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International Journal of Sports Science 2018, 8(3): 97-107
DOI: 10.5923/j.sports.20180803.05
Impact of Sodium Ingestion During Exercise on
Endurance Performance: A Systematic Review
Alan J. McCubbin*, Ricardo J.S. Costa
Department of Nutrition, Dietetics and Food, Monash University, Notting Hill, Victoria, Australia
Abstract Sports nutrition guidelines frequently encourage sodium ingestion during endurance exercise, and much work
has been undertaken to quantify sweat sodium losses during exercise. However, current guidelines for sodium do not
recommend specific quantities, nor provide justification for the effectiveness of sodium to improve endurance performance.
A systematic review was undertaken using six databases (CINAHL, Embase, Medline Ovid, Scopus, SPORTDiscus, and
Web of Science) to determine the effect of sodium ingestion during exercise on endurance performance. Five studies met
the inclusion criteria. They varied in quantity of sodium consumed (280 to 900mg/h), ingestion method (capsules or
solutions), fluid intake (programmed or ad libitum) and performance outcomes (time trial, distance-test, time to exhaustion
following steady state exercise, and finish time in an organized competition). Only one study reported a significant benefit
from sodium ingestion (504mg/h) of 7.8%. All other studies found no significant effect of sodium on performance. Several
limitations were found, including different ambient conditions across study days, ad libitum carbohydrate intake that was not
reported, and performance measured during an organized competition where other factors may have influenced finish time.
No study measured performance in hot ambient conditions (e.g., ≥30°C), and no study quantified each participant’s sweat
sodium losses beforehand, thus providing sodium intake as a proportion of expected losses. It is concluded that there is
currently minimal evidence that sodium ingestion during exercise improves endurance performance. The limited number and
quality of existing studies indicates a need for future work in this area.
Keywords Salt, Sweat sodium, Physical activity, Endurance exercise, Endurance performance
1. Introduction
During endurance exercise, the production of sweat for
purposes of thermoregulation results in significant losses of
body water, as well as electrolytes [1]. Because sodium is
the predominant electrolyte lost through sweating, the
quantification of these losses through sweat collection and
analysis has been the subject of much attention from
researchers [2-9]. It is generally accepted that optimal
replacement of fluid and sodium during endurance exercise
will enhance the rate of fluid absorption and gastrointestinal
comfort, maintain total body water (TBW), plasma volume
(PV) and serum sodium concentration (serum [Na+]) [1].
The maintenance of TBW and PV, through adequate
fluid and/or sodium intake, has been shown to prevent
a performance decline in high intensity endurance
efforts when they follow prolonged steady state exercise,
with TBW losses of as little as 1-2% of initial body mass
* Corresponding author:
alan.mccubbin@monash.edu (Alan J. McCubbin)
Published online at http://journal.sapub.org/sports
Copyright © 2018 The Author(s). Published by Scientific & Academic Publishing
This work is licensed under the Creative Commons Attribution International
License (CC BY). http://creativecommons.org/licenses/by/4.0/
resulting in performance decrements [10, 11]. In contrast,
larger TBW deficits appear to be required before a
reduction in performance is seen in longer, submaximal
endurance efforts [12]. In addition to the effect on body
water, maintenance of blood sodium concentration is
particularly important for the health of the athlete, with the
consequences of exercise-associated hyponatraemia (EAH)
a cause for great concern, especially in ultra-endurance
sports populations [13-15]. Perhaps due to its greater
overall influence on PV and serum [Na+] [13, 16], and
because it is easy to quantify in athletes [17], there has been
significant attention paid to fluid replacement guidelines
during exercise [1, 6]. Despite much effort to collect and
analyse sweat composition in athletes, there are currently no
specific recommendations for sodium replacement during
exercise, with current guidelines stating only that sodium
should be replaced “…when large sweat losses occur…”
[18].
Anecdotally, many endurance athletes and their support
teams seek to quantify sweat sodium losses through ‘sweat
testing’ services, with the belief that specific sodium
replacement will improve performance. This belief appears
to include both direct performance benefits from sodium
consumption, or indirect benefits through the prevention of
exercise-associated muscle cramps (EAMC) and/or EAH.
98 Alan J. McCubbin et al.: Impact of Sodium Ingestion During
Exercise on Endurance Performance: A Systematic Review
Commercial sweat composition testing services exist in
many countries, and much work has been undertaken to
improve the validity and reliability of field techniques for
sweat collection and analysis [2, 8, 9, 19, 20]. However, the
dichotomy between well validated sweat composition and
testing protocols, and lack of specific sodium replacement
guidelines, suggests an absence of research examining the
effect of specific sodium replacement during exercise on
subsequent endurance performance. Therefore, the aim of
this systematic review was to determine the impact of
quantifiable sodium replacement during exercise on
endurance performance, either directly or by attenuating
EAMC or EAH.
