Such deaths are rare, typically fewer than one per year in the United States. But according to family practice physician James Winger of the Loyola University Health System in Illinois, these fatalities are “essentially 100 percent preventable.” All runners have to do to minimize their danger is drink less. “The more volume they take in, the higher the risk,” said Winger, who recently co-authored a study of the phenomenon.
Overhydration floods the body with fluid and causes the concentration of sodium in the blood to drop to abnormally low levels. Cells absorb the excess water, which causes swelling, most threateningly in the brain, where it can result in seizures, coma, respiratory arrest and even death. Lesser symptoms of hyponatremia include nausea, vomiting and cramps.
Winger’s study, published last year by the British Journal of Sports Medicine, measured mistaken beliefs about hydration among runners by surveying 197 Chicago-area racers in 2009. Most — 58 percent — knew to drink only when thirsty.
That is one of two hydration approaches recommended by running authorities such as USA Track & Field, which changed its recommendations in 2003. The other approach, involving weighing yourself before and after exercising and drinking only enough to replace lost fluid, is considerably more complicated.
The rest of the runners in Winger’s survey were likely to follow the older guidelines, which called for drinking set amounts before, during and after a race, or drinking all they could hold. The thinking behind that was that even slight dehydration would hurt performance and that by the time a runner felt thirsty, the negative effects of dehydration had already begun.
Winger found that runners drinking according to a set schedule generally tended to be older, more experienced and faster. However, a New England Journal of Medicine report on the 2002 Boston Marathon found that women and slower runners were most likely to develop exercise-associated hyponatremia, probably because the former are smaller and the latter stay on the course longer, giving them more time to drink.
Exercise-associated hyponatremia was first described in the scientific literature in 1985 and has been extensively studied since. However, mistaken beliefs about its cause and prevention persist.
One Web site for marathoners, for instance, notes that runners need to avoid hyponatremia, but it also says that marathon runners should “drink 8 to 10 ounces of water or a sports drink every 10 to 20 minutes” and then “as much as they comfortably can” afterward.