Introduction
Salt is about 40% sodium and 60% chloride. Sodium is the major cation of the extracellular fluid and one of its primary functions is to maintain fluid equilibrium in the body. Humans have a sophisticated mechanism to control sodium and water homeostasis, and sodium is a critical nutrient in the maintenance of normal physiologic function and optimal exercise performance. In the adult human, water makes up approximately 60% of total body weight. This water is distributed between the intracellular space (ICF) and the extracellular space (ECF). Two thirds of the total body water is in the ICF and one third is in the ECF; 25% of the ECF is intravascular (plasma) fluid and 75% is interstitial fluid. The osmotic effect of sodium in the ECF allows it to function much like a sponge, drawing fluid into both the intravascular and interstitial spaces.
Because sodium can draw fluid into the intravascular space, excessive sodium intake can contribute to adverse health consequences such as hypertension. This leads many to believe that a high-sodium diet is always unhealthy, and indeed, the United States Recommended Daily Allowance (RDA) for sodium is relatively low. Although the typical American diet often contains more sodium than is needed, this may not be true for the athlete. Significant sodium and water losses can occur during exercise, exceeding the dietary intake and adversely affecting the fluid balance. The need to replace fluids in athletes who sweat profusely is well recognized and many position statements emphasize the importance of hydration [1–3,4••]. However, the salt deficit that accompanies dehydration from exercise often goes unrecognized, leading to inadequate salt replacement recommendations and therefore failure to adequately replace salt losses.
Fluid and Electrolyte Losses During Exercise
During athletic activity the magnitude of sweat loss can be considerable. This is especially true in athletes who practice or compete multiple times per day, often over consecutive days. To replace normal physiologic losses 2 g/d of sodium is necessary. The RDA for sodium is 2.4 g/d (roughly the equivalent of 1 tsp of table salt) and given that the normal dietary sodium intake in the United States is 6 to 8 g/d, most athletes will have an excess of 4 to 6 g/d of sodium. This is sufficient for most athletes; however, it may be inadequate in those with large sweat sodium losses [5].
Many factors influence sweat production during exercise, including environmental conditions, exercise intensity, clothing, duration of exercise, hydration status, and heat acclimatization [6•]. There is significant variation in sweat rates and sweat sodium concentrations among individuals. Three recent studies reflect this fact. Bergeron [7] evaluated a group of male tennis players prone to heat cramps and found sweat rates from 1.79 to 3.41 L/h (mean, 2.6 L/h), sweat sodium concentrations from 23.0 to 83.0 mmol/L (mean, 44.5 mmol/L), and sodium loss from 1.375 to 4.770 g/h (mean, 2.715 g/h). Shirreffs et al. [8] evaluated a group of male soccer players during a training session and found sweat rates from 0.99 to 1.93 L/h (mean, 1.46 L/h), and sweat sodium concentration from 15.5 to 66.3 mmol/L (mean, 30.2 mmol/L). Folkes Godek et al. [9] compared the sweat rates of American football players and cross-country runners participating in the same environmental conditions. The football players had higher sweat rates (2.14 L/h vs 1.77 L/h), sweat rate range (1.1–3.6 L/h vs 1.04–2.49 L/h), and mean total daily sweat loss (9.4 L vs 3.54 L).
Importance of Salt During Rehydration
Hydration during exercise is important to prevent heat-related illness and impairment in performance. Complete and rapid rehydration after exercise is especially important in the athlete who will soon return to practice or competition. Despite the knowledge of these facts, prolonged dehydration and incomplete rehydration frequently occur in sports, resulting in the athlete returning to practice in a dehydrated state [10]. Adequate rehydration can only be achieved if the sodium lost in sweat is replaced along with the fluid loss.
There are several reasons that sodium is needed in the rehydration process. The ingestion of plain water causes a rapid fall in plasma sodium concentration and osmolarity, leading to decreased aldosterone and vasopressin production; this increases urine output [11,12]. The addition of sodium to the ingested fluids maintains circulating vasopressin levels and prevents this diuretic effect [13]. Both Maughan and Leiper [11] and Shirreffs and Maughan [12] showed that the fraction of ingested fluid that was retained was directly related to the sodium concentration of the ingested fluid. Ingested sodium will enter the ECF and assist in restoring the extracellular fluid through its osmotic effect. Also, sodium will increase thirst and often the taste appeal of the drink and therefore increase the amount of fluids that an athlete will voluntarily consume [14].
