Every couple of miles, the 30,000 or so runners competing in the 36th Marine Corps Marathon on Sunday will pass stations stocked with water and sports drinks. Most, hopefully, won’t stop unless thirsty. Some, however, following outdated advice, will drink according to a preset schedule — even downing all they can hold — increasing their risk, doctors say, of a potentially fatal medical condition.

The condition, called exercise-associated hyponatremia, killed Hilary Bellamy of Bethesda two days after she competed in the 2002 Marine Corps Marathon.

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.

Another belief that Winter found among runners was that electrolyte-laden beverages, such as sports drinks, can prevent the condition by replacing sodium lost via sweating. In fact, Winger says in his study, overconsumption of sports drinks can lead to hyponatremia just as easily as drinking too much water can.

Estimates of the incidence of hyponatremia in marathoners vary widely, from fewer than one in 100 in a study of the 2000 Houston Marathon to more than eight in 100 in a study of the 2002 Boston Marathon.

Treatment ranges from intravenous saline solution in severe causes to medicines that can help manage the symptoms.

Based on past consumption from the Marine Corps Marathon’s 12 hydration stations, an average runner in the race downs 1.5 cups of water and 0.8 cups of sports drinks, according to Tami Faram, a spokeswoman for the Quantico-based race organization. “We never want to run out of water or sports drink,” Faram says. “That’s our goal.”

Faram notes that runners may also be drinking other fluids — many wear special water packs for such long races — and that the race organizers don’t attempt to monitor or prevent over-hydration. “That’s something the runners have to talk to their doctors and coaches and trainers about,” she says.

Faram says the race organizers will be prepared this year, as in the past, to help runners with medical issues, including hyponatremia. “We do have medical aid stations along the course to treat runners if they’re having issues, whether it’s over-hydration or under-hydration.” Faram says the medical staff of her marathon reports having helped six or seven runners with hyponatremia since 2003; one of them was taken to the hospital, treated and released.

Based on his research, Winger says runners can avoid hyponatremia by paying little attention to advertisements for sports drinks, outdated if well-meaning hydration advice and their own beliefs. “I like the simplicity of the message ‘Drink when you’re thirsty,’ ” he says. “It’s not something you have to tell your body to do.”

Henricks writes about health, technology, business and other topics from Austin.