2. Methods
A systematic literature search was undertaken by two
researchers, to determine the impact of sodium replacement
during endurance exercise on measures of performance in
accordance with the Preferred Reporting Items for
Systematic Review and Meta-Analyses (PRISMA) statement
[21].
2.1. Search Strategy
A three-step search was undertaken of published
English-language studies in six online scientific databases
from inception to March 2018 (CINAHL, Embase, Ovid
MEDLINE, Scopus, SPORTDiscus, and Web of Science). In
addition, the reference lists of all identified studies and other
known review papers relevant to the topic were searched to
identify additional studies that may have been missed by the
original search. In order to obtain the level of methodological
detail required, book chapters, opinion articles, reviews,
unpublished works, abstracts, short reports, and case-studies
were not considered. The keywords applied in the literature
search are shown in Table 1.
2.2. Eligibility Criteria
Eligibility criteria were established by the researchers
a priori in accordance with the Participant Intervention
Comparator Outcomes Study (PICOS) design format [21].
Original field observational studies and/or
laboratory-controlled trials, presenting quantified data on
endurance performance, within an exercise protocol of more
than 1 h duration, from participants exercising with two or
more levels of sodium intake during exercise were
considered for the review. Studies were suitable for inclusion
if they involved either a time trial or distance-test, or time to
exhaustion performance test, either as the entire exercise
bout or following a period of steady state exercise. After
duplicates were removed, the titles and abstracts were
reviewed by two researchers against the eligibility criteria
(Figure 1).
2.3. Data Extraction
Relevant data was extracted by two researchers and
cross-checked. Variables extracted were the number of
participants, age, training status (years of experience and
V
O2max where available); dietary intervention (sodium intake
during exercise); exercise protocol used, including
performance test; environmental conditions during the
exercise bout; hydration status before and changes during the
exercise bout; heart rate, perceived exertion, and outcomes
of the performance test. During the data extraction process
eligibility was again checked, and appropriate inclusion or
exclusion action was taken. Any difference of opinion
between researchers during the review process was resolved
by discussion and consensus. Where possible, units were
standardised by simple mathematical conversion. Data were
not considered appropriate for further synthesis into a
meta-analysis due to the absence of homogeneous outcome
measures.
2.4. Risk of Bias Assessment
Risk of bias assessment was performed using the
Cochrane ‘Risk of bias’ assessment tool [22]. The tool
assesses the risk of selection bias (due to random sequence
generation and concealment of allocation), performance bias
(from inadequate participant blinding), detection bias
(inadequate personnel blinding), attrition bias (incomplete
outcome data), reporting bias (selective reporting of
outcomes), and other potential forms of bias.
Table 1. Search strategy for the systematic review on the effect of dietary sodium intake during exercise on endurance performance
Field One (combine with OR) –
Population
Field Two (combine with OR) –
Intervention and Comparison
Field Three (combine with OR) - Outcome
Keywords: Athlet*, Physical
Exertion, Physical Activit*, Exercis*,
Sport*
MeSH headings: Athletes, Exercise,
Physical Exertion, Physical Activity,
Exercise, Sports
AND
Keywords: sodium*, sodium intake*,
salt intake*, diet* adj2 salt, salt adj2
restrict*
MeSH headings: Sodium (dietary),
Sodium, Diet (Sodium-restricted)
AND
Keywords:, athletic perform*, endurance
perfor., perform*
MeSH headings: athletic performance,
endurance, endurance capacity
* used to retrieve unlimited suffix variations.
International Journal of Sports Science 2018, 8(3): 97-107 99
Figure 1. PRISMA diagram, showing the inclusion and exclusion of papers in the review
3. Results
3.1. Search Results
The initial database search yielded 4108 non-duplicate
citations. 4097 of these were excluded on title and abstract
screening, leaving 11 papers. Following the full text
screening, five papers met all inclusion criteria and are
included in this review (Table 2).
3.2. Study Characteristics
The five studies investigated the effect of sodium
ingestion during exercise on different aspects of endurance
performance. One study [23] used a time trial, one a
distance-test [24], one time to exhaustion following steady
state exercise [25], and two provided known quantities of
sodium during an organized endurance competition [26, 27].
Each of these performance types will be described
separately. Participants across these studies included both
sexes. Mean or median age varied between studies, from 27
[23] to 40 years [24]. Training experience (in years) or
cardiorespiratory fitness (V
O2max) was not reported in three
of the five studies [24, 25, 27]. All studies were undertaken
in temperate ambient conditions with minimal variation
between interventions, except one study in which the
temperature varied from 2.6-20.6°C across trial days,
resulting in an effect of trial order on performance [24].
