UCSF study finds danger in marathon runners drinking too much water

May 01, 2000

Drinking too much water while running a marathon can kill you. That may sound like a rumor passed around on the Internet, but it does happen in some cases. Now researchers at the University of California, San Francisco think they know why. The excess water can help to cause the brain to swell, and fluid to leak into the lungs, either of which can be fatal. They confirm that the cure is a simple intravenous dose of salt water. The findings are published in the May 2 issue of the Archives of Internal Medicine.

Although marathon runners need to keep hydrated, in the last decade physicians have come to realize that a few hours of sweating away water and salt, but drinking only water, could put some runners into a danger zone known as hyponatremia. It was found that some runners who collapsed and died during a marathon had lost the normally well-balanced ratios of salt and water -- they had plenty of water, but far too little salt.

To understand how this might kill some runners, Allen Arieff, MD, a UCSF professor of medicine at the Veterans Affairs Medical Center in San Francisco, and two colleagues at Baylor College of Medicine, treated, or consulted on the cases of, seven athletes who suffered hyponatremia while running a marathon. They had all been nauseous, vomiting, or confused at some point during their run. Six of the seven patients survived after intravenous treatment with a high salt solution.

All the patients share several characteristics, which provide clues about the mechanisms of hyponatremia, Arieff said. In addition to having fluid in their lungs -- a hallmark of hyponatremia -- the patients all had low levels of sodium and oxygen in their blood. X-rays of six of the patients showed that they had significant brain swelling.

In the study, Arieff suggests the following mechanism. The body, in an attempt to keep a balance of salt and water levels between the blood and tissues, begins to draw water out of the blood, leading to puffiness in the skin and swelling in the brain. The brain responds to the pressure by sending an emergency distress call to release water into the lungs. The lung fluid, or the brain pressure, eventually kills most patients with hyponatremia, Arieff said. "They either suffocate or their brain herniates," he said. All the patients in this study had been taking ibuprofin-based pain relievers, such as Advil, which can make the body retain even more water. "These drugs could make (hyponatremia) more likely, and could make it more severe," Arieff said. However, he added that this apparent correlation needs to be confirmed in a larger study.

Women are likely to be more at risk for hyponatremia, Arieff said, because the female hormone estrogen can act in combination with another hormone called anti-diuretic hormone (ADH) to constrict the blood vessels in the brain. ADH is produced to save water in response to heavy perspiration.

In preparation for this year's marathon season, emergency room physicians should be educated about hyponatremia, and how to treat it, Arieff said. Most collapsed runners admitted to the emergency room would be thought to suffer from heart disease, so doctors might not suspect hyponatremia, especially since heart failure causes similar symptoms. But if they know to look for it by measuring blood sodium levels and giving a chest X-ray, doctors will be able to treat most cases of hyponatremia successfully, with a simple intravenous dose of high salt solution. Diagnosis is critical, Arieff said, because "most of these patients won't survive if they are treated for heart failure."

To avoid hyponatremia, Arieff recommends that athletes take salt tablets with their water before, and even during, marathons or longer endurance events. It's also possible that sports drinks containing electrolytes could help, he said. And, of course, it's important to drink water during a race, but only enough to replenish what is lost through perspiration.
Arieff's co-author's on the study were J. Carlos Ayus, MD, a professor of medicine, and Joseph Varon, MD, an assistant professor of medicine, both at Baylor College of Medicine in Houston.

University of California - San Francisco

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