Effects of hydration status on cognitive performance and mood

Abstract

Although it is well known that water is essential for human homeostasis and survival, only recently have we begun to understand its role in the maintenance of brain function. Herein, we integrate emerging evidence regarding the effects of both dehydration and additional acute water consumption on cognition and mood. Current findings in the field suggest that particular cognitive abilities and mood states are positively influenced by water consumption. The impact of dehydration on cognition and mood is particularly relevant for those with poor fluid regulation, such as the elderly and children. We critically review the most recent advances in both behavioural and neuroimaging studies of dehydration and link the findings to the known effects of water on hormonal, neurochemical and vascular functions in an attempt to suggest plausible mechanisms of action. We identify some methodological weaknesses, including inconsistent measurements in cognitive assessment and the lack of objective hydration state measurements as well as gaps in knowledge concerning mediating factors that may influence water intervention effects. Finally, we discuss how future research can best elucidate the role of water in the optimal maintenance of brain health and function.

Full text

Effects of hydration status on cognitive performance and mood
Natalie A. Masento*, Mark Golightly, David T. Field, Laurie T. Butler and Carien M. van Reekum*
School of Psychology and Clinical Language Sciences, University of Reading, Whiteknights Campus, Earley Gate,
Whiteknights Road, Reading RG6 6AL, UK
(Submitted 24 June 2013 – Final revision received 17 December 2013 – Accepted 19 December 2013)
Abstract
Although it is well known that water is essential for human homeostasis and survival, only recently have we begun to understand its role in
the maintenance of brain function. Herein, we integrate emerging evidence regarding the effects of both dehydration and additional acute
water consumption on cognition and mood. Current findings in the field suggest that particular cognitive abilities and mood states are
positively influenced by water consumption. The impact of dehydration on cognition and mood is particularly relevant for those with
poor fluid regulation, such as the elderly and children. We critically review the most recent advances in both behavioural and neuroima-
ging studies of dehydration and link the findings to the known effects of water on hormonal, neurochemical and vascular functions in an
attempt to suggest plausible mechanisms of action. We identify some methodological weaknesses, including inconsistent measurements in
cognitive assessment and the lack of objective hydration state measurements as well as gaps in knowledge concerning mediating factors
that may influence water intervention effects. Finally, we discuss how future research can best elucidate the role of water in the optimal
maintenance of brain health and function.
Key words: Hydration: Cognitive performance: Mood: Water consumption
Growing evidence suggests that the food and drink that
we consume affect mental and physical performance
(1)
. Food
and food components that exhibit physiological and mental
effects have been dubbed ‘functional foods’ or ‘nutraceuticals’
and are proposed as ways to help sustain good health and
protect against illness, disease and pathological ageing
(2)
.
Despite water constituting 60 –80 % of the human body, it is
often overlooked as a significant nutrient that can affect not
only physical performance, but also mental performance. In
this review, we evaluate evidence from studies that investigated
how cognitive performance is affected when water intake levels
are low (i.e. during dehydration) or optimal and beyond
(i.e. during acute water consumption). (To ensure that the
review was comprehensive, we carried out literature searches
using databases ‘Web of Science’ and ‘Google Scholar’ and
obtained published studies that investigated dehydration and
its effect on cognitive performance using search terms such as
‘dehydration & cognition’, ‘dehydration & performance’, and
‘dehydration & mental’. To obtain studies that investigated
water consumption and how it influences cognitive
performance, search terms such as the following were used:
‘hydration & cognition’; ‘hydration & performance’; ‘hydration
& mental’; ‘water consumption & performance’; ‘water
consumption & cognition’; ‘drinking water & cognition’.)
In addition to reviewing published research findings, we also
discuss previously proposed mechanisms of action as well as
new ones. Finally, based on the current state of the research
area, we propose avenues for future investigations.
Voluntary dehydration
Evidence from public surveys
(3,4)
and experimental
investigations
(5,6)
has indicated that the general public and
particularly groups such as children and older adults are at a
risk of voluntary dehydration
(7,8)
, such that individuals are
drinking insufficient amounts of fluid resulting in sustained
dehydration. Voluntary dehydration is likely to occur due to
a lack of awareness of how much fluid consumption is
required for a balanced hydration state (euhydration),
especially when not taking into account the amount of daily
activity; other external factors such as weather also contribute
to this day-to-day variability in hydration requirements.
Examples of voluntary dehydration have been reported in
school children living in hot climates
(5,6)
and also in a group
of experienced runners who, although aware that they
should rehydrate after exercise, drank insufficient amounts
of water due to an underestimation of their hydration state,
resulting in sustained dehydration
(9)
.
*Corresponding authors: N. A. Masento, email n.masento@pgr.reading.ac.uk; Dr C. M. van Reekum, fax þ44 118 378 6715,
email c.vanreekum@reading.ac.uk
British Journal of Nutrition, page 1 of 12 doi:10.1017/S0007114513004455
qThe Authors 2014
British Journal of Nutrition

Fluid balance within the body is maintained via homeostatic
mechanisms
(10)
; water conservation occurs via the renal
system, modifying urine production. Water intake is encour-
aged by thirst sensation. Although these mechanisms are
intrinsic in homeostatic maintenance, they are also fallible,
particularly in vulnerable groups such as children and older
adults who maintain their hydration state inadequately.
Inadequate hydration in young children and older adults
may be due to dependency on carers, making self-motivation
to seek fluid consumption difficult. There are also physiologi-
cal issues of interpreting the thirst response, prompted by
homeostatic mechanisms, which may be problematic due to
inexperience in children
(11,12)
and due to the deterioration
of osmoreceptor sensitivity in older adults
(13 – 15)
. These factors
preventing fluid consumption will over time result in individ-
uals sustaining dehydration. Older adults are also more likely
to have reduced kidney filtration function, resulting in less
efficient water conservation when dehydrated, further exacer-
bating difficulties in recognising a dehydrated state
(14)
.
