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Review

Role of Sodium in Fluid Homeostasis with Exercise

Rick L. Sharp, PhD

Exercise Physiology Laboratory, Department of Health & Human Performance, Iowa State University, Ames, Iowa

Key words: sodium, fluid, hyponatremia, exercise, physical activity, heat exhaustion

This paper provides a review of recent literature concerning the interactive effects of sodium and fluid

ingestion in maintaining fluid homeostasis during and following exposure to heat and exercise. Heavy sweating

during exercise combined with heat exposure commonly produces fluid deficits corresponding to 1–8% loss in

body mass. Thus, a great deal of attention has been focused on developing fluid replacement guidelines and

products for active people. Recently, there have been reports of more frequent cases of hyponatremia among

individuals who tend to over-ingest water during exercise lasting more than four hours, and inclusion of sodium

chloride in the fluid replacement beverage is often suggested as a potential means of reducing risk of

hyponatremia. Although hyponatremia is not likely to be a major risk factor for the general population,

ultra-endurance athletes and people with occupational physical activity and heat exposure may benefit from theserecommendations. Replacement of fluid deficits after exercise and heat exposure is another area that has received

considerable attention. Studies in this area suggest that if water is consumed, the volume ingested needs to

exceed the fluid deficit by approximately 150% to compensate for the urinary losses that will occur with water

ingestion. Inclusion of sodium chloride and other solutes in the rehydration beverage reduces urinary water loss,

leading to more rapid recovery of the fluid balance. Data are presented in this paper that suggest a quantifiable

interactive relationship between sodium content and fluid volume in promoting rapid recovery of fluid balance

after exercise and thermal-induced dehydration.

INTRODUCTION

In the 1960’s it was not uncommon to find salt tablet

dispensers in locker rooms at various sports venues. This wasbecause of the widespread belief that excessive losses of so-

dium in sweat during physical activity could lead to a depletion

of sodium and result in heat-cramps. Subsequent research,

however, showed that sweat is hypotonic and that the sodium

concentration is lower than plasma. This finding led to the

realization that the nutrient lost in greatest abundance during

exercise in the heat is water rather than sodium. Further re-

search confirmed this finding by showing that during exercise

in hot and humid conditions causes an increase in plasma

sodium concentration [1], implying that water replacement may

be more important than sodium replacement during exertional

heat stress.With the popularity of running in the 1970’s, it became

apparent that heat illness was a major risk for those individuals

running in hot and humid environments. Guidelines for fluid

replacement were developed and shared with the medical com-

munity, race organizers, and to the general public. Specialty

beverages were developed by food companies to provide fluid,

carbohydrate, and electrolyte replacement and were designed to

be used before, during and after exercise to help meet the

elevated demands for these nutrients in the exercising public.

The composition of sports beverages was adjusted over the next

30 years in response to both research findings and taste pref-

erences. It is the purpose of this paper to review the recent

scientific literature concerning sodium balance and its relation-

ship to hydration both during and following exercise, particu-

larly when performed under environmental heat stress.

WATER AND SODIUM LOSSES

DURING EXERCISE

Sweat production during exercise in the heat depends on

exercise intensity, duration, clothing, hydration status of the

Address reprint requests to: Rick L. Sharp, Ph.D., 250 Forker Building, Department of Health & Human Performance, Iowa State University, Ames, IA 50011. E-mail:

[email protected]

Journal of the American College of Nutrition, Vol. 25, No. 3, 231S–239S (2006)

Published by the American College of Nutrition

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individual, heat-acclimation of the individual, and environmen-

tal conditions [2–5]. When performing physical activity in high

environmental temperature, evaporation of sweat from exposed

skin is the predominant mechanism for heat loss. If heat loss is

not matched to the rate of metabolic heat production (intensity

of exercise), body heat storage rises and core temperature can

quickly reach dangerous levels. Maintaining a high capacity for

sweat production is therefore critical in thermoregulation and

prevention of heat illness. During high intensity athletic events,

sweat rates up to 3 L/hr are possible under hot and humid

conditions [6,7]. This leads to a loss of body water or dehy-

dration equivalent to 1–8% of body mass. Coupled with sweat

sodium concentrations ranging on average between 40– 60

mEq/L [6–9], such sweat rates can lead to sodium depletion

rates of about 150 mmol/hr with additional sodium losses in

urine production.

