
intake on cognitive performance was only observed in some
studies [55, 73], and there was no clear indication of greater
treatment efficacy on a particular cognitive domain.
However, some limitations to the current evidence exist. In 4
trials, the cognitive assessment was conducted ≤5 min after
concluding the dehydration protocol [48, 58, 68]; further 4
trials did not provide the necessary information to calculate
the amount of time between the conclusion of the
dehydration protocol and commencement of the cognitive tests
[58, 70, 73]. Prior research indicates that acute exercise has
a small positive effect on cognitive performance (typically
dissipating within ~15 min of exercise cessation) ,
whilst elevated core temperature via heat stress may
provide additional cognitive burden . Therefore, the
residual effects of physiological stressors used to induce
dehydration in these trials may obscure any influence of fluid
intake on cognitive performance. Investigations examining
the effect of hydration on cognitive performance should
also employ neuropsychological tests that have previously
demonstrated sensitivity to nutritional interventions [34,
83, 84]. Yet, only two studies included in the present review
selected cognitive tests on this basis [56, 73]. The majority
did not provide any rationale for their chosen method of
assessment [48, 55, 58, 67, 68, 70], increasing the likelihood
of false-negative reports. Fluid consumption positively
influenced mood state (measured as reduced anger, fatigue,
depression, tension and confusion) in 4 out of the 6 trials
where it was measured [55, 69, 73]. Whilst this may suggest
that self-reported mood state questionnaires are more
sensitive to the effects of fluid intake than objective tests of
cognitive function, subjective mood ratings were only
influenced by fluid intake during trials where significant
cognitive effects were also observed, i.e. effects on mood
and cognition were not independent of one another.
Collectively, it appears that the influence of fluid intake on
mood and cognitive performance is still poorly understood
and requires further research employing tasks with
This review does contain a number of limitations.
Firstly, only studies with accessible full text articles
written in English were included. Second, three of the
studies reviewed [69, 71, 77] examined rehydration in
combination with another placebo treatment (studies were
excluded if fluids were co-administered with another
experimental treatment). Thus, participants’ perceptions
regarding the expected treatment may have influenced
these results. Third, as oral fluid replacement cannot be
blinded, it is conceivable that the placebo effect may
account for a small amount of benefit observed with
rehydration. However, it was necessary to exclude research
studies that blinded participants to hydration status using
intravenous methods because the infusion does not
accurately mimic the physiological effects of oral rehydration.
Fourth, the present review elected to compare against a
“no fluid” or “negligible fluid” control condition, because a
euhydrated control may be confounded by the effects of
the dehydration protocol itself (i.e. hyperthermia or
fatigue). However, using this comparison, we cannot
determine whether fluid intake fully or partially restored
performance to euhydrated levels. Similarly, fluid
ingestion may have minimal or no effect on athletic or
cognitive performance if the outcome measured is not sensitive
to the effects of modest fluid losses. Fifth, where fluid was
administered at the time of dehydration (i.e. concurrent
fluid intake), rather than following dehydration (i.e.
subsequent fluid intake), different physiologic responses to the
dehydration protocol may occur on control and
intervention trials, e.g. decreased core temperature leading to
reduced central fatigue. This could have implications for
subsequent athletic performance, and consequently, the
magnitude of the overall treatment effect. Sixth, fluid
intake ≤200 mL was considered ‘negligible’ and included
within the definition of control conditions. However, one
study has reported that ingesting 100 mL of fluid (25 mL
boluses at 5-min intervals during exercise) increased TTE
following exercise-induced dehydration . Thus, trials
administering ≤200 mL fluid to dehydrated control
subjects may underestimate the true magnitude of the
Collectively, the results of the present review suggest
that individuals who have limited opportunity to
adequately rehydrate prior to performing continuous
exercise in a heated environment should consume fluid,
even if the body water deficit is modest (1.3% reduction
in BM) and fluid intake is inadequate for complete
rehydration (0.5 L kg BM lost−1). The influence of fluid
intake for those individuals performing intermittent,
resistance and sport-specific exercise or undertaking
cognitively demanding tasks is not as well understood, and
this review serves to highlight areas for future research.
Additional file 1: Supplementary Table S1 - S9. (DOC 725 kb)
No funding was received for the preparation of this manuscript.
All authors (DM, BD and CI) were involved in the conception and design of
this review. DM and CI were responsible for collating the manuscripts and
retrieving the data. DM conducted the analysis of the data. All authors (DM,
BD and CI) contributed to drafting and revising the article and the final
approval of the published version of the manuscript.
Danielle McCartney, Ben Desbrow and Christopher Irwin declare that they
have no conflict of interest.
Springer Nature remains neutral with regard to jurisdictional claims in
published maps and institutional affiliations.
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