Sustained dehydration is associated with poor health
(16,17)
;
chronic dehydration greatly increases the chances of kidney
stones and urinary tract infection
(16,18)
, whereas prolonged
vasoconstriction, as a result of chronic dehydration, can
increase the chances of hypertension and stroke
(10)
. These
physical consequences highlight the importance of preventing
voluntary dehydration and make it a public health issue.
Authoritative bodies such as the European Food Safety
Authority (EFSA) support the scientific opinion that water con-
tributes to the maintenance of normal physical and cognitive
function
(19)
and therefore have set recommended guidelines
of 2000 ml of fluids for females and 2500 ml for males to be
consumed per day
(20)
. These guidelines were set to encourage
more fluid consumption and reduce the risk of sustained
dehydration. There is some debate regarding the guidelines
and the apparent lack of empirical evidence concerning the
amount of additional fluids that individuals should actually
consume
(21,22)
. Based on the high individual variability regard-
ing fluid requirements, it is argued that the emphasis should
be on encouraging individuals to monitor their own hydration
levels using markers such as urine colour
(23)
and to be aware
of variables that may influence the amount of water they need
to consume, such as climate and physical activity. To identify
the best strategy for improving public water consumption,
it is important to understand the factors that lead to the
widespread neglect of water intake, as well as the impacts
of inadequate water intake on both physical and mental
performance.
Dehydration and cognitive function
Investigations into dehydration and mental performance
were first systematically carried out in a military popu-
lation
(24)
. Soldiers were exposed to extreme heat, inducing
varying severities of dehydration. Cognitive abilities such as
short-term memory, numerical ability, psychomotor function
and sustained attention were assessed to establish any particu-
lar deficits as a result of changes in hydration status. Cognitive
deficits were dependent on the severity of dehydration, which
affected performance in all cognitive tasks when soldiers were
in a severe state of dehydration (.2 % body mass loss). This
study was the first to emphasise that cognitive abilities were
sensitive to a suboptimal hydration state.
Subsequent studies both in a military population and in the
general population supported this initial evidence of detri-
ments in cognitive abilities with induced dehydration
(25 – 32)
.
However, relative to the study carried out by Gopinathan
et al.
(24)
, the cognitive deficits were more modest and only
found in particular cognitive domains such as short-term
memory and perceptual abilities, with preservation of other
cognitive abilities such as working memory and executive
function. Other studies
(33 – 38)
found no support of cognitive
impairment due to dehydration. These inconsistencies across
empirical studies make it difficult to conclude whether, and
how, dehydration affects cognitive performance (see Table 1
for all the dehydration and cognition function studies).
Indeed, some experts have questioned where there is suffi-
cient evidence to suggest that dehydration significantly affects
cognitive performance
(39)
.
Studies measuring self-reported changes in mental state
have consistently found associations between dehydration
and mood, in conjunction with changes in performance
(27,30)
or with limited to no performance changes
(35 – 37,40,41)
. Despite
variability in rating methods used, similar mood states were
reported such as ‘less alert’, ‘difficulty in concentrating’,
‘fatigue’ and ‘tension’
(27,35,36,40 – 42)
. Some studies also reported
that participants found completing the experimental tasks
more difficult
(35,36)
when in a dehydrated state. These findings
highlight that self-reported mood states are sensitive to
changes in hydration state and can occur independently
from any cognitive performance changes.
One possible source of the heterogeneity in the profile of
cognitive effects during dehydration may be the diversity in
methods used to induce dehydration and to measure cognitive
performance
(43)
. Early investigations used heat stress indepen-
dently
(24)
as well as a combination of strenuous exercise and
heat stress to create a severe dehydration state
(25 – 27,35,40,44)
,
whereas more recent investigations have used fluid restriction
to ascertain how mild dehydration influences perform-
ance
(32,36,45 – 47)
. These methods vary in the degree of
dehydration severity, which is probably a key determining
factor of deficits in cognitive performance
(24)
. This is sup-
ported by recent mild dehydration investigations reviewed
above, which found self-reported mood changes but the
preservation of cognitive abilities
(35,38,40)
. The use of different
methods also results in interpretive confounding factors.
Specifically, evidence suggests that exercise alone improves
cognitive performance
(48)
, which could counteract any poten-
tial deficit caused by dehydration. Increased core temperature
via heat stress has also been shown to cause cognitive deficits,
more so than dehydration
(44)
. Therefore, studies that use exer-
cise and heat stress to induce dehydration may confound the
mechanisms responsible for any effect found, placing into
question whether these methods are optimal to investigate
the influence of dehydration on cognitive performance.
Studies that have used fluid restriction to induce
dehydration
(32,36,45 – 47)
are comparatively free of such
N. A. Masento et al.2
British Journal of Nutrition

Table 1. Characteristics of dehydration and cognitive function and/or mood studies
Authors
Sample
size (n) Sample age Design Dehydration method
Self-reported
measures Other measures Cognitive tasks
Cognitive
performance/MRI change
Self-reported
changes
Sharma
et al.
(49)
8 men 21– 24 years
old
Repeated-measures
cross-over
Heat chamber with moderate activity – %BMC Symbol substitution
test – processing speed
Slower processing –
speed at 3 %
–
Targeted varying severities of
dehydration (1 – 3 % BMC)
Concentration test – WM Reduced psychomotor
functionEye– hand coordination test –
psychomotor function
Gopinathan
et al.
(24)
11 men 20– 25 years
old
Repeated-measures
cross-over
Heat chamber with moderate activity – %BMC Word recognition – STM Global deficits at 2 %: STM –
Targeted varying severities of
dehydration (1 – 4 % BMC)
Serial addition –
mathematical efficiency
Mathematical efficiency
Trail-making test –
visuomotor processing
Visuomotor processing
Cian et al.
