Journal of the American College of Nutrition, Vol. 25, No. 90003, 231S-239S (2006)
Published by the American College of Nutrition
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
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: rlsharp{at}iastate.edu
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ABSTRACT
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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 these recommendations. 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.
Key words: sodium, fluid, hyponatremia, exercise, physical activity, heat exhaustion
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INTRODUCTION
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In the 1960's it was not uncommon to find salt tablet dispensers in locker rooms at various sports venues. This was because of the widespread belief that excessive losses of sodium 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 research 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 community, 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 preferences. It is the purpose of this paper to review the recent scientific literature concerning sodium balance and its relationship to hydration both during and following exercise, particularly when performed under environmental heat stress.
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WATER AND SODIUM LOSSES DURING EXERCISE
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Sweat production during exercise in the heat depends on exercise intensity, duration, clothing, hydration status of the individual, heat-acclimation of the individual, and environmental 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 dehydration 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 conducted in the heat (32–37C, 30–50% relative humidity) on eight days [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 accounted 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 rate response) was similar among the three trials.
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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].
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HYPONATREMIA
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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 dehydration, 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]. Hyponatremia 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 beverages ingested during exercise [21–24].
Prevalence of Hyponatremia
Several authors have described cases of hyponatremia during endurance exercise in the heat. Speedy et al. have published the largest field-based study of the occurrence of hyponatremia [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 hyponatremic. Eleven of these subjects were described as severely hyponatremic (< 130 mmol/L) and seven of these were symptomatic. 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 physical activity 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 widespread 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 excessive retention of ingested fluids [25]. Findings of greater prevalence of hyponatremia among women suggests either a biological sex effect on fluid homeostasis or behavioral differences 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 development of hyponatremia, one would expect plasma sodium concentration 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 exercise for 3 hr in an environmental chamber kept at 34C. The subjects 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.
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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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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] inclusion 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 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 replacement 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 temperature 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 temperature response between water and saline ingestion and both trials resulted in smaller rise in these variables than was observed when no fluid was ingested. Plasma volume dropped less 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 trials. 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 suggest 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 hyponatremia or that this improves either exercise performance or thermoregulation. 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 exercise or occupations that involve prolonged physical activity in hot, humid environments.
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ROLE OF SODIUM IN REHYDRATION AFTER EXERCISE
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Despite efforts to replace fluid losses during exercise, mild dehydration after exercise remains a common finding. Dehydration equivalent to less than 2% loss of body mass is associated with reduced performance and impaired thermoregulation 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 advantage of prescribing fluid intake equal to fluid lost restores plasma volume while total body water remains somewhat contracted. The rationale for the approach that involves prescribing fluid 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 allowed 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 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. Calculation 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) compared 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 to achieve 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 beverage (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 beverage. 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 adequate 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, continued sweat water loss, and water loss through respiration. These studies fail to demonstrate complete body water restoration 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 achieving euhydration after mild dehydration of approximately 2%. Their approach involved ingestion of 150% of the fluid lost during a 30 minute period after a dehydration protocol consisting of intermittent cycling exercise in a 32C environment. Recovery of physiological markers of dehydration was followed 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 considerably larger than used in the prior research, neither the 2 mmol/L nor 26 mmol/L beverages resulted in complete recovery 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.
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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].
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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 complete (91% for both) with the lower sodium fluid when consumed in both 150% and 200% excess but was incomplete with either 50% volume (39% recovery) or 100% volume (60% 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.
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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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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 between different volumes and between different intakes of sodium, there have been no attempts to use the combined data from several studies in estimating the independent and interactive contributions of fluid volume and sodium concentration to the rehydration process. The data displayed in Table 2 summarizing 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 rehydration (dependent variable) was expressed as the percentage recovery of the fluid loss that had occurred during the dehydration protocol. The reported sodium concentration of the rehydration solution and the volume of this solution were used as independent variables. Initially, additional variables were entered into the regression model but none of the other variables 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).
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Table 2. Summary of Papers Used in Multiple Regression to Describe Relationship between Fluid Volume and Sodium Concentration of the Rehydration (RH) Solution
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The final regression model included both sodium concentration (mmol/L) of the rehydration fluid and volume of this solution consumed during the rehydration period (ml) as significant predictors of percent recovery of fluid balance (Table 3). The resulting regression equation was
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, calcium, 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 incomplete but does support the contention that both fluid volume and sodium concentration are important considerations in the selection and/or design of optimal rehydration solutions.
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Table 3. Multiple Regression of Percent Recovery of Fluid Balance as a Function of Both Volume and Sodium Concentration of Fluid Ingested during Rehydration
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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 dehydrating 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 carbohydrate-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 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 rehydration drink or in food consumed during rehydration. In addition, these findings show that it may not be necessary to include sodium in every aliquot of fluid ingested during rehydration if sufficient sodium is provided early in the rehydration period either as a constituent of fluid or food.
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SUMMARY AND CONCLUSION
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Both sodium and fluid ingestion play important roles in maintaining health and physiological function during physical activity in hot environments. Whether people engage in prolonged 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 hyponatremia in some individuals participating in at least 4 hr of exercise. Thus, many authors now recommend sodium concentration of 20–50 mmol/L in beverages consumed during the physical activity.
In designing a nutritional strategy for recovery from exercise 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. Alternatively, 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.
Received January 9, 2006.
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ABSTRACT
INTRODUCTION