Pre-exercise hydration status, by measurement of any
combination of TBW, plasma osmolality, plasma or serum
[Na+], or urine osmolality was reported in all but one study
[25]. The rate of sweat loss and subsequent changes in body
mass were reported in all studies and were not significantly
different between interventions. PV changes were reported
only in one study [23], in which sodium supplementation
increased PV by 1.8%, whilst placebo resulted in a 0.9%
reduction (p < 0.05).
3.3. Dietary Intervention
All studies provided sodium in the form of sodium
chloride. In four of the studies this was provided in capsules
and compared to a placebo capsule that contained no
sodium in a blinded manner [23, 25-27], whilst in the fifth
study [24] sodium was provided unblinded in a solution,
with a high and low sodium concentration as well as a water
only control. The quantity of sodium provided varied
between studies from 280 to 900 mg/h. The quantity
provided per hour was not related to the exercise duration or
ambient conditions. In all studies the amount of sodium
provided was the same for every participant per hour of
exercise, with no individualisation of sodium dose based on
the participant’s body mass, sweat sodium concentration, or
expected sweat sodium losses.
100 Alan J. McCubbin et al.: Impact of Sodium Ingestion During
Exercise on Endurance Performance: A Systematic Review
Table 2. Systematic review search results, showing included papers to determine the impact of sodium ingestion during exercise on endurance
performance
Reference
Participants
Sodium intake
intervention,
fluid & CHO
intake during
exercise
Environmental
conditions
Exercise
Protocol
Change in hydration
status
Heart Rate (bpm)
and Rating of
Perceived Exertion
Performance,
EAH and EAMC
Cosgrove &
Black
(2013)
9 well trained
cyclists
(male: 5, female:
4),
Age 27 ± 9 years,
V
O2max 62 ± 8
mL/kg/min
Crossover trial:
Na: 280mg/h
given in
capsules (NaCl
or placebo)
Fluid: Ad
libitum
Na: 428 ± 166
mL/h
Placebo: 269 ±
65 mL/h *
CHO: Ad
libitum,
quantities
consumed not
reported
Na: Tamb 14.0
± 2.1oC,
63% RH
Placebo: Tamb
13.5 ± 2.1oC,
63% RH
72km
cycling TT
outdoors
(undulating
course)
Sweat Rate:
Na: 570 ± 220 mL/h
Placebo: 710 ± 290
mL/h NS
Body Mass Loss:
Na: 1.0 ± 0.8%
Placebo: 1.0 ± 0.6%
NS
PV Change:
Na: +1. 8 ± 2.2%
Placebo: -0.9 ±
1.8% *
Pre-ex Plasma
[Na+]:
Na: 142 ± 2 mmol/L
Placebo: 140 ± 1
mmol/L *
Mean HR:
Na: 158 ± 9
Placebo: 157 ± 9
NS
72km TT
performance:
Na: 171.3 ± 23.5
min
Placebo: 172.3 ±
23.3 min NS
EAH: None in
either group
EAMC: Not
reported
Del Coso et
al. (2016)
26 well trained
male triathletes
Age 37 ± 7 years
Experience 8 ± 3
years
Randomised
trial:
Na: 504mg/h,
given in
capsules
(Saltstick caps
or placebo)
consumed
during
transitions &
bike leg.
Fluid: Ad
libitum
Na: 371 ± 78
mL/h
Placebo: 273 ±
109 mL/h*
CHO: Ad
libitum
Na: 32 ± 14 g/h
Placebo: 40 ±
17 g/h NS
Tamb 22.5 ±
2.7 °C,
36.8 ± 8.3%
RH
Half IM
distance
triathlon
race
Sweat Rate:
Na: 781 ± 214 mL/h
Placebo: 721 ± 198
mL/h NS
Body Mass Loss:
Na: 3.4 ± 1.3%
Placebo: 2.8 ± 0.9%
NS
PV not reported
POsm (Na
intervention):
Pre-ex: 291 ± 4
mOsm/kg
Post-ex: 304 ± 5
mOsm/kg †
POsm (Placebo):
Pre-ex: 291 ± 6
mOsm/kg
Post-ex: 300 ± 4
mOsm/kg †
Serum [Na+] (Na
intervention):
Pre-ex: 142 ± 2
mmol/L
Post-ex: 145 ± 2
mmol/L †
Serum [Na+]
HR not reported
Whole-race RPE
obtained
post-exercise:
Na: 17 ± 2
Placebo: 16 ± 3 NS
Race finish time:
Na: 307 ± 32min
Placebo: 333 ±
40 min *
EAH: Not
reported
EAMC: Not
reported
International Journal of Sports Science 2018, 8(3): 97-107 101
(Placebo):
Pre-ex: 141 ± 1
mmol/L
Post-ex: 143 ± 2
mmol/L †
Earhart et
al. (2015)
11 experienced
runners/ cyclists
(male: 4, female:
7),
2 cycling, 9
running
Age 31 ± 12
years
Crossover trial:
Na: 900mg/h,
given in
capsules (NaCl
or placebo)
Fluid: 400
mL/h for body
wt up to 70kg,
600 mL/h to
body wt
70-89kg,
800 mL/h for
body wt >90kg.
