Hyponatremia

Exercising for long periods may cause low blood sodium (hyponatremia), a potentially fatal condition.⁴⁰ Low blood sodium can occur if you drink excessive amounts of water and dilute the sodium content of your blood. This may lead to rapid and dangerous swelling of the brain. Inadequate sodium intake also may play a role. Low blood sodium is most likely to occur during prolonged exercise in dehydrated athletes who lose large amounts of sodium through sweating. It is also possible in athletes who habitually restrict sodium consumption in the foods and beverages they consume. In general, unless it is contraindicated because of a medical condition and the athlete is under the careful supervision of a physician, adding salt to meals and beverages is a desirable strategy for avoiding low blood electrolytes and reducing hyponatremia risk.

Signs and symptoms of low blood sodium include the following:

• Headache Swollen fingers and ankles
• Confusion Pulmonary edema
• Nausea Seizures
• Cramping Coma
• Bloated stomach

Just before the 2003 Boston Marathon, USA Track & Field announced fluid-replacement guidelines for long-distance runners that are designed to lower hyponatremia risk.⁴¹ Earlier guidelines encouraged runners to drink as much as possible to "stay ahead" of their thirst, but the new guidelines advise runners to drink only as much fluid as they lose through sweat during a race. This recommendation suggests that athletes consume 100 percent of fluids lost through sweat and no more. Higher levels of consumption, particularly of plain water, could cause a drop in blood sodium concentration, leading to hyponatremia. Athletes at increased risk of hyponatremia include those who

• take NSAIDs;
• are on a low-sodium diet;
• drink water or other no-sodium beverages during exercise;
• aren't acclimatized to warm weather or are poorly trained; and
• run slowly, taking longer than 4 hours to complete endurance events.

The highest risk of hyponatremia appears to be in athletes who produce a large volume of sweat with a relatively high concentration of sodium and who consume large volumes of plain water (which contains no sodium).⁴² Sports beverages contain approximately 20 milliequivalents (mEq) of sodium chloride (table salt), but even higher levels of sodium are recommended by a number of researchers who have assessed plasma changes during prolonged exercise in the heat.^43,44 These researchers have recommended 20 to 50 milliequivalents per liter, but it appears that most athletes with normal sweat rates and normal sweat sodium concentrations who consume commercial sports beverages and avoid consumption of plain water during endurance events are protected.⁴⁵

If you eat during exercise, salty foods such as pretzels are a good choice. Sports drinks also are a good source of sodium, water, and carbohydrate. Dehydration during prolonged exercise is still far more common than low blood sodium. It is important, therefore, to start exercise well hydrated and to drink appropriate amounts of fluid during exercise.⁴⁶