Hyponatremia

Hyponatremia (British English: hyponatraemia) is when a human body does not have enough salt (sodium) in its blood. It is mostly caused by the presence of too much water.
Hyponatremia is most often a complication of other medical illnesses in which excess water collects in the body faster than can be excreted (for example in congestive heart failure, syndrome of inappropriate antidiuretic hormone (SIADH), or polydipsia). Some runners who drink a lot of water before running a marathon also suffer this illness. Drinking too much water can impair the ability of marathon runners.