A recent study by Mao et al. measured sweat electrolyte and

urinary electrolyte concentration and excretion in 13 adolescent

(16–18 yr) soccer players during 1-hour soccer practices con-

ducted in the heat (32–37C, 30–50% relative humidity) on eightdays [10]. Mean sodium concentration in sweat was 55 mmol/L.

Average sweat loss during the 1-hour practice sessions was 1.54 L

(SD 2.06 L). Calculated sweat loss of sodium averaged

82 mmol (SD 62 mmol). Urinary loss of sodium averaged 110

mmol (SD 36 mmol). Thus average sodium excretion ac-

counted for by sweat and urinary excretion was 192 mmol (Table

1). Because no dietary intake data were reported for these subjects,

sodium and fluid balance could not be calculated. Likewise, no

data were obtained to assess either performance or physiological

consequences of these fluid and electrolyte losses. Nonetheless,

these observations suggest large losses of both sodium and water

during exercise in the heat.It is possible that the sweat collection method used by Mao

et al. overestimated the whole body sodium losses in sweat due

to regional variations in sodium concentration of sweat [11,12].

In the Mao et al. study, sweat was collected from the backs and

chests of the subjects for 5 min during the exercise sessions.

The measured sodium concentration of 55 mmol/L is similar to

the Na concentration of sweat collected by Shirreffs using a

whole-body washdown method [12]. Shirreffs et al. measured

sweat sodium concentrations of 51.6 mmol/L during exercise

producing a 2% dehydration of subjects. It is therefore unlikely

that the data obtained by Mao et al. are grossly overestimated.

In a study by Sanders et al. [13], water and sodium losses

were measured during 4 hr cycling exercise at 20 C at exercise

intensity equivalent to 55% of peak VO2. During the exercise

subjects ingested 3.85 L of an 8% carbohydrate-electrolyte

drink containing 5, 50, or 100 mmol/L of sodium. Sweat losses

averaged between 3.7 and 3.9 L for each of the trials. Sodium

concentration of sweat ranged from 43–48 mmol/L, producing

a sweat sodium loss between 150 and 190 mmol over the 4 hr

of exercise. Combined with the urinary sodium losses, subjects

experienced a negative sodium balance of 198 mmol when

ingesting the 5 mmol/L Na beverage, 36 mmol when ingesting

50 mmol/L Na beverage, and experienced a positive sodium

balance of 159 mmol when ingesting the beverage containing

100 mmol/L sodium (Fig. 1). In addition to assuring a positive

sodium balance throughout exercise, ingestion of the beverage

containing 100 mmol/L sodium reduced total fluid lost during

exercise in comparison to the other beverages. Calculation of

water compartment changes revealed a significant loss of fluid

from ECF (1.1 L) in the 5 mmol/L sodium trial, no change in

ECF in the 50 mmol/L sodium trial, and expansion of ECF

volume (0.5 L) in the 100 mmol/L sodium trial. Despite the

better maintenance of hydration status in the 50 and 100

mmol/L sodium trials, cardiovascular responses (e.g. heart rateresponse) was similar among the three trials.

HYPONATREMIA

During the last 20 years, persons engaged in long duration

endurance exercise in the heat have been advised to drink as

much fluid as possible during the exercise to prevent dehydra-

tion, preserve the sweating response and thereby maintain

thermoregulatory capacity [14]. Unfortunately, this advice has

led to an increase or at least a recognition of hyponatremia in

many athletes competing in these events [15–19]. Hyponatre-

mia may result because of excessive loss of sodium due to a

heavy sweating response, or alternatively, due to a dilution of

plasma sodium as a consequence of overzealous hydration [16].

Various recommendations for preventing hyponatremia are

made in the literature and include reducing the emphasis on

fluid ingestion [20] and/or increasing sodium content of bev-

erages ingested during exercise [21–24].

Prevalence of Hyponatremia

Several authors have described cases of hyponatremia dur-

ing endurance exercise in the heat. Speedy et al. have published

the largest field-based study of the occurrence of hyponatremia

Table 1. Body Fluid and Sodium Losses during 1-Hour Soccer Practices among Adolescent Boys

Body Mass

(kg)

Fluid Loss

(L)

Sweat [Na]

(mmol/L)

Sweat Na Loss

(mmol)

Urinary Na Loss

(mmol)

Total Na Loss

(mmol)

Mean 62.5 1.54 55 82 110 192

SE 6.8 0.57 27 62 36 —

Data were derived from Mao et al. [10].

