Hydration and Fluid Balance
Lamprecht M (ed): Acute Topics in Sport Nutrition.
Med Sport Sci. Basel, Karger, 2013, vol 59, pp 113–119
Salt and Fluid Loading: Effects on Blood
Volume and Exercise Performance
Ricardo Mora- Rodriguez ⭈ Nassim Hamouti
University of Castilla- La Mancha, Exercise Physiology Laboratory, Toledo, Spain
During prolonged exercise, fluid and salt losses through sweating reduce plasma volume which
leads to heart rate drift, hyperthermia and reductions in performance. Oral rehydration with water
reduces the loss of plasma volume and lessens heart rate drift and hyperthermia. Moreover, the
inclusion of sodium in the rehydration solution to levels higher than those in sweat, restores
plasma volume when ingested during exercise (e.g. 100 mmol/l Na+), and expands plasma volume
if ingested pre- exercise (e.g. 164 mmol/l Na+). Pre- exercise salt and fluid ingestion with the inten-
tion of expanding plasma volume has received an increasing amount of attention in the literature
in recent years. In four studies, pre- exercise salt and fluid ingestion improved performance, mea-
sured as time to exhaustion, either during exercise in a hot or thermoneutral environment. While
in a hot environment, the performance improvements were linked to lowering of core tempera-
tures and heart rate, the reasons for the improved performance in a thermoneutral environment
remain unclear. However, when using pre- exercise saline concentrations above 0.9% (i.e. >164
mmol/l Na+), osmolality and plasma sodium increase and core temperature remain at dehydration
levels. Thus, too much salt counteracts the beneficial effects of plasma volume expansion on heat
dissipation and hence in performance. In summary, the available literature suggests that pre-
exercise saline ingestion with concentrations not over 164 mmol/l Na+ is an ergogenic aid for sub-
sequent prolonged exercise in a warm or thermoneutral environment.
Copyright © 2012 S. Karger AG, Basel
While the importance of water replenishment during and after prolonged exercise
is commonly accepted, the need of salt replacement is debatable. Sodium in blood
exerts osmotic forces that defend plasma volume during prolonged exercise induc-
ing dehydration. One of the prominent adaptations to chronic exercise in a hot cli-
mate (i.e. acclimation) is to reduce sodium excretion in sweat . By doing so, more
sodium is kept in the blood, increasing osmotic forces that help to maintain blood
volume during progressive dehydration. The role of sodium in blood volume mainte-
nance is masterly illustrated in a recent experiment comparing cystic fibrosis patients
who excrete a lot of sodium in sweat (133 mmol/l) with control subjects with average
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114 Mora- Rodriguez · Hamouti
sweat sodium of 44 mmol/l . Both groups exercised in a hot environment to 3%
dehydration. Due to their large sweat Na+ losses, the patients finished the exercise
with a lower blood sodium concentration than the controls (146 vs. 150 mmol/l).
With less sodium in blood to exert osmotic forces, the cystic fibrosis group had larger
plasma volume reductions during exercise than control subjects.
While curtailing sodium losses could benefit the cardiovascular system by reduc-
ing blood volume losses, adding sodium to the blood holds the promise to maintain
blood volume during prolonged exercise. This is the main focus of this review. We
will present current information about the acute use of water and salt as a nutritional
aid that can help cardiovascular function. The impact of salt and water loading on
reducing cardiovascular and thermal strain and its consequences in exercise perfor-
mance will also be discussed. Most of the review will deal with oral ingestion although
intravenous delivery of saline solutions is also presented with the aim of clarifying
the physiological mechanisms. Human sweat during exercise contains ~45 mmol/l
of Na+. Thus, studies using drinks with sodium concentrations below that range of
concentrations (i.e. sports drinks) are not considered salt loading and are beyond the
scope of this review. Lastly, we focused on studies in a normotensive population with
normal renal function and appropriate ADH secretion. Salt and water loading is obvi-
ously discouraged in hypertensive populations.
Saline Delivery after Dehydration but prior to a Subsequent Exercise Bout
Fortney et al.  rehydrated with saline subjects that had lost 200 ml of their plasma
volume after exercise in the heat combined with water restriction. They used intra-
venous (IV) infusion of 3% saline (0.4 ml/kg body mass/min) to successfully restore
plasma volume. However, due to the high osmolality of the infusate (i.e. 1,026 mosm/
kg H2O), plasma osmolality remained at dehydrated values and thermoregulation
did not benefit from intravascular rehydration. A series of studies have followed
(reviewed by van Rosendal et al. ), using isotonic (0.9%; ~308 mosm/kg H2O) or
hypotonic (0.45%; ~154 mosm/kg H2O) IV saline infusion to rehydrate subjects that
have lost fluid during prolonged exercise in the heat. In general, these IV saline solu-
tions restored plasma volume to euhydrated conditions without differences between
the iso- and hypotonic saline . The acute expansion of blood volume with the IV
saline infusion seemed to restore central venous pressure improving heat dissipation.