(25)
8 men Mean age:
27·4
years
Repeated-measures
cross-over
Cond 1: heat chamber – passive
hyperthermia to approximately
2·8 % BMC
VAS: fatigue and
mood
%BMC; heart rate;
blood samples; core
body temperature
Pictures recall – long-term memory
task
Slow perceptual
discrimination RT
Increased
fatigue
Cond 2: 60 % VO
2max
exercise to
approximately 2·8 % BMC
Four-choice serial RT – visual
attention
Reduced STM recall
Perceptive discrimination –
perceptual processing
Psychomotor errors
Digit span memory – STM
Unstable tracking –
psychomotor skills
Cian et al.
(26)
7 men Mean age:
25 years
Repeated-measures
cross-over
Cond 1 and 2: heat chamber –
passive hyperthermia with or
without FR to approximately
2·8 % BMC
VAS: fatigue and
mood
%BMC; heart rate;
core body
temperature
Pictures recall – long-term
memory
Slower perceptual RT Increased
tiredness
Cond 3 and 4: 65 % VO
2max
exercise
to approximately 2·8% BMC
Judgement of line length –
perceptual discrimination
Impaired STM
performance
RT – processing speed
Digit span test – STM
Unstable tracking –
psychomotor skills
Ainslie
et al.
(86)
17 men:
9 younger
and 8 older
Mean age:
24 years
Independent sample
–YA
v. OA
Exercise – 10 d walking activity – Uosm; %BMC; daily
dietary record;
energy expenditure;
blood samples
Choice RT OA progressive dehy-
dration over 10ds
–
Mean age:
56 years
Grip strength – motor
function
YA sustained euhydration
Flexibility and vertical
jump – muscle power
Psychomotor function
deficit for OA
Suhr et al.
(46)
28 adults Mean age:
63·7
years
Correlational FR approximately 12h – %BMC – bioelectrical
impedance
RBANS – range of
cognitive abilities
Slower psychomotor
processing speed with
low %BMC
–
Trail-making task –
psychomotor function
Grooved Pegboard
Test – manual dexterity
Shirreffs
et al.
(41)
15 adults Mean age:
30 years
Repeated-measures
cross-over
FR for 37h VAS: thirst, mouth
dry, mouth
pleasant,
headache,
concentration,
tiredness and
alertness
%BMC; Uosm; blood
samples
– – Increased
headaches
Reduced
concentration
and alertness
at 24 and
37 FR
Bar-David
et al.
(45)
51 children Mean age:
11 years
Independent
samples,
.800 mosm/kg
H
2
O (dehydrated)
v. ,800 mosm/kg
H
2
O (euhydrated)
No intervention, natural hydration
state for comparison
– Uosm Hidden figures – visual attention/
perceptual speed
Reduced STM at afternoon
testing for dehydrated
children
–
Auditory number span – WM
Making groups – semantic
flexibility
Verbal analogies – semantic
memory
Number addition – perceptual
speed and numerical reasoning
Hydration status, cognition and mood 3
British Journal of Nutrition

Table 1. Continued
Authors
Sample
size (n) Sample age Design Dehydration method
Self-reported
measures Other measures Cognitive tasks
Cognitive
performance/MRI change
Self-reported
changes
Szinnai
et al.
(36)
16 adults Mean age:
26 years
Repeated-measures
cross-over
FR for 28h VAS: thirst,
effort and
concentration
Blood samples; Uosm;
auditory ERP
Choice RT task – sustained visual
attention
No cognitive differences Increased
tiredness
Likert scale:
tiredness and
alertness
Auditory serial addition task –
sustained/divided attention
Less alertness
Stroop task – verbal response time
Smooth pursuit rotor task –
manual tracking
Petri et al.
(32)
10 men Mean age:
25 years
Repeated measures FR for 24h 10-point Likert
scale mood:
depression,
working
energy,
anxiety and
self-confidence
– Complex Reactionmeter Drenovac:
light signal position
discrimination
Slower total solving time
found from 9 h of FR
and onwards
–
STM
Simple visual orientation
Simple arithmetic
Complex motor coordination
Patel et al.
(28)
24 men Mean age:
21·9
years
Repeated-measures
cross-over
FR for 15h plus 45 min 65 –70 %
VO
2max
exercise
Concussion
measures
Balance error scoring
system
ANAM Reduced visual memory
performance
Increased fatigue,
‘feeling slowed
down’ and
‘difficulty
in concentrating’
Sleep scale
test – fatigue
measure
NeuroCom sensory
organisation test –
postural stability
Simple RT
USG Mathematics processing test
Match-to-sample test
Sternberg memory test
Baker
et al.
(30)
11 males Mean age:
21·3
years
Repeated measures Cond 1: exercise þplacebo drink VAS lightheaded-
ness, hotness
and total body
fatigue
%BMC; blood samples;
core body tempera-
ture
Test of Variables in
Attention – continuous
performance test
Slower RT and increased
errors compared
Increased fatigue,
lightheadedness
and overheating
Cond 2: exercise þcarbohydrate
drink
Two versions: first half
frequent targets and second
half infrequent.Cond 3– 6: degrees of dehydration
1–4%
Adam
et al.
(34)
8 adults Mean age:
24 years
Repeated-measures
cross-over
Cond 1: exercise-induced
dehydration
POMS;
NASA-TLX
%BMC Sentry duty simulation –
marksmanship simulation with
weapon
No cognitive differences Not reported
Cond 2: passive heat dehydration Scanning visual vigilance
Cond 3: exercise plus fluids and
cold environment
Cond 4: exercise plus fluids and
temperate environment
Ackland
et al.
(30)
52 adults Mean age:
62 years
Independent
measures:
colonoscopy
surgical patients
v. sigmoidoscopy
surgical patients
Medical procedure – bowel
preparation
Quality of
life – SF8
%BMC – bioelectrical
impedance
Trail-making test A and B No cognitive differences Colonoscopy
patients more
anxiousSpielberger
State-Trait
Anxiety
Inventory
Rey Auditory Verbal Learning Test
Subjective
cognition scale
D’Anci
et al.