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[18]. In this study, 330 finishers of a triathlon competition (6 –9

hr) were studied. Based on plasma sodium concentration less

than 135 mmol/L, 58 (18%) of the finishers were hyponatre-

mic. Eleven of these subjects were described as severely hy-

ponatremic ( 130 mmol/L) and seven of these were symp-

tomatic. The authors also noted that those subjects with the

most severe cases of hyponatremia had less change in body

weight during the race, implying that fluid overload was the

cause of the hyponatremia in most of the cases.

Other authors suggest that hyponatremia may only be a

significant risk factor in extraordinarily long duration physicalactivity such as marathon running and triathlon lasting 4 hours

or more. Noakes et al. [20] point out that most cases of

hyponatremia are observed in the less well trained participants

who take considerably longer to finish the race than do the top

finishers. The longer duration of exercise coupled with greater

total fluid intake as a result of the longer duration, therefore

puts these persons at greater risk of developing hyponatremia.

Because the cases of exercise-induced hyponatremia are

mostly confined to extraordinary physical efforts lasting longer

than 4 hr, hyponatremia is not likely to be particularly wide-

spread among the general population who engage in exercise

lasting less than 2 hrs per day. Various mechanisms have been

proposed to explain the development of hyponatremia in some

individuals. These causes include fluid overload or dilution

effect [17], excessive sodium loss during the exercise [21], and

inappropriate response of arginine-vasopressin leading to ex-

cessive retention of ingested fluids [25]. Findings of greater

prevalence of hyponatremia among women suggests either a

biological sex effect on fluid homeostasis or behavioral differ-

ences between men and women that may lead women to be

more compliant with advice to drink as much fluid as possible

during endurance exercise [27].

Prevention of Hyponatremia

If fluid overload is an important contributor to the develop-

ment of hyponatremia, one would expect plasma sodium con-

centration to fall during exercise in proportion to the volume of

low- or no-sodium fluid ingested. Vrijens and Rehrer [24] have

examined this question by recruiting 10 male subjects to exer-

cise for 3 hr in an environmental chamber kept at 34C. Thesubjects performed this exercise on two separate days; once

while ingesting sodium-free water every 15 minutes to match

fluid loss, and once while ingesting a commercial sodium-

containing (18 mmol/L Na, 63 g/L carbohydrate, 3 mmol/L

potassium) beverage to match fluid loss. During the water

ingestion trial, average plasma sodium concentration declined

from 140 mmol/L before exercise to 134 mmol/L by the end of

exercise (Fig. 2). In the carbohydrate-electrolyte trial, plasma

sodium concentration did not decrease significantly (140

mmol/L before exercise, 138 mmol/L at end of exercise). The

authors conclude that hyponatremia is possible even when fluid

intake matches fluid loss during long duration exercise when

sodium is not included in the fluid replacement beverage.

Other authors have also recommended inclusion of sodium

in beverages consumed during exercise [7,22,23,26]. Gisolfi

[26] recommended that persons exercising for 1–3 hr should

consume between 800–1600 ml/hr of fluid containing 10–20

mmol/L sodium and that persons exercising for more than 3 hr

should consume 500–1000 ml/hr of fluid containing 20 –30

mmol/L sodium. Lutkemeier et al. [22] suggested that saline

ingestion before exercise can help preserve the plasma volume

and may lead to beneficial changes in endurance exercise

performance. In a review article published by Rehrer [7] inclu-

sion of sodium in a fluid replacement beverage at concentration

ranging between 30 and 50 mmol/L was suggested as possibly

beneficial to those engaged in long duration exercise (3 hr or

more) in the heat.

Consistent with the hypothesis that excessive sodium loss is

Fig. 1. Sodium balance at the end of 4-hr cycling exercise in 20C

(dry-bulb) environment. Trials were repeated with ingestion of 3.85 L

of an 8% carbohydrate-electrolyte beverage with either 5, 50, or 100

mmol/L sodium concentration. Adapted from Sanders et al. [13].