CHO: Not
reported
Na: 21.0 ±
0.6°C,
34.5 ± 11.0%
RH
Placebo: 21.3
± 0.3°C
34.88 ± 10.6%
RH
2hr
treadmill
running or
stationary
cycling at
60% HRR,
followed
by TTE at
incremental
increasing
intensity
Sweat Rate:
Na: 1016 ± 239
mL/h
Placebo: 1054 ±
278 mL/h NS
Body Mass Loss:
Na: 2.0 ± 0.4%
Placebo: 2.3 ± 0.7%
NS
PV not reported
Peak HR:
Na: 181 ± 13
Placebo: 180 ± 12
NS
Mean RPE during
steady state:
Na: 13 ± 1
Placebo: 13 ± 1 NS
Time To
Exhaustion:
Na: 6.88 ± 3.88
min
Placebo: 6.96 ±
3.61min NS
EAH: Not
reported
EAMC: One
participant in the
evening after the
Na trial
Hew-Butler
et al. (2006)
114 IM
triathletes
(male: 104,
female: 10)
Age 34 ± 7 years
Randomised
trial:
Na: 284 ± 160
mg/h, given in
capsules (NaCl
or placebo)
Fluid: ad
libitum and not
reported
CHO: ad
libitum and not
reported
Range: Tamb
15.6-20.9°C,
48-79% RH
IM
triathlon
race
Sweat Rate:
Na: 229 ± 102 mL/h
Placebo: 236 ± 134
mL/h NS
Body Mass Loss:
Na: 3.6 ± 1.4%
Placebo: 3.9 ± 2.1%
NS
PV not reported
Serum [Na+] (Na
intervention):
Pre-ex: 141 ± 2
mmol/L
Post-ex: 142 ± 3
mmol/L NS
Serum [Na+]
(Placebo):
Pre-ex: 141 ± 2
mmol/L
Post-ex: 141 ± 4
mmol/L NS
HR not reported
Exercise intensity
(1-10 scale,
converted to 6-20
RPE scale):
Na: 16 ± 9
Placebo: 16 ± 9 NS
Race finish time:
Na: 758 ± 88 min
Placebo: 762 ±
101min NS
EAH: One
participant in the
placebo group
developed EAH.
EAMC: Not
reported
Twerenbold
et al. (2003)
13 well trained
female
runners
Age: Median
(range)
40 (22-53) years
Crossover trial:
High Na – 680
mg/h
Low Na – 410
mg/h
Water – 0 mg/h
Taken in water
with different
NaCl
concentrations
Fluid: 1000
mL/h in all
Range Tamb:
2.6-20.6oC,
large
differences on
different trial
days, RH not
reported
4 hour
running DT
on outdoor
athletics
track
Sweat Rate:
High Na: 525 ± 250
mL/h
Low Na: 450 ± 300
m/h
Water: 475 ± 475
mL/h NS
Body Mass Loss:
High Na: 3.6 ±
1.7%
Low Na: 3.1 ± 2.1%
Water: 3.3 ± 3.3%
HR and RPE not
reported
4hr Distance
Trial:
High Na: 39.9 ±
4.3 km
Low Na: 42.0 ±
4.8 km
Water: 40.6 ± 5.2
km NS
Mild EAH
(plasma [Na+]
130-135mmol/L):
High Na: 46% of
102 Alan J. McCubbin et al.: Impact of Sodium Ingestion During
Exercise on Endurance Performance: A Systematic Review
trials
CHO: 60 g/h in
Low Na and
water trials, 63
g/h in High Na
trial.