Exertional Heat Cramps
Heat cramps are severe muscle spasms that occur during or after exercise, usually accompanied by excessive sweating. They begin as subtle twitches or fasciculations in one or more muscle groups and can rapidly progress to severe widespread debilitating muscle spasms that leave the athlete writhing in pain [7,15]. Exertional heat cramps are thought to be due to salt loss, dehydration, and muscle fatigue [16]. A contracted ECF due to fluid and sodium loss may cause changes in the ionic concentrations in the ECF and mechanical deformation of motor nerve terminals during muscle shortening. These conditions may initiate hyperexcitability in selected motor nerve terminals [15,17].
Scientific evidence that sodium and fluid losses are factors in exertional heat cramps is limited. Stofan et al. [18] found that athletes who were prone to cramps lost more total sodium, were more dehydrated, and had higher sweat rates than those who did not cramp. Case studies have shown that cramping can be reduced or eliminated in tennis players by increasing dietary salt, ingesting beverages containing salt, and remaining well hydrated during play [7,17]. Some authors have cited normal serum sodium levels as evidence that sodium depletion does not cause exertional heat cramps. However, serum sodium concentration is a better measure of total body water than it is of total body sodium; and thus an athlete with severe sodium and fluid loss may actually have an increased serum sodium level due to hemoconcentration or intravascular volume contraction [19].
Exercise-associated Hyponatremia
Exercise-associated hyponatremia has gained much attention in the past 20 years. The condition of low plasma sodium concentration and osmolarity can lead to cerebral edema, pulmonary edema, and death. In mild cases it is asymptomatic, but symptoms develop as the plasma sodium level drops below 130 mmol/L. Symptoms include headache, vomiting, swollen hands and feet, restlessness, confusion, wheezy breathing, and fatigue, and if undiagnosed or improperly treated can lead to seizure, coma, brainstem herniation, respiratory arrest, and death [20•]. The risk of symptoms is greater as the sodium level falls and is associated with the rate of decline and the length of time the patient is hyponatremic.
Athletes with hyponatremia will gain weight during an exercise session because they consume more fluids during exercise than they lose in sweat. This combination of excessive drinking and possibly large sweat sodium loss is the root of hyponatremia. It is seen more commonly in women, those of smaller stature, and those with a longer race time [21•]. Hypovolemic hyponatremia can also occur in a dehydrated athlete who has large losses of both sodium and fluids.
Practical Applications and Guidelines
There is significant variation in both sweat rate and sweat sodium concentration among individuals participating in the same event or practice. Thus, it is difficult to put together a guideline for fluid and electrolyte replacement that applies to everyone. Each athlete must be treated as an individual [22]. An athlete exercising 4 hours a day who has a sweat rate of 3.0 L/h with a sweat sodium concentration of 80 mmol/L will lose 12 L of fluid and 960 mmol of sodium in 1 day. This equates to over 22 g of sodium or over 55 g of salt. It must also be considered that an athlete may continue this pattern for several days. At the other end of the spectrum, an athlete with a sweat rate of 1.5 L/h with a sweat sodium concentration of 25 mmol/L exercising for 4 hours will lose only 6 liters of fluid and 150 mmol of sodium. This athlete would need only about 3.5 g of sodium or 8.6 g of salt, which could easily be replaced with normal dietary salt intake; however, the losses in the first athlete would likely result in a severe deficit in total body exchangeable sodium.
It is difficult to assess when a deficit in total body exchangeable sodium occurs. It is not uncommon for a clinician to assume that a sodium deficit is not present based on a normal serum sodium measurement. However, the serum sodium level is not a valid marker of total body exchangeable sodium. A sodium-depleted athlete may present with a normal or slightly elevated serum sodium concentration. Acute loss of fluid and sodium from sweat will cause body fluid compartment changes in an attempt to normalize the plasma osmolarity. The serum sodium level and to some extent the plasma volume is maintained, but the volume in the interstitial space will be altered. A better alternative to determine exchangeable sodium deficit is a 24-hour urine collection to monitor renal sodium conservation [7]. However, this method is not very practical for use on a regular basis.