Fig. 2. Plasma sodium concentration before and after 3-hr exercise in

34C (dry bulb) environment with ingestion of either plain water to

match fluid loss or a commercial carbohydrate-electrolyte beverage to

match fluid loss. Adapted from Vrijens and Rehrer [24].


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the primary cause of exercise-induced hyponatremia, Hiller et

al. [21] suggested 1–2 g sodium ingestion per hour of exercise

to prevent hyponatremia. Assuming fluid ingestion of 1 liter per

hour to match fluid lost through sweating, this amount of

sodium requires a beverage containing 43– 87 mmol/L sodium.

This recommendation is slightly higher than that recommended

by Rehrer and represents a sodium concentration roughly 2–4

times as high as that found currently in most commercial fluid

replacement beverages. Barr et al. argues that the reduced

palatability of such beverages would likely lead to less fluid

consumption among the general population and result in a

greater risk of dehydration [28].

There are also several studies that provide evidence that

sodium supplementation during exercise along with fluid re-

placement is not necessary [28–32]. Barr et al. had 8 subjects

perform 6 hr exercise at 55% VO2max in a heat chamber held

at 30C [28]. Each subject completed this exercise on separate

occasions to evaluate the possible effects of water ingestion,

water plus sodium (25 mmol/L), or no fluid. When the subjects

were not provided with fluid during the exercise, core temper-ature and heart rate rose rapidly while plasma volume declined

throughout exercise. Under this condition, only one subject was

able to complete the full 6 hr exercise and the mean time of

exercise was 4.5 hr. The subjects who failed to complete the

exercise did so because heart rate exceeded 95% maximum

heart rate (n 1), core temperature exceeded 40C (n 1), or

volitional exhaustion (n 5). In the water and saline trials,

seven of the eight subjects completed the 6 hr of exercise.

There were no differences in either heart rate or core temper-

ature response between water and saline ingestion and both

trials resulted in smaller rise in these variables than was ob-

served when no fluid was ingested. Plasma volume droppedless when ingesting the saline beverage than when ingesting

water. Plasma sodium concentration decreased by small

amount in both the saline (change 3.0 mmol/L) and water

(change 3.9 mmol/L) trials but there were no significant

differences in plasma sodium concentration between these tri-

als. Calculation of overall sodium balance revealed a sodium

deficit in the water trial (207 mmol) that was significantly

larger than observed in the saline trial (91.3 mmol). Based on

these results, the authors concluded that sodium concentration

equivalent to that found in commercial sports drinks do not

prevent the fall in plasma sodium during exercise when fluid

intake matches fluid lost through sweating. They further sug-

gest that sodium replacement is not necessary in exercise

lasting less than 6 hr.

Based on these reviewed studies, it is apparent that inclusion

of sodium in fluid replacement beverages can offset some of

the losses of sodium that occur during prolonged and heavy

sweating. It is less clear that doing so will prevent hyponatre-

mia or that this improves either exercise performance or ther-

moregulation. As suggested by Sanders et al., however, sodium

ingestion likely preserves the plasma volume during exercise at

the expense of the intracellular fluid volume. What effect this

relative dehydration has on muscle metabolism and function

has not yet been studied. An additional finding common to

most of these studies is that even if sodium ingestion does not

affect plasma sodium concentration, it does reduce the sodium

deficit that occurs during prolonged exercise in the heat. This

may be significant for people who are involved in daily exer-

cise or occupations that involve prolonged physical activity in

hot, humid environments.

ROLE OF SODIUM INREHYDRATION AFTER EXERCISE

Despite efforts to replace fluid losses during exercise, mild

dehydration after exercise remains a common finding. Dehy-

dration equivalent to less than 2% loss of body mass is asso-

ciated with reduced performance and impaired thermoregula-

tion during subsequent exercise if the fluid deficit is not

corrected. Thus, considerable research has been devoted to

understanding the rehydration process and the role played by

sodium in restoring body fluids lost during prior exercise.

In studying rehydration after exercise-induced body water

loss, investigators have employed three models for rehydration:

allow subjects to drink fluids ad lib during the rehydration

period [33–35], prescribe fluid intake during the rehydration

period to match the fluid lost during the prior exercise [36 –38],

and prescribe fluid intake in excess of the fluid lost in the prior

exercise [39 – 43]. The advantage of allowing ad lib rehydration

is that factors regulating thirst can be studied while the advan-

tage of prescribing fluid intake equal to fluid lost restores

plasma volume while total body water remains somewhat con-

tracted. The rationale for the approach that involves prescribingfluid intake in excess of that lost in the prior exercise is that

both plasma volume and total body water are restored by the

end of the rehydration period. Finally, there are also hybrid

models in which varied amounts of fluid and sodium content

are studied to allow for evaluation of independent effects of

sodium and fluid volume on the rehydration process.

