According to conventional wisdom, athletes should down fluids even before they feel thirsty. But excessive hydration can be deadly, says a study of Boston marathoners. It may lead to hyponatremia, the potentially fatal depletion of sodium, an electrolyte essential for cellular function.

Normal number of millimoles of sodium per liter of blood: 135 to 145

Number of marathoners studied: 488

Number with hyponatremia at the finish: 65

Percent who drank every mile: Among runners with normal levels: 54

Among those with hyponatremia: 75

Percent who drank more than three liters during race: Among normals: 26

Among those with hyponatremia: 42

Percent who weighed more at the finish (from fluid intake): Among normals: 29

Among those with hyponatremia: 71

SOURCE: New Eng land Journal of Medicine, April 14