(27)
54 adults;
study
1: 31 adults
Mean age:
19·8
years
Repeated-measures
cross-over
Study 1 Thirst sensation
scale; POMS
%BMC Digit span forward task – STM STM improvement Increased anger,
fatigue,
depression,
tension and
confusion
Cond 1: 60 min exercise plus FR Simple RT Decreased vigilance over
timeCond 2: 60 min exercise plus water Choice RT
Kit of Factor-Referenced Cognitive
Test – map planning
Mathematical addition
Continuous performance task
Mental rotation task – visual
perception
N. A. Masento et al.4
British Journal of Nutrition

Table 1. Continued
Authors
Sample
size (n) Sample age Design Dehydration method
Self-reported
measures Other measures Cognitive tasks
Cognitive
performance/MRI change
Self-reported
changes
Kempton
et al.
(87)
7 men Mean age:
23·8
years
Repeated-measures
cross-over
Thermal – exercise-induced
dehydration
– Uosm; %BMC;
structural MRI
– Ventricular expansion
following dehydration
–
%BMC correlated with
ventricular volume
Serwah &
Marino
(88)
8 men Mean age:
24·5
years
Repeated-measures
cross-over
Heat chamber plus exercise and
varying fluid replacement
Perceived exertion
and thermal
comfort
%BMC; heart rate; skin
temperature
Choice RT: one, two or four
choices – varying complexities
processing speed
Reduced accuracy while
performing complex
processing
Increased
exertion due to
exercise
Bandelow
et al.
(44)
20 men Mean age:
20 years
Repeated-measures
cross-over
Exercise-induced dehydration with
or without fluid replacement
– %BMC; core
temperature; blood
samples
Visual sensitivity – visuomotor RT Improved fine motor speed
and complex WM RT
during dehydration
–
Corsi block design test –
visuospatial WM
Slower RT for simple
WM task
Sternberg test – WM
Finger-tapping test – fine motor
speed
Suhr et al.
(47)
21 women Mean age:
60·3
years
Correlational No intervention, natural hydration
state
– %BMC – bioelectrical
impedance; blood
pressure
Auditory consonant trigrams – WM Worse hydration state
related to worse
declarative memory
ability
–
Auditory verbal learning –
declarative memory
Ganio
et al.
(40)
26 men Mean age:
20 years
Repeated-measures
cross-over
Cond 1: exercise-induced
dehydration plus diuretic
VAS: task
difficulty,
concentration
and headache;
POMS
%BMC; USG Psychomotor vigilance test –
simple RT
Increased false alarms in
vigilance task and
slower RT for spatial
WM
Increased tension
and fatigue
Cond 2: exercise plus placebo Four-choice visual RT test
Cond 3: exercise with fluid
replacement
Match-to-sample test – short-term
spatial memory
Repeated acquisition test –
learning and STM
Grammatical reasoning – logical
reasoning.
Kempton
et al.
(38)
10 adolescents Mean age:
16·8
years
Repeated measures Two conditions: exercise-induced
dehydration or euhydration.
Structural and functional MRI
at baseline
and after thermal exercise
protocol to induce dehydration
VAS: physical
sedation and
mental
sedation
Uosm; %BMC; body
core temperature;
structural MRI
Tower of London task – executive
function
No cognitive performance
change
Physical and mental
sedation
increased due to
exercise
irrespective of
dehydration
condition
BOLD functional MRI;
ASL MRI
Ventricular enlargement
due to dehydration
BOLD MRI increased
activation for task-
related areas after
exercise
Armstrong
et al.
(35)
25 women Mean age:
23 years
Repeated-measures
cross-over
Cond 1: exercise-induced
dehydration plus diuretic
VAS task difficulty,
concentration
and headache;
POMS
%BMC; USG Psychomotor vigilance test –
simple RT
Increased false alarms in
vigilance task
Increased anger,
vigour and
fatigue, difficulty
in concentrating,
headaches and
difficulty in
doing task
Cond 2: exercise plus placebo Four-choice visual RT test
Cond 3: exercise with fluid
replacement
Match-to-sample test – short-term
spatial memory
Repeated acquisition test –
learning and STM
Grammatical reasoning – logical
reasoning
Smith et al.
(31)
7 adults Mean age:
21·1
years
Repeated-measures
cross-over
FR for 12h – %BMC; Ucol Motor performance task – golf
simulation
Impairments in shot
distance and target
accuracy
–
Golf shot distance judgement and
perceptual depth judgement
Increased errors in
distance judgement
Lindseth
et al.
(29)
89 adults Mean age:
20·3
years
Repeated-measures
cross-over
Fluid diet intervention: low-fluid
diet v. high-fluid diet
– %BMC; sleep activity General aviation trainer – flight
simulator
Impairments in spatial WM
for those who reached
1–3% BMC
–
Vandenberg mental rotation
test – spatial WM
Sternberg Item Recognition Test
Hydration status, cognition and mood 5
British Journal of Nutrition

confounding factors. Interestingly, these studies tend to show
modest cognitive deficits, if any, possibly reflecting that the
participants were less severely dehydrated, although this is
speculative; to date, no objective hydration state measure
has been used to establish whether dehydration was induced.
However, fluid restriction closely resembles routine voluntary
dehydration behaviour found in the general population. Future
research should focus on the extent to which fluid restriction
may influence cognitive performance and ideally include objec-
tive measures of hydration state; this would not only advance
the field of dehydration research but also benefit public
health initiatives to encourage adequate fluid consumption.