Ad Libitum Rehydration

Nose et al. dehydrated six subjects by 2.3% using thermal

and exercise induced dehydration [34]. Over the next 3 hr,

subjects were seated in a thermoneutral environment and al-

lowed to rehydrate ad libitum using tap water (15C), placebo or

capsules containing NaCl to produce sodium concentration of

75 mmol/L. The purpose of this approach was to examine the

effect of sodium on drinking behavior and restoration of body

fluid compartments. Average fluid loss in the dehydration

period was 1550 ml and was followed by ingestion of 1100 ml

in the water trial and 1216 ml in the water plus sodium trial,

leaving the subjects in a fluid deficit after 3 hr of rehydration.

When urine production is subtracted from fluid ingestion, net

fluid gain during rehydration was 826 ml in the water trial and


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1045 ml in the water plus sodium trial. Despite the persistent

negative fluid balance even after 180 min, plasma volume had

returned to pre-dehydration by 90 min of recovery in the water

plus sodium trial while plasma volume remained slightly below

the pre-dehydration level even at 180 min of recovery. Calcu-

lation of fluid compartment recovery based on chloride space

showed that by the end of the rehydration period, total body

water had recovered by 52% in the water trial and by 76% in

the water plus sodium trial. Recovery of intracellular fluid was

not different between water and water plus sodium trials. Both

ECF and PV were more completely restored in recovery in the

water plus sodium trial (84% and 100%, respectively) com-

pared with water only (44% and 77%, respectively). These

findings illustrate the following points: 1) thirst is inadequate to

assure complete recovery of total body water deficits likely due

to early restoration of plasma volume, thereby removing the

volume dependent dipsogenic drive, 2) the presence of sodium

in the rehydration beverage stimulates greater drinking likely

due to greater osmotic dipsogenic drive, 3) the presence of

sodium in the rehydration beverage accelerates the recovery of extracellular fluid and plasma volume in particular, and 4)

sodium in the rehydration beverage reduces urinary losses of

water, allowing a greater fraction of the ingested fluid to be

retained. These findings were later confirmed by Wemple et al.

using a similar dehydration and rehydration protocol [35].

Rehydration with Fluid Intake Sweat Loss

Several studies have examined recovery of body water

losses after exercise by providing an amount of fluid to subjects

that is equal to the amount of water lost during the exercise as

a consequence of sweating. Most of these studies attempted toachieve complete rehydration within a relatively short period

lasting between 2 and 4 hours. The early study by Costill and

Sparks [36] dehydrated eight male subjects using intermittent

exposure to dry heat (70C) until 4% of body mass was lost.

Once the prescribed dehydration was reached, the men returned

to a thermoneutral environment to begin the rehydration period.

At the beginning of rehydration and at 15-min intervals the

subjects drank a volume of fluid equal to 7.7% of the volume

lost during the dehydration. This was continued for 3 hr so that,

by the end of the 3 hr rehydration period, the subjects had

ingested the same total volume of fluid as lost in dehydration.

The procedure was repeated once when ingesting plain water as

the rehydration fluid and once using a carbohydrate-electrolyte

(CE) drink for rehydration. The CE drink contained 22 mmol/L

sodium, 17 mmol/L chloride, 2.6 mmol/L potassium, 3.9

mmol/L phosphate, and 10.6 g/100ml glucose with osmolality

of 444 mOsm/L.

Urine production was significantly higher when subjects

rehydrated with water (602 ml) than when using the CE bev-

erage (367 ml). Despite drinking a volume of fluid equal to that

which was lost in dehydration these subjects were only able to

recover 62% of their body mass loss during the rehydration.

This was mostly due to urinary and insensible loss of water

during the rehydration period. Plasma volume had dropped by

an average of 12% with dehydration and 38% of this loss was

recovered during rehydration with water while 67% of the loss

in plasma volume was recovered when drinking the CE bever-

age. The authors concluded that the presence of electrolytes and

carbohydrate in the rehydration favored a more complete re-

filling of plasma volume, but that neither beverage was ade-

quate for completely restoring either plasma volume or total

body water when 100% of the dehydration volume is consumed

over a 3 hr period.