NS
PV not reported
Plasma [Na+]
Pre-ex:
High Na: 137 ± 1
mmol/L
Low Na: 137 ± 1
mmol/L
Water: 138 ± 2
mmol/L NS
Plasma [Na+]
Pre-ex:
High Na: 135 ± 3
mmol/L
Low Na: 133 ± 2
mmol/L
Water: 131 ± 2
mmol/L §
participants
Low Na: 69%
Water: 92%
Severe but
asymptomatic
EAH (plasma
[Na+]
<130mmol/L):
High Na: 0% of
participants
Low Na: 0%
Water: 17%
Symptomatic
EAH: None in
any group
EAMC: Not
reported
EAH: Exercise-associated hyponatraemia, EAMC: Exercise-associated muscle cramps, Na: Sodium, NaCl: Sodium chloride, CHO: Carbohydrate Tamb: ambient
temperature, RH: relative humidity, TT: time trial, TTE: time to exhaustion, DT: distance-test, IM: Ironman, HRR: Heart Rate Reserve, RPE: Rating of Perceived
Exertion (6-20), PV: Plasma volume, NS: Not significant, † Difference pre- to post-exercise, * Difference between sodium intakes (p < 0.05), § Difference between high
Na and water interventions (p < 0.05).
3.4. Effect of Sodium Intake on Time Trial Performance
One study assessed time trial performance in a 72 km
cycling time trial on an outdoor course, including both male
(n= 5) and female (n= 4) athletes. Capsules provided either
280 mg/h or 0 mg/h of sodium, and mean heart rate (p=
0.86) and time trial performance was not different between
interventions (p= 0.46).
3.5. Effect of Sodium Intake on Distance-test
Performance
One study assessed the distance covered in four hours
around an outdoor running track in female ultramarathon
runners (n= 13) [24]. In a crossover design, a high and low
concentration of sodium solution were consumed as well as
water in separate trials, in an unblinded fashion. There were
no significant differences in the distance completed
between interventions, however it should be noted that an
effect of trial order was found (p < 0.0001), most likely due
to the variable ambient conditions on each trial day
(temperature range of 2.6-20.6°C across trials).
3.6. Effect of Sodium Intake on Time to Exhaustion
following Steady State Endurance Exercise
One study assessed the time to exhaustion of male (n= 7)
and female (n= 4) endurance athletes during an incremental
exercise test using either stationary cycling with increasing
power output (n= 2), or treadmill running with increasing
gradient (n= 9) in a laboratory setting [25]. The incremental
test was preceded by two hours of steady state cycling or
running at 60% of heart rate reserve. Capsules provided
either 900 mg/hr or 0 mg/hr sodium, and time to exhaustion
was not different between interventions (p= 0.919).
3.7. Effect of Sodium Intake on Performance during
Organized Endurance Competitions
Two studies assessed the effect of sodium intake on
performance during an organized half- (n= 26, all male) [26]
and full-Ironman (IM) triathlon (n= 114, 104 male) [27].
Participants were randomised to consume either sodium or
placebo capsules throughout the race (half-IM 504 mg/h,
IM 284 mg/h). Fluid and carbohydrate (CHO) were
consumed ad libitum in both studies. In the half-IM, fluid
intake was significantly greater in the sodium compared to
placebo group (371 ± 78 and 273 ± 109 mL/h respectively,
p= 0.05) with no significant difference in CHO intake (p=
0.39) [26]. Fluid and CHO intake were not reported in the
IM study [27]. In the Half-IM, mean finish time was 7.8%
faster in the sodium group compared to placebo (p= 0.04)
[26], whereas there was no difference in finish time
between groups in the IM (p= 0.14) [27].
3.8. Effect of Sodium Intake on EAMC and EAH
Only one study reported on the incidence of EAMC,
noting only one participant reported EAMC, in the evening
after the exercise bout was completed, having consumed the
high Na intake during exercise [25]. Three of the five
studies reported incidence of EAH. One reported no cases
of EAH [23]. Another reported a single participant
developing symptomatic EAH requiring hospitalisation,
following excessive fluid ingestion and substantial body
mass gain [26]. A third study reported an incidence of more
than 40% for asymptomatic EAH in participants with no,
low and high sodium ingestion [27], when fluid was
ingested at around double the rate of sweat losses. The
incidence of mild EAH (plasma sodium concentration
International Journal of Sports Science 2018, 8(3): 97-107 103
130-135 mmol/L) was lower with increasing sodium
intake (High Na: 46%, Low Na: 69%, Water: 92%) and
post-exercise plasma sodium concentration was
significantly lower with water compared to low and high
sodium intakes (p < 0.001) [27].
3.9. Risk of Bias Assessment
Results of the risk of bias assessment are shown in Table
3. Selection bias was not evident in any study. Potential
performance and detection bias was observed in one of the
five papers [24], due to a lack of blinding of the sodium
intervention. Potential reporting bias was seen due to
uneven or unreported participant drop outs in two studies
[25, 27], although the effect of this was unclear. The main
sources of bias however came from inadequate control of
ambient conditions and dietary intake, with at least one of
these factors evident in all included studies. Overall none of
the studies were deemed low risk of bias, with three deemed
unclear [25-27] and two high risk of bias [23, 24].