As with any assessment of cognition, the tests chosen have
a profound impact on the sensitivity to observe effects of the
manipulation. There has been a propensity within the current
collection of studies investigating dehydration and cognitive
performance
(28,44,46,49)
to either use broad cognitive measure-
ments from neuropsychological batteries or to select cognitive
measurements based on previous use. These include batteries
such as the Repeatable Battery for the Assessment of Neuro-
psychological Status and the Automated Neuropsychological
Assessment Metrics. These neuropsychological tests often
show little sensitivity to performance changes due to nutri-
tional intervention, have inherently large variability in the
detection of these performance differences and therefore
lack validity in nutrition research. These issues with general
neuropsychological tests that were not specifically designed
for nutritional studies may lead to subtle effects being over-
looked and an increase in false-negative reports. To avoid
such issues, a number of researchers in this field, including
Lieberman
(43,50)
and Edmonds et al.
(51,52)
, have recommended
that standardised cognitive measures, i.e. those that have pre-
viously shown sensitivity to nutritional interventions, should
be utilised in future studies. Cognitive batteries that were
designed for nutritional interventions such as phytochemicals
have in recent years become popular
(53)
, and the identification
of sensitive cognitive measurements in nutritional intervention
contexts can be found in published reviews (see Macready
et al.
(54)
). Standardising the method of cognitive testing
would also make it easier to corroborate evidence across
different empirical studies in the future.
Mechanisms of action
Despite the inconsistent evidence of the impact of dehydration
on cognitive performance, sustained dehydration is character-
ised by specific physiological changes. These physiological
changes are part of a highly complex and variable system,
making it particularly difficult to establish a unified baseline
for hydration states across individuals. This issue perhaps
underlies the variability in the findings of the influence of
dehydration on cognition. Nevertheless, these physiological
mechanisms of action may further inform us as to which
aspects of mental performance are probably affected by
dehydration. Reflecting on the homeostatic responses of
dehydration, when the body is in a state of dehydration,
many substrates and neurotransmitters are influenced by
circulating vasopressin (also known as antidiuretic hormone)
Table 1. Continued
Authors
Sample
size (n) Sample age Design Dehydration method
Self-reported
measures Other measures Cognitive tasks
Cognitive
performance/MRI change
Self-reported
changes
Pross et al.
(42)
20 women Mean age:
25 years
Repeated-measures
cross-over
FR for 23h or euhydration POMS Uosm; plasma
measures; saliva
(Cognitive data not included in
the paper)
Increased ratings of
sleepiness,
fatigue, reduced
vigour, alertness
and confusion
Bond Lader scale Driving simulator After ad libitum
water states
reversed
VAS: thirst Mackworth Clock Test
Karolinska
sleepiness
scale
Rey Auditory Verbal Learning Test
Ely et al.
(37)
32 adults Mean age:
22 years
Mixed design Exercise –heat procedure with
varying temperatures
(10– 408C) with
or without fluid
replacement
POMS %BMC; USG Psychomotor vigilance task No cognitive differences Increased anger,
confusion,
depression and
fatigue when
dehydrated
Four-choice RT test
Match-to-sample test
Grammatical reasoning
%BMC, body mass change; WM, working memory; STM, short-term memory; Cond, condition; VAS, visual analogue scale; RT, reaction time; FR, fluid restriction; YA, younger adults; OA, older adults; RBANS, Repeatable Battery
for the Assessment of Neuropsychological Status; Uosm, urine osmolality; ERP, event-related potential; ANAM, Automated Neuropsychological Assessment Metrics; USG, urine specific gravity; POMS, profiles of mood states;
NASA-TLX, National Aeronautical Space Administration, task load index; SF8, Health Survey by QualityMetric Inc.; BOLD, blood oxygen level-dependent; ASL, arterial spin labelling; Ucol, urine colour.
N. A. Masento et al.6
British Journal of Nutrition

and angiotensin II
(17,55)
. These are key hormones involved in
the homeostatic response of fluid imbalance
(10)
. One possible
mechanism, proposed by researchers in this field
(6,11,17)
, for
cognitive deficits during dehydration could be increased
levels of cortisol, often released during a stress response. It
has been shown that higher levels of cortisol can lower
memory function and processing speed
(56)
and consequently
cause memory-related cognitive deficits
(57)
.
Other neurotransmitter systems have been shown to act
differently as a consequence of dehydration, potentially
mediating the cognitive deficits reported. Serotonergic and
dopaminergic systems modify blood–brain barrier per-
meability, which, if sustained, causes central nervous system
dysfunction
(58)
. Findings also indicate that d-aminobutyric
acid and glutamate levels increase during chronic dehydration,
influencing both inhibitory and excitatory activities of the
brain
(59)
. These modulations due to dehydration, however,
are still unclear in relation to how they may influence func-
tional brain activation and therefore cognitive performance.
To better understand the mechanisms of action of dehydration
on cognitive performance, studies directly manipulating dehy-
dration and measuring the impact on neurotransmitter function
should be carried out. For instance, positron emission tomogra-
phy or magnetic resonance spectroscopy can be employed to
uncover how the functioning of these neurotransmitter systems
changes as a result of dehydration.
As described above, mild dehydration studies so far have
failed to show a replicable impact on cognitive performance.
Whether this is due to insufficiently sensitive cognitive
measurements and issues of variability, discussed previously,
or due to a genuine lack of impact of mild dehydration on
cognitive performance remains unclear. The evidence for
reported mood state changes is more consistent across studies.
Despite the lack of behavioural changes in cognition, neural
activity in brain regions involved in attention and executive
function has been shown to increase when individuals are
mildly dehydrated than when they are euhydrated
(38)
. One
explanation is that individuals compensate for dehydration
at both the neural and behavioural levels through investing
greater effort and mental energy
(60)
, thus producing no net
performance changes. Others suggest that NO production is
increased during dehydration
(61,62)
. Indeed, studies have
shown that NO production is associated with increased cer-
ebral blood flow and vasodilation
(63)
and could ultimately
counteract any potential impairment to cognitive perform-
ance, leading to a sustained level of ability. These theorised
processes need to be investigated further, focusing on the criti-
cal point at which the brain can no longer compensate for
dehydration and at what point cognitive deficits begin.