Rehydration With Fluid Intake > Fluid Loss

Based on the earlier observations of incomplete body water

restoration when either thirst regulates fluid intake or fluid

intake matches the fluid lost in the prior dehydration, most

recent studies have provided fluid in excess of that which was

lost in dehydration [39– 43]. Authors recognized that additional

fluid was needed to offset the obligatory urinary losses, con-tinued sweat water loss, and water loss through respiration.

These studies fail to demonstrate complete body water resto-

ration during rehydration lasting up to 6 hours unless the

ingested fluid is coupled with sodium ingestion. A convenient

method of providing both fluid and sodium during rehydration is

to select a rehydration beverage or food providing both fluid and

sodium with other nutrients (carbohydrate and potassium, e.g.)

that may be vital in restoring normal function after dehydration.

Maughan and Leiper [39] examined the role of varied

concentrations of sodium in the rehydration beverage in achiev-

ing euhydration after mild dehydration of approximately 2%.

Their approach involved ingestion of 150% of the fluid lostduring a 30 minute period after a dehydration protocol consist-

ing of intermittent cycling exercise in a 32C environment.

Recovery of physiological markers of dehydration was fol-

lowed for 5.5 hr after ingesting the rehydration beverages. The

four beverages compared included sodium concentrations of 2,

26, 52, and 100 mmol/L. Although the fluid intake was con-

siderably larger than used in the prior research, neither the 2

mmol/L nor 26 mmol/L beverages resulted in complete recov-

ery of body water (66% and 82% recovery of body mass loss,

respectively) (Fig. 3). Both of the higher sodium beverages

resulted in complete (100%) rehydration by the end of the 5.5

hr monitoring period.

In an ambitious study designed to assess the interactive

effects of both sodium content and volume of fluid ingested in

rehydration, Shirreffs et al. [41] rehydrated subjects using

either 50%, 100%, 150%, or 200% of the volume lost and each

of these volumes contained either low sodium (23 mmol/L) or

higher sodium (61 mmol/L) concentration. Based on the net

fluid balance presented, body water recovery was nearly com-

plete (91% for both) with the lower sodium fluid when con-

sumed in both 150% and 200% excess but was incomplete with

either 50% volume (39% recovery) or 100% volume (60%



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recovery) (Fig. 4). With the higher sodium content in ingested

fluid, recovery of the fluid deficit was complete with ingestion

of 150% of volume lost (107% recovery) while ingestion of

200% of volume lost resulted in a surplus of fluid (127%

recovery). Neither the 50% volume nor 100% volume fully

restored whole body fluid balance (38% recovery and 81%

recovery, respectively). Urine volume was positively related to

the volume of fluid ingested and inversely related to the content

of sodium in the rehydration beverage.

Multiple Regression of Sodium Concentration and

Fluid Volume

That recovery of total body water would depend on both the

sodium intake and the volume of fluid ingested may seem

intuitively obvious. The above reviewed studies provide an

evidentiary framework for quantifying this interactive effect.

Although each of these studies has compared rehydration be-

tween different volumes and between different intakes of so-

dium, there have been no attempts to use the combined data

from several studies in estimating the independent and interac-

tive contributions of fluid volume and sodium concentration to

the rehydration process. The data displayed in Table 2 summa-rizing the findings of several rehydration studies were therefore

used in a multiple regression analysis to assess the relative

contributions of sodium concentration and fluid ingestion. In

each study, the data that were presented in the published paper

were either used directly (when provided by the authors) or the

relevant data were calculated from other results reported by the

authors. For the purpose of this analysis, whole-body rehydra-

tion (dependent variable) was expressed as the percentage

recovery of the fluid loss that had occurred during the dehy-

dration protocol. The reported sodium concentration of the

rehydration solution and the volume of this solution were used

as independent variables. Initially, additional variables wereentered into the regression model but none of the other vari-

ables achieved statistical significance (p 0.05). The variables

which did not significantly contribute to the prediction of fluid

recovery included urine volume during dehydration (likely due

to colinearity with sodium concentration), body mass (due to

low range of body mass in the reported studies), and duration

of rehydration period (which ranged from 2–6 hr).

