Sweat is important for cooling the body. Sweat is mostly water. For most activities, sweat losses can be replaced with ordinary water. However, sweat also contains some minerals—primarily sodium and chloride, with a smaller amount of potassium—and during prolonged exercise, the loss of sodium through sweat can be great enough to affect health and performance.
A reduction in the level of sodium in the blood is referred to as hyponatremia. This condition can occur if an athlete loses large amounts of water and salt in sweat and then tries to replace the loss with water alone. This dilutes the sodium in the blood. It is something like dumping out half of a glass of salt water and replacing what was poured out with plain water.
Athletes can lose 2 to 3 grams of salt per liter of sweat. It is not unusual for an athlete to lose a liter (4 cups) of sweat per hour, so he or she may sweat away 20 or more grams of salt during a 10-hour competition. It is also possible to develop hyponatremia even when salt losses from sweating are not excessive. This can occur if an athlete drinks too much water, which dilutes the sodium in the system.
For example, an athlete may overhydrate while exercising in a cooler climate, where sweat losses are lower. It is the concentration of sodium that is important, not the absolute amount.
Hyponatremia causes a number of problems. Sodium in the blood helps hold fluid in the blood vessels. As sodium concentration drops, fluid will leave the bloodstream by osmosis. It builds up in the tissues, causing swelling. In the lungs, fluid accumulation interferes with the exchange of oxygen and carbon dioxide.
In the brain, excess fluid causes disorientation, seizure, coma, and even death. The early symptoms of hyponatremia may be similar to those of dehydration: nausea, muscle cramps, disorientation, slurred speech, and confusion. But drinking water will make the problem worse and can lead to seizure, coma, or death.
Hyponatremia is a serious concern in endurance events that take place in hot environments. About 30% of the finishers of the Hawaii Ironman Triathlon are both hyponatremic and dehydrated. Mild symptoms of hyponatremia can be treated by eating salty foods or drinking a sodium-containing beverage, such as a sports drink. People with more severe symptoms need medical attention.
Athletes can reduce the risk of hyponatremia by using sodium-containing sports drinks during long-distance events and increasing their sodium intake several days prior to competition. Athletes also should avoid Tylenol, aspirin, ibuprofen, and other nonsteroidal anti-inflammatory agents. These drugs interfere with kidney function and may increase the risk of hyponatremia.
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