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Can too much water harm marathon runners? Reuters Health recently reported just that. When runners collapse or get sick at the end of a long race, it seems logical to give them fluids. Sometimes, however, water is the last thing these athletes need, researchers report.

In the current issue of Annals of Internal of Medicine, investigators report ( Report 1 and Report 2) on marathon runners who developed brain swelling as a result of "water intoxication." All had taken in too much water during their races, causing sodium levels in the blood to drop. From there, according to Dr. J. Carlos Ayus, excess water is absorbed into blood and fluid builds up in the brain. Eventually, fluid accumulates in the lungs, and athletes become breathless and nauseated.

Ayus, a professor of medicine at Baylor College of Medicine in Houston, Texas, told Reuters Health that when runners collapse or become ill, the natural assumption may be that they are having a heart attack. Yet, rather than being a sign of heart attack, fluid build-up in the lungs--called pulmonary edema--may signal brain swelling, according to Ayus. "I believe," he said, "that this is what happens to many people who collapse and die after a race. It should be diagnosed more often now." In their report, Ayus and his colleagues describe the cases of seven marathoners who collapsed and had nausea and vomiting after their races. When brain scans revealed swelling, six of the patients were treated with an intravenous solution containing high amounts of sodium--a water-depleting treatment that is directly the opposite of the low-sodium solutions that runners may receive if they are misdiagnosed, Ayus noted. The seventh patient, who was not diagnosed with brain swelling, later died; an autopsy revealed that there had indeed been fluid on the brain. Five of the patients were female, suggesting that women may be more prone to water intoxication and its effects on the brain and lungs. And, Ayus pointed out, all seven had a history of using nonsteroidal anti-inflammatory drugs--painkillers that include aspirin. These drugs can block the excretion of water from the body, he explained. Whether athletes other than marathoners experience this series of events is unclear, but Ayus said that it is possible. Runners who become breathless and nauseated after drinking large amounts of water during a race should go the hospital, he advised. There, he added, doctors should check blood sodium levels.