The final regression model included both sodium concen-

tration (mmol/L) of the rehydration fluid and volume of this

solution consumed during the rehydration period (ml) as sig-

nificant predictors of percent recovery of fluid balance (Table

3). The resulting regression equation was

% rehydration 22.7 0.406 * Na 0.021 * volume

In the example of a 75 kg person who dehydrates by 2.5% and

ingests 100% of the volume lost during rehydration, a sodium

concentration of approximately 93 mmol/L would be required

to achieve fluid balance within 6 hr. On the other hand, if fluid

intake is increased to 150% of that lost in prior dehydration, the

regression model predicts that full rehydration could be

achieved with a sodium concentration of approximately 50

mmol/L. However, it must be noted that the regression model

accounts for only 66% of the variance in body water recovery.It is likely that additional variables including temperature of the

ingested fluid, presence of other electrolytes (potassium, cal-

cium, magnesium) and nutrients (carbohydrate, amino acids),

arginine vasopressin and aldosterone, and osmolality of the

rehydration fluid also play important roles but are not included

in this regression model. Thus the present analysis is incom-

plete but does support the contention that both fluid volume and

sodium concentration are important considerations in the se-

lection and/or design of optimal rehydration solutions.

Fig. 3. Percent recovery of fluid balance during a 5.5-hr rehydration

period in which fluid was ingested at a volume equal to 150% of the

fluid deficit that was incurred. Rehydration was compared between

beverages containing 2–100 mmol/L sodium. Adapted from Maughan

and Leiper [39].

Fig. 4. Percent recovery of fluid balance during 6-hr rehydration period

in which both volume and sodium concentration of beverage were

varied. Adapted from Shirreffs et al. [41].


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Rehydration with Food

One study from our laboratory [38] examined the question

of whether ingestion of food containing fluid and sodium is

effective in restoring fluid and sodium balance after a dehy-

drating bout of exercise and heat. Subjects were dehydrated by

2.5% using intermittent exposure to heat and exercise. Once the

prescribed fluid loss was achieved, subjects ingested 355 ml of

either chicken broth, chicken soup with noodles, a carbohy-

drate-electrolyte beverage, or tap water. Thereafter, the subjects

ingested an average of 290 ml water every 20 min so that total

fluid intake by 2 hr matched fluid loss. The decision to choose

Table 2. Summary of Papers Used in Multiple Regression to Describe Relationship between Fluid Volume and Sodium

Concentration of the Rehydration (RH) Solution

Reference Body Mass

(kg)

Change in Body

Mass† (kg)

Volume Ingested

During RH (ml)

Sodium

Concentration

(mmol/L)

Urine Volume

(ml)

% Recovery of

Fluid Balance‡

Costill & Sparks 1973 [36] 71.7 2.74 2740 0 602 73

71.7 2.74 2740 60 367 73Maughan & Leiper 1995 [39] 71.8 1.36 2045 2 1350 66

71.8 1.36 2045 26 940 82

71.8 1.36 2045 52 610 100

71.8 1.36 2045 100 580 100

Maughan et al. 1996 [40] 66.1 1.36 2042 21 940 75

66.2 1.36 2042 21 935 73

Shirreffs et al. 1996 [41] 71.5 1.49 746 23 135 41

71.5 1.45 1448 23 493 69

71.5 1.50 2255 23 867 101

71.5 1.46 2927 23 1361 103

73.2 1.52 758 61 194 40

73.2 1.52 1522 61 260 83

73.2 1.50 2243 61 602 106

73.2 1.59 3180 61 1001 136

Shirreffs & Maughan 1998 [42] 69 1.27 1912 0 1182 5069 1.29 1938 25 970 69

69 1.31 1968 50 800 80

69 1.36 2035 100 578 101

Ray et al. 1998* [38] 72.0 1.80 1800 0 232 76

72.3 2.00 2000 21 310 76

72.0 1.80 1800 18 188 75

72.2 1.70 1700 35 231 78

Mitchell et al. 2000 [43] 79.6 2.26 2280 25 300 71

79.6 2.26 2280 50 180 104

79.6 2.28 3390 25 600 76

79.6 2.28 3390 50 540 101

* Sodium concentration calculated based on amount of sodium provided by ingestion of soup broth and soup diluted by additional water ingested during rehydration period.