The ideal method to assess fluid and electrolyte balance is to perform a comprehensive individual assessment of sweat rate, sweat sodium concentration, pre- and postexercise hydration status, and dietary fluid and sodium intake [17]. The clinician can then implement a detailed plan for food, fluid intake, and electrolyte intake. Although this method has been used with great success to assist individual athletes, it is impractical for use in a large number of athletes and there are a limited number of professionals that have the expertise to collect these data in an accurate manner [7,17].
A second method to determine fluid and electrolyte need is to assess the sweat rate and then make sodium recommendations based on average sweat sodium concentration. To assess a sweat rate, weigh the athlete before and after a 1-hour exercise session and calculate the fluid loss based on the weight loss (also account for fluid intake and urine output during this hour). For example, if an athlete loses 2.5 L of fluid per hour, assuming a sweat sodium concentration of 50 mmol/L, the athlete would need to replace (2.5 L × 50 mmol) 125 mmol of sodium per hour of exercise. This is about (125 mmol × 23 g/1000 mmol) 2.9 g of sodium or 7 g of salt. Replace this amount of salt by dietary manipulation; consultation with a nutritionist with experience in this area is valuable.
A third method is to replace salt based on weight loss during exercise. An estimate of salt needs can be made assuming an average dietary salt intake and sweat sodium concentration. This guideline can be used in real time, with a large number of athletes, and yet still allows for significant individual variation. With a formula developed by Dimeff and Gimre (Unpublished data), 1.3 g of salt is given for each pound lost in excess of 5 pounds. The salt lost through sweat during exercise is calculated by multiplying weight loss in pounds by the sodium sweat concentration in mmol/L by the constant of 0.0263Lg/#mMNa. Assuming an average sweat sodium concentration of 50 mmol/L, each pound of sweat would result in a loss of (1 × 50 × 0.0263) 1.3 g of salt. Given the average dietary salt excess of 4 to 6 g/d, the loss of 5 pounds would result in a 6.5-g salt loss, thus exceeding the daily salt excess. For example, if an athlete lost 9 pounds during an exercise session, he would add (1.3 × [9–5]) 5.2 g of salt to his normal “high-salt” diet.
It is practical to recommend increased dietary salt intake in athletes, especially those exercising in the heat. In some athletes this increased sodium is not needed and will be harmless. However, in many it could avert heat-related illness or enhance performance. The additional salting of everyday foods is an inexpensive and effective method of increasing sodium intake. Food and snacks high in sodium include pickles, pretzels, tomato juice, canned soups, baked beans, and pizza. Sports drinks also contain sodium and may be used for fluid replacement instead of plain water; adding 2.5 mL (0.5 tsp) of table salt to each 960 mL (32 fl oz) of a sports drink will further increase sodium concentration without adversely affecting taste or absorption. Increased salt intake should always include an adequate amount of fluids to replace those fluids lost during exercise. Following a specific plan of consuming a certain amount of sodium-rich beverages at regular intervals throughout the day may help assure adequate fluid and sodium intake [23]. Another tip to avert full blown heat cramps is to have the athlete drink 480 mL (16 fl oz) of a sports drink with 2.5 mL (0.5 tsp) of extra salt added at the first signs of muscle twitching and continue this throughout the remainder of the contest [24].
Conclusions
In athletes who have large sweat losses, there is much emphasis placed on fluid replacement; however, sodium replacement is often overlooked. Those who work with athletes should continue to emphasize fluid replacement but not encourage fluid replacement in excess of fluid loss. In addition, salt supplementation should be considered, as the athlete may not attain proper rehydration without adequate sodium replacement. Replacement needs to be individualized based on the fact that there is a significant variance in sweat rates and sweat sodium concentrations. The clinician must take measures to assure proper rehydration by monitoring daily weights and paying special attention to athletes who are heavy sweaters, salty sweaters, or those who have had previous heat-related illness.
Acknowledgment
The author would like to thank Michael F. Bergeron, PhD, and Robert J. Dimeff, MD, for their insight and mentorship in this area.
References and Recommended Reading
Papers of particular interest, published recently, have been highlighted as: • Of importance •• Of major importance
1. American College of Sports Medicine, American Dietetic Association, Dietitians of Canada:
Joint position statement: nutrition and athletic performance.Med Sci Sports Exerc 2000,
32:2130–2145.