4. Discussion
The ingestion of sodium during endurance exercise is
commonplace and is reflected in the specific inclusion of
sodium in sports nutrition guidelines and many sports
nutrition targeted commercial products [1, 6, 18]. This
systematic review aimed to determine the effect of
quantifiable sodium ingestion during exercise on endurance
performance outcomes. Our search of the literature found
only five studies that met the inclusion criteria, and only
one that suggested a benefit from sodium replacement on
endurance performance [26], at least for the exercise
intensities, durations, and sodium intake doses studied. It
should also be noted that the quality of the included studies
was generally poor, due to variations in ambient conditions
between trial days, and differences in CHO or fluid intake
between interventions.
Table 3. Risk of bias assessment for the systematic review to determine the impact of sodium ingestion during exercise on endurance performance, using
the Cochrane Collaboration’s ‘Risk of bias’ tool [20]
Criteria
Cosgrove & Black (2013)
Del Coso et al. (2016)
Earhart et al. (2015)
Hew-Butler et al. (2006)
Twerenbold et al.
(2003)
Random
sequence
generation
Randomized crossover
design
Participants were
pair-matched for age,
anthropometry,
experience and best race
time, then randomly
allocated
Randomized crossover
design
Random allocation
Randomized
crossover design
Allocation
concealment
Randomized crossover
design
Not stated
Randomized crossover
design
Not stated
Randomized
crossover design
Participant/
personnel
blinding
Participants and
personnel blinded
Participants and
personnel blinded
Participants blinded.
Personnel blinding not
stated
Participants blinded.
Personnel blinding not
stated
Not blinded.
Outcome
assessment
blinding
Personnel blinded
Personnel blinded
Not stated
Not stated
Not stated
Incomplete
outcome data
Complete
Drop outs reported and
even across both groups
Incomplete – one drop
out due to GI distress
from NaCl capsules, two
due to schedule conflicts
Drop outs described but
not clear from which
group
Not stated
Selective
reporting
N/A - only one outcome
measure reported
N/A - only one outcome
measure reported
N/A - only one outcome
measure reported
N/A - only one outcome
measure reported
N/A - only one
outcome measure
reported
Other
potential
sources of
bias
Outdoor time trial on two
separate occasions,
possible differences in
wind speed/direction.
CHO intake ad libitum
but intake not reported.
Fluid intake greater in Na
trial compared to placebo
(p < 0.05).
Fluid intake greater in Na
trial compared to placebo
(p < 0.05). Performance
measured in organized
race with other
competitors, which may
influence performance
CHO intake not reported
– presumed to be none
CHO and fluid intake ad
libitum but not reported –
possible differences
between groups.
Performance measured in
organized race with other
competitors, which may
influence performance
Large differences
in ambient
temperature
between trial days
(by 15oC). Effect
of trial order on
performance.
Judgement
High risk of bias
Unclear risk of bias
Unclear risk of bias
Unclear risk of bias
High risk of bias
104 Alan J. McCubbin et al.: Impact of Sodium Ingestion During
Exercise on Endurance Performance: A Systematic Review
4.1. Study Methodologies
Sweat sodium losses vary significantly between and
within individuals, with sweat fluid losses and sweat
sodium concentration both determinants [2]. These factors
in turn are influenced by exercise mode, intensity, duration
and ambient conditions; body composition; sweat gland
density and distribution; heat acclimation and/or
acclimatisation, and possibly dietary sodium intake [2, 4,
15]. Although four of the five studies provided sodium or
placebo in capsules during exercise, in temperate ambient
conditions, the sodium dose consumed per hour varied by a
factor of three between studies. The amount was not related
to participant body mass, ambient conditions or exercise
duration. We were surprised to find no studies have
attempted to measure the participant’s sweat sodium losses
in well-controlled laboratory conditions, and/or provide
sodium in quantities that represented a specific proportion
of their expected losses. It cannot be determined from the
included studies if the sodium ingested was adequate to
replace a given proportion of sodium losses in some, all or
none of the participants. The arbitrary nature of the sodium
interventions would make interpretation of results difficult,
if indeed there was a dose-dependent effect of sodium
ingestion on performance.
