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. 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.
Many factors influence sweat production during exercise, including environmental conditions, exercise intensity, clothing, duration of exercise, hydration status, and heat acclimatization. There is significant variation in sweat rates and sweat sodium concentrations among individuals.
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. 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. The addition of sodium to the ingested fluids maintains circulating vasopressin levels and prevents this diuretic effect.
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. Exertional heat cramps are thought to be due to salt loss, dehydration, and muscle fatigue. 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.
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. 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. 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. Hypovolemic hyponatremia can also occur in a dehydrated athlete who has large losses of both sodium and fluids.
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.