Acute water intervention and cognitive function
With evidence to suggest that individuals are routinely at a
risk of mild dehydration day to day
(8)
, particularly vulnerable
populations such as children and older adults, there has been
an increased interest in studying whether additional water
consumption might benefit cognitive performance. The small
collection of published water intervention studies involving
either young adults or school children report consistent
positive effects of water intervention on particular cognitive
abilities.
Acute water intervention and visual sustained attention
Visual sustained attention has shown sensitivity to water
consumption: the first study to investigate this
(64)
employed
a between-group design randomly allocating young adults to
a no-water, 120ml water or 330ml water condition. Using a
sustained attention task (rapid visual information-processing
task), the participants were asked to locate target numbers
among successive sequences. The researchers found a dose-
related improvement in performance, with those in the
330 ml water condition performing the best of the three
groups and the no-water group performing the worst. How-
ever, this response was only found for those participants
who reported thirst before the water intervention. These
results suggest that visual sustained attention was sensitive to
water consumption depending on the baseline hydration state
of the individual. Interestingly, the task used in this study
(rapid visual information processing) has not shown consistent
results with acute water intervention. A subsequent study
(65)
using a repeated-measures design and an overnight fast – the
latter to minimise variability in baseline hydration – included
this task and found no improvement in sustained attention
or in other cognitive performance measures after a water
intervention. The inconsistencies found between these two
studies could be due to the differences in experimental
design: Rogers et al.
(64)
employed a between-group design,
whereas Neave et al.
(65)
used a within-subjects design control-
ling for baseline hydration state. Due to repeated exposure to
the cognitive tasks, within-subjects designs are likely to suffer
from practice effects, which can diminish the sensitivity of the
cognitive measurements. As discussed above, insensitive
cognitive assessments can increase false-negative reports and
make it difficult to ascertain whether there is a genuine
effect of water intervention. Further investigation is still
needed to understand these inconsistencies and how the
role of experimental design may interact with any influence
that water consumption has on cognitive abilities.
Other water intervention studies have reported similar sus-
tained attention performance changes without a dependency
on prior thirst/hydration state. In a mixed design
(66)
, young
adults were given 200 ml of water and performance was
found to increase from baseline in a sustained attention task
(letter cancellation), which involved searching for a target
letter within a grid. This was the only task to show improve-
ment out of a battery of tasks including working memory
assessments and simple reaction time. Other studies carried
out by the same research group
(67 – 69)
have replicated these
improvements in visual sustained attention after water con-
sumption in groups of school children. Despite these studies
varying in the amounts of water ingested and experimental
design, consistently these studies have shown visual sustained
attention to improve after acute water consumption.
The corroborating evidence regarding visual sustained
attention improvement after water intervention clearly
Hydration status, cognition and mood 7
British Journal of Nutrition

highlights this as a key cognitive domain sensitive to water
intervention. Further empirical studies should establish what
particular component of this cognitive ability is benefiting
from water intervention. One question that remains is whether
attentional processes in other modalities such as auditory sus-
tained attention would be similarly affected by acute water
consumption. These recommendations have been made by
researchers in the field
(39,50,51,66)
, but have thus far not been
implemented in empirical research. Testing these different
sensory modalities would help teasing apart whether water
intervention improvement is specific to the visual system, as
has been found in some flavonoid intervention studies, for
example
(70)
, or whether higher-level, cross-modal attentional
mechanisms are affected.
Acute water intervention and short-term memory
Short-term memory has also been shown to improve after water
consumption. Short-term memory improvements after water
consumption were found by three studies that investigated
acute water intervention in school children
(6,68,71)
. The study
carried out by Benton & Burgess
(71)
used a repeated-measures
design with school children, assessing changes in the cognitive
domains of short-term memory and sustained attention (using
the recall of objects task and an auditory reaction time task).
Interestingly, the authors failed to replicate water-induced per-
formance improvements in the sustained attention task
observed in other studies, possibly due to the task relying on
auditory sustained attention rather than on visual sustained
attention. However, the researchers did find that the children’s
short- and long-term recall of a list of objects improved after
water consumption compared with no-water condition. A
study carried out by Edmonds & Burford
(68)
testing 7–9-year-
old children in London schools found that water consumption
improved visual attention and visual memory. Using a ‘spot
the difference’ task, the researchers found that water interven-
tion significantly influenced children’s visual memory: children
who consumed water were able to identify more differences
between two pictures compared with those who did not con-
sume water. When taking into account the dosage of water con-
sumed (250 v. ,250 ml), they also found that short-term
memory performance was improved, but only for those who
drank more water (250ml). The differential effect of dosage
highlights that there may be a minimum amount of water
consumption required to cause a significant impact on
particular cognitive abilities and this no doubt will be related
to the baseline hydration state of the children. Future studies
should consider whether water dosage might have any differen-
tial effect on performance between various cognitive domains.
Relatedly, a study that investigated Italian school children
(6)
found that children who were less hydrated were more likely
to perform worse in an auditory number memory task, also
implying that optimal hydration leads to better performance
in the auditory number memory task. This study further high-
lights the importance of including metrics of baseline hydration
state of children, in this case urine osmolality, which is an
objective hydration state measure. Inclusions of hydration
state markers such as urine osmolality provide valuable
information related to the day-to-day hydration levels of
individuals and the extent to which these levels change after
acute water consumption. Taking these measurements into
account when assessing cognitive performance after water
ingestion would contribute towards the understanding of the
underlying mechanisms at work.
Acute water intervention and simple reaction time
A recent study carried out by Edmonds et al.
