50% of recreational runners may be drinking too much fluid during races. 37% of runners drink according to a preset schedule or to maintain a certain body weight and 9% drink as much as possible. Expert guidelines recommend runners drink only when thirsty. 30% incorrectly believe they need extra salt while running, and 57% drink sports drinks because the drinks have electrolytes that prevent low blood sodium. However, main cause of low sodium is drinking too much water or sports drinks during exercise, which can dilute the sodium content of blood to abnormally low levels. This can cause a potentially fatal condition called exercise-associated hyponatremia. Symptoms of hyponatremia include: nausea, vomiting, headache, confusion, loss of energy, muscle weakness, spasms or cramps. In extreme cases, the condition can lead to seizures, unconsciousness and coma.
British Journal of Sports Medicine, June 2011