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Exercise-associated hyponatremia, is a fluid-electrolyte disorder caused by a decrease in sodium levels during or up to 24 hours after prolonged physical activity. This disorder can develop when marathon runners or endurance event athletes drink more fluid, usually water or sports drinks, than their kidneys can excrete. This excess water can severely dilute the level of sodium in the blood needed for organs, especially the brain, to function properly.
The incidence of EAH in athletes has increased in recent years, especially in the United States, as marathon races and endurance events have become more popular. A recent study showed 13% of the Boston 2002 marathon runners experienced EAH; most cases were mild. Eight deaths from EAH have been documented since 1985.