In fact, in these studies, core temperature was lower during subsequent exercise than
when subjects did not receive saline infusions.
The main goal of the above- cited studies was to compare the rehydration effects of
IV vs. oral solutions and thus 0.45% saline solutions were also ingested. The main con-
clusion of these studies is that although IV rehydration seemed to be faster at restor-
ing plasma volume, the cardiovascular, thermoregulatory and performance benefits
during subsequent exercise were similar to when rehydrating orally . While the
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Saline Ingestion Improves Performance 115
mode of delivery of the saline (IV vs. oral) was nicely addressed in these studies, no
conclusions can be derived about the effects of adding salt to a rehydration fluid since
a water ingestion control trial was not included.
Saline Delivery during Dehydrating Exercise
We found three articles investigating the role of IV saline infusion delivered during
a dehydrating exercise on the cardiovascular, thermoregulatory and performance
responses to exercise. In two of them, saline was infused while pedaling in a hot envi-
ronment (30°C) while the other study was held in a thermoneutral environment but
at higher workload (i.e. 84 vs. 60–65% V2 max) and thus all of them caused moderate
levels of heat accumulation. The three studies coincided in that infusion of isotonic
saline (0.9%; 0.3–0.9 ml/kg body weight/min) during exercise restored plasma vol-
ume to pre- exercise levels [6–8]. The infusions suppressed the gradual drift in heart
rate and reduced the hyperthermia observed when no fluid was delivered. IV infu-
sion of isotonic saline neither raised plasma sodium  nor blood osmolality .
Interestingly, performance measured by endurance time was not improved by the IV
saline infusion . However, there was a wide spread of times to fatigue (8–42 min)
among the subjects participating in that study which together with the low reliability
of time to exhaustion to measure performance makes those results inconclusive.
We found three studies using saline ingestion during dehydrating exercise. In one
study , subjects ingested slightly hypertonic saline (1%; ~342 mosm/kg H2O) as a
rehydration fluid prior to and during exercise avoiding dehydration (i.e. <0.5% body
weight loss). The ingestion of saline raised blood osmolality to the levels of when
no fluid was ingested (i.e. dehydration trial). However, saline ingestion maintained
plasma volume at pre- exercise levels, well above the dehydration trial and somewhat
above the water ingestion trial. Despite this positive cardiovascular effect of saline
ingestion, aural temperature increased above the water trial to levels similar to the
dehydration trial. Similar to what happened with hypertonic IV saline infusions ,
the ingestion of hypertonic saline negated the thermoregulatory benefits that may
bring about the expansion of plasma volume.
Finally, in two experiments by Sanders et al. [10, 11], subjects ingested a 400- ml
saline bolus before exercise and 100- and 150- ml aliquots every 10 min during 3–4 h
exercise at 55–65% of V2 max. In one study held in a 32°C environment , the fluid
ingested only replaced 50% of the fluid losses and there was no difference between
ingesting water or a saline solution of 100 mmol Na+ (~0.58% saline). For instance,
plasma volume and even plasma sodium concentration were equally maintained with
water than with the saline ingestion and heart rate drift was similarly attenuated. The
low sodium dose and volume ingested was possibly not enough to act as a plasma
volume expander. In their second experiment, fluid intake matched sweat losses dur-
ing prolonged exercise in a themoneutral environment (i.e. 20°C). On this occasion,
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116 Mora- Rodriguez · Hamouti
the ingestion of a 100- mmol/l Na+ solution maintained plasma volume (i.e. extracel-
lular fluid) better than when ingesting a 5- mmol/l Na+ solution. This was achieved by
increased fluid retention with reduced urine production that however did not affect
the heart rate drift or increase in rectal temperature. Due to the low exercise intensity
and environmental heat stress, heart rate drifted only 6% during 4 h of pedaling and
rectal temperature 1.8°C from the 15- min value . It is possible that in a hotter
environment the effects of saline ingestion may have been revealed.
Saline Delivery Pre- Exercise: Effects on Plasma Volume
Greenleaf et al.  investigated the optimal composition of sodium beverages for
increasing plasma volume prior to exercise in euhydrated individuals. Using a com-
bination of sodium chloride and sodium citrate, they delivered drinks containing 55
mmol/l of Na+, however hypertonic (i.e. 365 mosm•kg–1 H2O) and 164 mmol•l–1 of
Na+ but hypotonic (i.e. 253 mosm•kg–1 H2O). After a drinking and resting period
(i.e. ~100 min), they found that the higher the sodium concentration, the higher the
increase in plasma volume (i.e. 5 vs. 8%, respectively). They concluded that drink-
ing a Na+ concentration appears to be more important than its osmolality to expand
plasma volume. These results were then confirmed by the same research group in
another study using a similar drink formulation .
Since then, other investigators have used oral saline solutions of 164 mmol/l of
Na+ to induce pre- exercise hypervolemia and to study its effects in performance dur-
ing subsequent exercise under different environmental conditions [14–17]. Unlike
Greenleaf et al. , these authors found lower levels of plasma volume expansion (i.e.