The performance outcomes measured in the included
studies also varied considerably. Notwithstanding the
differences in validity between time to exhaustion and time
trial or distance-tests [28], there was no particular effect of
the type of performance measured on whether or not
sodium intake was beneficial. The single study to show a
performance improvement did so by measuring the effect of
ingestion of sodium or placebo capsules, in a blinded
manner, on finish time of a half-IM triathlon [26]. In
contrast, the study of sodium supplementation on
performance in a full distance Ironman triathlon failed to
find a difference in performance [27]. The inclusion of
these two studies is also of interest, with advantages and
disadvantages of measuring performance during an
organized competition. Ultimately, performance during
actual endurance events is the outcome of most importance
to athletes, so it can be argued that these studies represent
ecologically valid performance measures. On the other hand,
this design introduces the possibility that performance
outcomes are influenced by factors other than the
intervention studied (e.g. sodium ingestion), reducing the
reliability of this study design [28]. As both studies that
occurred during organized competitions were triathlon
events, variables such as the time spent in transition
between legs, stopping or slowing down at aid stations,
race tactics in response to other competitors and changes
in race scenario could all conceivably influence the primary
outcome, namely event finish time. We note that the
performance improvement attributed to sodium
supplementation in the half-IM event was 7.8% [26],
a performance difference that would seem implausibly
high from a single nutritional intervention alone. In
laboratory-controlled studies measuring time trial
performance, the beneficial effects of varied CHO, fluid,
and/or caffeine ingestion are rarely greater than 5% [29-31].
Therefore, we suggest that future studies use an
ecologically valid exercise model, but do so in a more
controlled laboratory or field-based setting to improve their
reliability.
Four of the five included studies were conducted
outdoors [23, 24, 26, 27]. This means that ambient
temperature, relative humidity, wind speed and direction
could vary considerably between intervention groups. In the
organized competition studies, participants competed at the
same time, taking either sodium or placebo supplementation.
Therefore conditions were not different between groups at
race start, but may have differed as event duration increased
in time. For the crossover trials however, participants
completed trials on different days. One study [24] observed
an effect of trial order, likely due to large differences in
ambient temperature between trial days. Cosgrove and
Black [23] reported no significant difference in ambient
conditions between trial days but did not report wind
conditions, other than to state the course and time of day
was chosen to minimise the influence of the wind. Cycling
time trial performance is significantly affected by wind
speed and direction, whereby even subtle changes could
conceivably have measurable effects on performance [32].
To our surprise, none of the included studies were
completed in hot ambient conditions (e.g., ≥30°C), where
any theoretical benefit of sodium ingestion on performance
would be expected to be greater, due to increased fluid and
sodium losses. Moreover, in hot ambient conditions the
total sodium losses would be expected to be greater than
temperate conditions due to increased sweating rate,
especially for individuals who are not heat acclimated [2].
Although excessive fluid intake has been identified as the
main cause of EAH in most cases, there are suggestions that
excessive sweat sodium losses even in the absence of fluid
excess can increase the risk of a hypovolaemic EAH, the
development of which, if symptomatic, would almost
certainly impair performance [13]. Ideally, studies of
sodium intake during exercise and performance would be
conducted in a laboratory setting, in both hot and temperate
ambient conditions, with careful control of all
environmental variables.
Two other major control variables when measuring
endurance exercise performance are the intake of CHO and
fluid during the exercise bout. The effect of CHO intake on
performance is well established, with increased CHO
ingestion during the exercise durations of the included
studies likely to be beneficial to performance [31]. Despite
this, only one of the included studies provided a
standardised CHO intake across intervention groups (60g/h
in low and no sodium trials, 63g/h in high sodium trial) [24],
while one of the included studies conducted during an
organized competition allowed ad libitum CHO intake and
reported no difference between groups (sodium intervention:
International Journal of Sports Science 2018, 8(3): 97-107 105
32 g/h, placebo: 40 g/h) [26]. Differences in fluid intake
between intervention groups may also be of significance to
the performance outcomes of the included studies. It is
therefore perhaps surprising that only two of the included
studies standardised fluid intake between interventions [24,
25], with the other three allowing ad libitum fluid intake [23,
26, 27]. Of the three studies where fluid intake was allowed
ad libitum, two reported actual fluid intake, and in both
cases fluid intake was significantly greater with sodium
supplementation compared to placebo [23]. Some authors
have suggested that one of the benefits of sodium ingestion
during exercise is the stimulation of thirst and subsequently
greater voluntary fluid intake and fluid retention [1].
Therefore, it could be argued that the lack of standardisation
of fluid intake represents a feature rather than a flaw in
study design. In future studies, comparison of sodium intake
with both standardised and ad libitum fluid intakes would
be helpful in understanding whether sodium has a direct
(when fluid intake is standardised) or an indirect effect (due
to stimulation of greater fluid intake) on performance in
different scenarios.
Finally, the method of sodium ingestion may also be
important in determining any performance effect. Four of
the five included studies provided sodium in capsule form.
This provides the distinct advantage of participant blinding,
since the distinctive taste of sodium chloride cannot be
detected and compared to the placebo. The use of capsules
assumes that any effect of sodium ingestion occurs from
absorption of sodium from the gastrointestinal tract, and
subsequent effects on PV, osmolality and thirst drive. Given
that there appear to be effects of both CHO and caffeine in
the oral cavity on performance, without ingestion [33, 34],
it is conceivable that there could be an independent effect of
sodium in the oral cavity that is independent of consuming
it in capsule form. Future studies may be able to examine
this relationship by comparing the consumption of sodium
capsules and solutions on endurance performance.