2. Convertino VA, Armstrong LE, Coyle EF,
et al.:
American College of Sports Medicine position stand. Exercise and fluid replacement.Med Sci Sports Exerc 1996,
28:i–ix.
3. Casa DJ, Armstrong LE, Hillman SD,
et al.:
National Athletic Trainers' Association position statement: fluid replacement for athletes.J Athlet Train 2000,
35:212–224.
4.•• Sawka MN M, Burke L, Eichner ER,
et al.:
American College of Sports Medicine position stand: exercise and fluid replacement.Med Sci Sports Exerc 2007,
34:377–390.
This ACSM position stand provides an excellent evidence-based review on all areas surrounding the topic of exercise and fluid replacement.
5. Coyle EF:
Fluid and fuel intake during exercise.J Sports Sci 2004,
22:39–55.
6.• Sharp RL:
Role of sodium in fluid homeostasis with exercise.J Am Coll Nutr 2006,
25:231S–239S.
This paper provides a review of recent literature concerning the interactive affects of sodium and fluid ingestion in maintaining fluid homeostasis during and following exposure to heat and exercise.
7. Bergeron MF:
Heat cramps: fluid and electrolyte challenges during tennis in the heat.J Sci Med Sport 2003,
6:19–27.
8. Shirreffs SM, Aragon-Vargas LF, Chamorro M,
et al.:
The sweating response of elite professional soccer players to training in the heat.Int J Sports Med 2005,
26:90–95.
9. Folkes Godek SF, Bartolozzi AR, Godek JJ:
Sweat rate and fluid turnover in American football players compared with runners in a hot and humid environment.Br J Sports Med 2005,
39:205–211.
10. Folkes Godek SF, Godek JJ, Bartolozzi AR:
Hydration status in college football players during consecutive days of twice-a-day preseason practices.Am J Sports Med 2005,
33:843–851.
11. Maughan RJ, Leiper JB:
Sodium intake and post-exercise rehydration in man.Eur J Appl Physiol Occup Physiol 1995,
71:311–319.
12. Shirreffs SM, Maughan RJ:
Volume repletion after exercise-induced volume depletion in humans: replacement of water and sodium losses.Am J Physiol Renal Physiol 1998,
274:868–875.
13. Rehrer N:
Fluid and electrolyte balance in ultra-endurance sport.Sports Med 2001,
31:701–715.
14. Clapp A, Bishop P, Smith J, Mansfield E:
Effects of carbohydrate-electrolyte content of beverages on voluntary hydration in a simulated industrial environment.AIHAJ 2000,
61:692–699.
15. Layzer, RB:
The origin of muscle fasciculations and cramps.Muscle Nerve 1994,
17:1243–1249.
16. Casa DJ, Clarkson PM, Roberts WO:
American College of Sports Medicine roundtable on hydration and physical activity: consensus statements.Curr Sports Med Rep 2005,
4:115–127.
17. Bergeron MF:
Heat cramps during tennis: a case report.Int J Sport Nutr 1996,
6:62–68.
18. Stofan JR, Zachwieja JJ, Horswill CA,
et al.:
Sweat and sodium losses in NCAA Division I football players with a history of whole-body muscle cramping.Med Sci Sports Exerc 2003,
35:S48.
19. Ruth JL, Wassner SJ:
Body composition: salt and water.Pediatr Rev 2006,
27:181–188.
20.• Murray B, Eichner R:
Hyponatremia of exercise.Curr Sports Med Rep 2004,
3:117–118.
This commentary provides an excellent viewpoint of exercise-associated hyponatremia.
21.• Seto C, Way D, O'Connor N:
Environmental illness in athletes.Clin Sports Med 2005,
24:695–718.
This article offers a review of environmental illness in athletes.
22. Shirreffs SM, Sawka MN, Stone M:
Water and electrolyte needs for football training and match-play.J Sports Sci 2006,
24:699–707.
23. Bergeron, MF:
Sodium: the forgotten nutrient.Sports Science Exchange 2000,
13:1–4.
24. Bergeron M:
Averting heat cramps.Phys Sportsmed 2002,
30:14.