3–4.5% above resting values) despite ingesting a similar Na+ solution of 164 mmol/l.
We have recently found a similar low level of plasma volume expansion after inges-
tion of 10 ml/kg body mass of 164 mmol/l Na+ solution (1% expansion; Hamouti et
al., unpubl. data). This disparity in the level of plasma volume expansion between
Greenleaf et al. and other authors may possibly be due to a lower aerobic fitness level
among participants they recruited (V2 max 40 vs. 55 ml O2/kg/min in the other stud-
ies). Aerobically fit subjects are hypervolemic as a consequence of endurance training
adaptations . It is then possible that training reduces the amount of plasma vol-
ume available to be expanded since they are close to the ceiling for expansion. Figure
1 depicts what level of plasma expansion could be expected when ingesting saline
solutions with regard to the aerobic fitness level of the subject.
Saline Delivery Pre- Exercise: Effects on Performance
We could find only two studies in a thermoneutral environment that report the
effects of pre- exercise saline ingestion on exercise performance. Greenleaf et al. 
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Saline Ingestion Improves Performance 117
found that, compared with a moderate sodium beverage (i.e. 55 mmol/l of Na+), pre-
exercise ingestion of 164 mmol/l Na+ improved cycling time to exhaustion by 24%.
However, this improvement was not associated with changes in the cardiovascular
(i.e. heart rate) or thermoregulatory responses (i.e. core temperature or total whole
body sweat rate) regardless of higher blood availability. Coles and Luetkemeier 
found an 8% improvement in time- trial performance upon a 3% pre- exercise blood
volume expansion with a 164- mmol/l Na+ drink compared to a non- sodium placebo
drink. Similar to Greenleaf et al. [12, 13], they did not find differences in heart rate,
core temperature, rate of perceived exertion or total body sweat rate. It is then intrigu-
ing what could have caused the improvements in performance. It is possible that the
increased plasma volume may have permitted an increase in V2 max and thus in time
to exhaustion . Supporting this possibility, Coles and Luetkemeier  reported
that the individuals with the lower maximal aerobic fitness level (and probably lower
initial blood volume) had the greatest increase in performance upon plasma volume
expansion with saline ingestion.
In a hot environment, only two studies report performance results after pre- exercise
saline ingestion [15, 16]. There is one other study on the heat using saline ingestion to
expand plasma volume, but performance is not reported . In agreement with the
thermoneutral studies just discussed, performance time to exhaustion was improved
by 22% after the ingestion of a 164- mmol/l Na+ versus a 10- mmol/l Na+ solution.
These rather large improvements in performance occurred despite moderate pre-
exercise plasma volume expansion (i.e. 4.5% [15, 16]). In the case of exercise in the
heat, the performance effects could be related to the lower core temperature and per-
ceived exertion associated with the increased plasma volume. We have recently found
similar findings during a cycling time trial under hot environmental conditions (i.e.
Pre-exercise plasma volume
expansion (% from baseline)
20 40 60 80 100 120 140 160 180
Na+ concentration in beverage (mmol·l–1)
Untrained; r = 0.98
Trained; r = 0.87
Fig. 1. Relationship between sodium concentration in the beverages from several studies and the
percent of pre- exercise plasma volume expansion. Data is separated in untrained (VO2 max 40 ml O2/
kg/min) and trained individuals (VO2 max 55 ml O2/kg/min). Each point represents one experimental
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118 Mora- Rodriguez · Hamouti
7.4% improved performance with a 1% plasma volume expansion; Hamouti et al.
From the available data using pre- exercise saline ingestion it can be concluded that,
independently of the environment in which the studies were performed, the level of
pre- exercise plasma volume expansion seems to play an important role in endurance
performance. In figure 2, we show available data which suggest that the higher the
pre- exercise plasma volume expansion, the higher the increase in endurance perfor-
mance. Again, a large plasma volume expansion is only possible in subjects with a low
aerobic fitness level and thus the ergogenic power of pre- exercise saline ingestion for
a trained athlete may not exceed 8% in a thermoneutral environment and somewhat
higher in a hot environment.
The authors have no conflicts of interest to disclose.
Increase in performance
(% from control trial)
Pre-exercise plasma volume expasion
(% from baseline)
Cloes and Luetkemeier (2005)
Sims et al. (2007) Greenleaf et al. (1997)
r = 0.84
Hamouti et al.
Fig. 2. Relationship between the percent of pre- exercise plasma volume expansion induced by the
ingestion of 164 mmol/l Na+ solution (10 ml/kg body mass) and the percent of increase in perfor-
mance with respect to the control trial being the ingestion of the same volume of a low sodium
solution or plain water. Each point represents one study.
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Saline Ingestion Improves Performance 119
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Ricardo Mora- Rodriguez
University of Castilla- La Mancha
Avda Carlos III, s/n, ES–45071 Toledo (Spain)
Tel. +34 925 26 88 00, ext. 5510
E- Mail Ricardo.Mora@uclm.es
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