(52)
has found
improvements in simple reaction time after acute water inter-
vention. The study consisted of thirty-three adults within a
repeated-measures design. Cognitive performance changes
were measured using the Cambridge Neuropsychological
Test Automated Battery. When taking into account prior
thirst of individuals, the researchers found that performance
in the simple reaction time task was different between those
who were thirsty and those who were not thirsty, with non-
thirsty individuals exhibiting a relatively similar performance
independent of water intake, whereas thirsty individuals per-
formed significantly worse in the no-water condition. Even
though thirst was measured subjectively, these results suggest
that such subjective reports provide valuable information
regarding hydration state; individuals who reported being
thirsty during the experiment and were not provided with
water supplementation were potentially mildly dehydrated,
resulting in slower reaction times. This study helps us to
understand how experienced variations in hydration state may
interact with changes in cognitive performance. The majority
of previous studies on this topic have lacked a measurement
of baseline hydration status of their participants
(52,64 – 69,71)
,
and with the findings from this study highlighting that thirst
mediates the performance change in specific cognitive abilities,
it is evident that we need to further our understanding of the
relationship between hydration state and change in cognitive
performance in future work.
Acute water intervention and real-world settings
The importance of an optimal hydration state for adequate
cognitive performance has been highlighted by a range of
studies reviewed above, all of which have been carried out
within a laboratory setting testing individuals’ performance
using relatively controlled neuropsychological tasks that are
impoverished in comparison with real-life demands. There-
fore, it is important to test the effects of changing hydration
states in real-world settings that require a complex array of
cognitive abilities. Such studies have already been carried
out, testing the effects of dehydration on performance in
real-life tasks such as airplane piloting and playing golf
(29,31)
.
An attempt has been made by one study to investigate how
drinking-water may be related to performance in examin-
ations in university students: Pawson et al.
(72)
observed the
number of people who took drinks to university examination
sessions and compared the performance of these students
with that of those who sat the same examination but did
not take a drink. The results revealed a positive relationship
between water taken to the examination session and
N. A. Masento et al.8
British Journal of Nutrition

performance in examination. Although these findings are
correlational and do not include a measure of students’ prior
hydration state or of the amount of water consumed during
the examination, these results support the notion that water
consumption, or preventing dehydration, can have cognitive
benefits. Further studies should investigate how drinking
habits influence real-world settings, particularly for tasks that
require a multitude of cognitive processes at once, such as
driving and airline traffic control.
Acute water intervention and mood
Self-reported mood has been reported to show particular sensi-
tivity to water consumption. One study that tested young adults
on a range of cognitive tasks, including attention and working
memory
(65)
, failed to find any significant impact of water con-
sumption on cognitive performance. However, mood ratings
were shown to significantly change when individuals were
given water. Individuals reported feeling more ‘calm’ and ‘alert’
immediately after water consumption. These results are in line
with those of other young adult studies that found similar reports
of ‘alertness’ after water consumption
(64)
. The recentstudy carried
out by Edmonds et al.
(52)
, which found that thirst mediated
cognitive effects, also tested mood using visual analogue mood
scales. The authors found that particular mood states were
influenced by the counterbalanced order of water conditions.
An example is that individuals were more confused when
exposed to the control condition first, in which they did not
receive any water, compared with when they were given water
in the experimental condition. Interestingly, this relationship
was moderated by drinking, indicating that water consumption
was associated with lower reported confusion levels, irrespective
of the condition order. This perhaps highlights that self-reported
mood may be influenced by the expectancy of the experimental
procedure itself and may have consequences for cognitive
performance; therefore, it is important that participants are
blind to the aims of the study to avoid such issues. Keeping
participants blind in water intervention studies is particularly
difficult when explicit instructions to consume water are
provided. Considerations should be made to mask the true
intentions of water consumption in such studies; one novel
study was carried out by Edmonds et al.
(66)
,inwhichtheexperi-
menter had a drinkher/himself and providedan additional cup of
water without explicitly instructing the participants to consume
the drink. The participants still consumed an adequate amount
of water (approximately 167 ml) that was enough to exert a
significant impact on cognitive performance.
The majority of studies that have investigated acute water
intervention in children have either not included a mood
measure or asked children to rate their ‘happiness’, which pro-
vides a measure of mood similar to standardised mood assess-
ments, and yet have so far failed to show any change after
water consumption. This is possibly because self-reported
happiness may not be sensitive enough, particularly as happi-
ness does not usually capture a state of arousal that has shown
sensitivity to water intervention
(64,65)
. A more recent study
involving children has used an adapted version of the Profiles
of Mood State questionnaire designed for children
(6)
and
found a significant correlation between better hydration and
reports of ‘vigour’, further supporting young adult mood
reports. Studies that have included mood measures of alert-
ness and other arousal states reveal that water consumption
does have a significant impact on alertness and arousal; how-
ever, the extent to which this consumption sustains these
mood changes is still inconclusive. Current findings suggest
that these mood effects are short-lived and occur immediately
after water consumption
(64,65)
. Future studies should consider
investigating the temporal pattern of mood changes before
and after water consumption.
Mediating factors
Despite only a relatively small collection of published empiri-
cal studies, evidence on acute water intervention hydration
and cognition suggests that both cognitive performance and
self-reported mood benefit from water consumption. As the
field of water intervention is still in its infancy, there is some
uncertainty as to how mediating factors such as water
temperature and time of cognitive testing can influence sub-
sequent intervention effects. To date, no water intervention
studies have standardised water temperature and a majority
of them have failed to report water temperature, despite evi-
dence suggesting that particular chilled water temperatures
(58C) are most pleasant and thirst quenching
(73 – 75)
. This
may be a critical mediating factor, as individuals have shown
preference for chilled water when deprived for a period of
time
(76)
. This preference for chilled water may result in
improved motivation and mood after consumption, more so
than room temperature, potentially resulting in different out-
comes due to water intervention.