4.2. Mechanisms for a Potential Effect of Sodium Intake
during Endurance Exercise on Performance
Guidelines that encourage the consumption of sodium
during endurance exercise suggest two mechanisms that
could theoretically improve performance. Firstly, sodium
intake increases thirst and promotes greater voluntary fluid
consumption [1]. Secondly, sodium ingestion promotes
greater PV retention and reduces urine output [1, 6]. These
mechanisms are often cited by sports nutrition companies
commercialising sodium containing products, implying
exercise performance benefits from quantified sodium
ingestion during endurance exercise. Of the three studies
included in this review where fluid was consumed ad
libitum during exercise, two found increased fluid
consumption with sodium supplementation [23, 26], whilst
the other did not report fluid intake [27]. But despite the
theoretical rationale that increased fluid intake would
improve performance, only one of these three studies found
a performance benefit from increased sodium (and
subsequently increased fluid) intake [26]. It is possible that
the exercise duration and subsequent relative exercise
intensity required to optimise performance in these
scenarios was too long and too low respectively; such that
TBW and PV are not limiting factors on performance, or
that because the rate of sweating is less with lowered
exercise intensity, ad libitum fluid intake is adequate to
maintain PV even in the absence of sodium ingestion. Only
one of the included studies reported change in PV during
exercise, and although there was an effect of sodium
ingestion, PV in the placebo group showed a reduction of
less than 1% [23]. Studying the effect of sodium ingestion
during exercise in hot ambient conditions (e.g., ≥30°C)
would provide more insight into whether the greater
maintenance of TBW and PV with sodium ingestion
subsequently improves performance.
4.3. Sodium Ingestion during Exercise, Endurance
Performance and Current Sports Nutrition
Guidelines
Current sports nutrition guidelines specifically include
sodium as a key nutrient of interest, especially for
endurance athletes [1, 18]. However unlike CHO and fluid,
no specific sodium consumption recommendations are
advised, and there is minimal evidence to draw a link
between sodium ingestion and endurance performance. This
is not surprising given the findings of this systematic review,
which found only a small number of studies on the topic,
with methodologies that resulted in an unclear or high risk
of bias in all cases, and with no participant-specific sodium
intake levels used. This points to the need for more
controlled and robust research in this area, in order to better
inform sports nutrition recommendations and guidelines in
the future. Such research should include careful dietary
standardisation of both macronutrient and sodium intake in
the days prior to exercise, careful control of exercise
intensity and duration, pre-exercise hydration status, heat
acclimation status and ambient conditions [4]. In addition,
sweat sodium losses should be measured in an identical
prior exercise bout, with whole body washdown the
preferred method of sweat collection [2], and sodium
replacement provided during exercise as a proportion of the
expected losses, which can vary considerably [3].
4.4. Study Limitations
One potential limitation to this systematic review is that
other existing studies were missed during our search. We
attempted to minimise this risk by searching the reference
lists of the identified papers, and of known review papers
relevant to the topic. We were also unable to perform a
meta-analysis of the included papers, due to the large
differences in study design, particularly the exercise
performance outcome measures used.
106 Alan J. McCubbin et al.: Impact of Sodium Ingestion During
Exercise on Endurance Performance: A Systematic Review
5. Conclusions
Endurance athletes anecdotally consume sodium
containing foods, fluids, and supplements during endurance
exercise, partly in the belief that it will improve their
exercise performance. In addition, commercial sweat testing
services allow quantification of sweat sodium losses during
exercise, providing athletes with an information guide to
replace expected losses during training or competition.
However, our systematic review failed to find any studies
that examine the effect of quantified sodium replacement
according to expected losses during endurance exercise.
In addition, we found that arbitrary sodium ingestion
during endurance exercise does not improve endurance
performance compared to no sodium intake in temperate
ambient conditions, albeit that all of the identified studies
were found to have either a high or unclear risk of bias, and
participants were not impaired during exercise by either
EAMC or symptomatic EAH from either the presence or
absence of sodium intake during the exercise bout. There
are currently no studies that describe the effect of sodium
ingestion during exercise on endurance performance in hot
ambient conditions. Future research should attempt to
address these issues, with valid and reliable quantification
of sweat sodium losses to inform sodium replacement
strategies, both programmed and ad libitum fluid ingestion,
during well controlled exertional-heat stress models, whilst
ensuring adequate control of study blinding and potentially
confounding variables such as CHO intake, hydration status,
wind speed and direction.
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