As the story goes, in 490 B.C., Philippides, the first marathon runner, ran 26 miles from the Greek city of Marathon to Athens to announce the victory of the Athenian army over Persian forces. He delivered the news, keeled over and died. It was summer, so perhaps heat stroke felled him. Or maybe the hero had a heart attack. But it's also possible that the legendary runner drank himself to death - with water.

According to the Greek historian Herodotus, Philippides had run from Marathon to Sparta the previous day. If so, he would have been tired and his progress to Athens slow - slow enough to allow him time to drink more water than a human body can take in without fatally diluting sodium levels in the blood. Too little sodium and too much water and the cells of the brain can swell enough to press against the skull and interfere with the brain stem's ability to regulate breathing.

The condition, called hyponatremia, has killed more long-distance runners, hikers and others who engage in sustained physical exercise than physicians, coaches or athletes realized. [...]