Water temperature has also been shown to influence the rate of
water absorption into the bloodstream from the gut
(77)
.This
change in absorption could mediate the critical time at which
cognitive performance measures should be taken. Based on the
current evidence, water absorption in the gut reaches its
peak into the bloodstream between 20 and 60 min after
ingestion
(78,79)
. Water intervention studies thus far have found
cognitive performance changes within a critical window of
20– 45 min
(64,66,68,69,71)
.Thiswindowiscloselyrelatedtothe
peak absorption rates, suggesting that the critical time for
cognitive testing should be in conjunction with this peak
absorption point. Should water temperature vary, this peak
absorption window is likely to be shifted and subsequently cog-
nitive testing time would need to be altered. These are important
considerations that need to be further investigated and con-
sidered in future empirical studies to truly identify how important
these mediating factors are for water intervention effects.
Mechanisms of action
Despite the expansion of this research area, we still do not have
a clear understanding as to how acute water intervention may
influence mental performance and its associated neural activity.
Researchers have suggested psychological mechanisms related
to limited attentional resources during thirst
(51,52,80)
. However,
evidence has also highlighted the importance of physiological
Hydration status, cognition and mood 9
British Journal of Nutrition

mechanisms, with findings that the expectancy of water alone
does not influence cognitive performance
(66)
. Herein, we not
only discuss previously proposed mechanisms but also intro-
duce new potential physiological mechanisms that we think
have been previously overlooked.
Psychological mechanisms have been commonly
proposed
(51,52,80)
to explain the effect of water consumption
on cognitive abilities and mood states. The global workspace
model
(81)
is a well-known generalised model of cognitive
processes that postulates that there are limited amounts of
cognitive resources and parallel processes often compete to
obtain these resources. Applied to the topic at hand, states
such as thirst and dehydration compete for these resources,
resulting in limited capacity for other mental processes
(80)
.
Within the context of acute water intervention, by alleviating
the state of thirst and dehydration, these states no longer require
allocation of resources, thus allowing parallel processes to
recruit the required resources. This shift in cognitive resource
allocation may provide the mechanism for performance
change in cognitive tasks after water consumption. Support
for this mechanism can be found in studies that demonstrated
improvements in cognitive performance after hydration, with
the level of thirst mediating the effect
(52,64)
. However, an alterna-
tive interpretation is that the state of thirst could be an indication
of mild dehydration that could subsequently induce physiologi-
cal changes, similar to those observed in brain imaging data
(38)
such as total brain volume shrinkage. Future studies focusing
on this potential mechanism will help us to decipher whether
it is the influence of thirst itself or the consequence of dehy-
dration that underlies any changes in cognitive performance.
Potential physiological mechanisms for performance
improvements after water intervention are based on theorised
physiological changes as a result of water consumption.
To date, researchers have not explored these mechanisms.
The importance of physiological mechanisms, in addition to
psychological mechanism, is underscored by a recent study
carried out by Edmonds et al.
(66)
. The researchers manipulated
expectancy by informing half of the participants about the
beneficial effects of water consumption on cognitive perform-
ance during either a no-water or water consumption period.
Cognitive improvements were found after water intervention,
with no influence of expectancy. The authors posit that these
findings reveal the lack of influence that expectancy has on
cognitive improvements after water intervention and provide
support for physiological mechanisms. To date, it is still
unclear as to what hydration state participants in empirical
studies reviewed above actually experience. With a lack of
objective measurement, it is not known whether individuals
experience mild dehydration at baseline or a euhydrated
state. With evidence to suggest that even mild dehydration
states are associated with significant changes at the neural
level, such as total brain volume shrinkage and over-recruitment
of specific brain areas during cognitively demanding tasks
(38)
,
it may be possible that providing mildly dehydrated
participants with water may be reversing this effect. With a
lack of data related to baseline hydration states of individuals
and no further published work using imaging techniques to
examine hydration state, these proposed mechanisms are
merely speculative.
Another physiological mechanism to consider is the reactiv-
ity of the cardiovascular system after acute water consump-
tion
(82)
. Reduced heart rate and vasodilation have been
found in young adults after drinking 500 ml of water, whereas
a significant blood pressure increase has been observed in
healthy older adults
(83)
. This cardiovascular reactivity probably
promotes cerebral blood flow, which will encourage the circu-
lation of substances such as oxygen and glucose known to
stimulate neural activity and associated behavioural perform-
ance
(84)
, a mechanism similar to that suggested for cognitive
function improvements due to physical exercise
(85)
. Future
studies should consider using neuroimaging techniques such
as functional MRI and perfusion to understand how cardio-
vascular changes can be related to neural activity changes
and thus how these influence cognitive performance.
Conclusion
Accumulating evidence supports the notion that hydration
state affects cognitive ability and mood. Severe dehydration
has been shown to cause cognitive deficits such as short-term
memory and visual perceptual abilities as well as mood distur-
bance, whereas water consumption can improve cognitive
performance, particularly visual attention and mood. This
research field is still in its infancy and fundamentally there is
still a high amount of variability with regard to cognitive find-
ings in both dehydration and acute water intervention studies.
Researchers should investigate why this variability occurs and
what the optimum conditions are for hydration state to affect
cognitive performance. In this review, we have highlighted
the importance of controlling for any potential confounding fac-
tors that may occur due to experimental design, exercise/heat
stress protocols used in dehydration studies or conditions
related to acute water intervention such as water temperature.
Other advancements include taking into account the mechan-
isms that may underlie the observed performance changes:
conducting behavioural studies with physiological markers to
monitor hydration state such as urine indices and neuro-
imaging studies to discover the underlying neural events
during hydration state change. Standardising cognitive testing
would also help advance knowledge in this field. The topic is
highly relevant for public health and engages with a wide
audience, and this research has the potential to pave the way
for intervention programmes in public arenas, improving
people’s quality of life.
Acknowledgements
The authors thank Professor David Richardson for his assist-
ance in the preparation of the final manuscript.
Britvic Soft Drinks Plc (grant number: F3408400) partially
funded the PhD studentship of N. A. M. Britvic Soft Drinks
had no role in the design and analysis or writing of this article.
All authors contributed to the manuscript equally.
None of the authors has any conflicts of interest to declare.
N. A. Masento et al.10
British Journal of Nutrition

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