† Change in body mass from pre-dehydration to pre-rehydration.

‡ Calculated as percentage recovery in body mass lost or net fluid balance depending on how the data were expressed in referenced paper.

Table 3. Multiple Regression of Percent Recovery of Fluid Balance as a Function of Both Volume and Sodium Concentration of

Fluid Ingested during Rehydration

Coefficient Std Error t P

Constant 22.70 9.17 2.48 0.020

Na conc 0.406 0.093 4.38 0.001

Volume 0.021 0.004 5.46 0.001

DF SS MS F P

Regression 2 7764 3882 23.9 0.0001

Residual 25 4055 162

Total 27 11819 438Y 22.7 0.406Na conc 0.021vol intake)

R 0.81 R2 0.66

Data were extracted from references shown in Table 2.



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these products was based on commercial availability to

consumers as well as their varied amounts of electrolytes and

osmolality. With regard to sodium intake, chicken noodle soup

and chicken broth treatments provided a total sodium ingestion

of 50 mmol and 39 mmol, respectively. This is considerably

less than the sodium intake associated with the prior studies in

which subjects ingested 150% of the fluid loss with a sodium

concentration of 50 –100 mmol/L. Using the regression model

from above, it is expected that the chicken broth and the

chicken noodle soup treatments would not fully restore the

fluid deficit in 3 hr (estimated % rehydration 73% for both).

Measured fluid recovery was 76% and 78% for the chicken

broth and chicken noodle soup, respectively. Although total

body fluid balance was not fully recovered in rehydration,

plasma volume was fully restored with the chicken broth and

the chicken noodle soup trials, but not with either a commercial

carbohydrate-electrolyte beverage or with water.

These findings illustrate the importance of ingestion of

sodium during the rehydration period not only for encouraging

increased retention of ingested fluids but also for restoration of the plasma volume, which can be re-filled ahead of total fluid

balance when sufficient sodium is provided either in the rehy-

dration drink or in food consumed during rehydration. In ad-

dition, these findings show that it may not be necessary to

include sodium in every aliquot of fluid ingested during rehy-

dration if sufficient sodium is provided early in the rehydration

period either as a constituent of fluid or food.

SUMMARY AND CONCLUSION

Both sodium and fluid ingestion play important roles inmaintaining health and physiological function during physical

activity in hot environments. Whether people engage in pro-

longed endurance exercise such as marathons and triathlons or

if they are involved in occupational heat exposure during

physical activity, it is important that both fluid and sodium are

provided to offset the losses in both nutrients that occur as a

consequence of heavy sweating. People involved in vigorous

exercise in hot environments lose up to 3 liters of water and 3.5

grams of sodium per hour through sweating. Preventing these

fluid and sodium deficits helps to maintain both performance

and thermoregulation in such environments. The evidence from

published literature shows that fluid intake during exercise in a

warm environment is absolutely essential to attenuate the rise

in core temperature. These studies also demonstrate that unless

sodium is provided in the fluid replacement beverage, fluid

intake that matches or exceeds fluid loss may cause hypona-

tremia in some individuals participating in at least 4 hr of

exercise. Thus, many authors now recommend sodium concen-

tration of 20–50 mmol/L in beverages consumed during the

physical activity.

In designing a nutritional strategy for recovery from exer-

cise and heat exposure that results in mild dehydration, the dual

and interactive roles of fluid and sodium intake should be

considered. This synergistic association between fluid volume

and sodium intake is reflected in recommendations to consume

fluid in excess of that lost during the prior exercise and to

include sodium to increase the retention of the ingested liquids

by minimizing urine production. The papers reviewed here

suggest that plasma volume can be fully restored before total

body water deficits are fully corrected when sodium intake is

consumed either as a component of the rehydration beverage

with sodium concentration of approximately 20 mmol/L or

with food consumed in the early part of a rehydration period.

Using the meta-analysis presented in this paper, full recovery of

the fluid deficit within 6 hrs requires ingestion of a rehydration

solution containing 100 mmol/L sodium if consuming the same

volume of fluid that was lost in the prior dehydration. Alter-

natively, correction of the fluid deficit can also be achieved by

ingesting 150% of the volume lost if the rehydration solution

contains 50 mmol/L sodium.

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Received January 9, 2006.



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