According to The New England Journal of Medicine, 13% of 488 Boston Marathon finishers studied had hyponatremia, which is a dangerous (and even fatal in severe cases) condition that is caused by an abnormally low level of sodium in the blood.
The strongest single predictor of hyponatremia is substantial weight gain during the race, which correlates with excessive fluid intake. Runners should drink only as much fluid as they lose with sweating. You can test your body’s sweat rate, but thirst is usually a good guide. Replacing lost electrolytes—for example, salt—should also be part of your fueling plan.