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In hindsight, Elaine Vescio says she should have recognized something was wrong partway through her half-Ironman triathlon. The seasoned marathon runner and long-distance cyclist had unusual trouble controlling her bike last summer during the 90-km cycling portion. By the time she set off on the 21-km run, she felt generally "lousy" and her hat felt strangely tight.

What Ms. Vescio didn't realize was that her brain was swelling. "It was actually pushing against my skull at that point," says the 47-year-old triathlon coach from Millbury, Mass.

Ms. Vescio pushed on despite her discomfort and finished the course, guzzling plenty of liquid whenever she could. But driving home, she felt extremely dizzy. Her hands were tingling. She felt confused and feared she might have a seizure.

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At a nearby hospital, a doctor identified the problem: Ms. Vescio was suffering from exercise-associated hyponatremia, a potentially fatal condition that occurs when sodium levels in the blood are diluted because of excessive fluid intake. In an effort to stay hydrated during the race, she had inadvertently consumed too much water, causing her blood sodium to fall to a dangerous level.

While such severe cases as Ms. Vescio's are rare, researchers, sports coaches and physicians are increasingly warning athletes about exercise-associated hyponatremia. Endurance athletes are at greater risk of the condition because they typically drink more and lose electrolytes through sweat while racing for long periods of time. The danger occurs when the excess fluid in the blood transfers into the cells of the brain, causing it to swell. Symptoms can range from nausea, spasms and cramps to vomiting, confusion and seizures, but because signs of hyponatremia are non-specific, those who have mild cases may not realize it. Health experts detect it by measuring sodium levels in the blood to see if they have dropped.

Conventional wisdom dictates that athletes should take in plenty of fluids, but nearly half of recreational runners may actually be drinking too much, according to a new study published in June by researchers at the Loyola University Health System in Maywood, Ill. In recent years, experts have begun revising their guidelines to recommend that runners drink only when thirsty. Yet in a survey of nearly 200 runners, the Loyola researchers found 9 per cent drink as much as possible during racing and training, while 37 per cent drink according to a preset schedule, whether they feel thirsty or not.

Study author Jim Winger says when exercise-associated hyponatremia was first identified in 1985, dissenting voices had argued the problem was due to dehydration, not overhydration. And sports drink marketers in the 1980s and 1990s promoted the idea that it was impossible for athletes to drink too much, which contributed to the confusion.

"Slowly, but I think surely, that has changed," he says, noting that in the past five to 10 years, "we're starting to swing this pendulum from 'drink as much as you can' to safer, more responsible methods of hydrating."

Hyponatremia can occur regardless of whether athletes drink plain water or electrolyte-enhanced sports drinks, since sports drinks are typically quite dilute, Dr. Winger says.

A common misconception, he says, is that people need to drink before they feel thirsty to avoid dehydration. When the body loses fluid through sweat, the brain stimulates a hormone called vasopressin, which causes the kidneys to reabsorb pure water from one's urine, thus, naturally balancing the blood to normal levels. Only when the body continues to lose fluid does the brain stimulate thirst.

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"When we hear, 'Oh, it's too late when you're thirsty,' that's not true because things are already working when you're thirsty," Dr. Winger says.

He adds that the guideline to drink only when thirsty can also apply off the race course in regular daily life. "Thirst as a mechanism is many, many hundreds of millions of years old and is finely tuned," he says. "I sort of chuckle when I hear you need to drink this many versus this many glasses of water a day, because your body has a pretty good handle on that, actually."

Chris Woollam, medical director of the Mississauga Marathon, says drinking too much fluid has even been found to slow runners down. Although he has seen only a handful of cases of hyponatremia over the years, he says he has become extra vigilant of the condition.

Marathon medics now rarely give intravenous fluids to athletes who aren't feeling well, he says. The worry is "we're going to take a person who's already overhydrated, give them some fluids and do them in ourselves."

While some runners take sodium tablets to ensure their blood concentrations are balanced, Dr. Woollam says the jury is still out on them. He notes such pills can potentially irritate the stomach and cause nausea. Like Dr. Winger, he says the safest bet is to hydrate according to your thirst. "Even if you let yourself get a little bit thirsty, a little bit dry, that's fine."

Fortunately, exercise-associated hyponatremia is easy to treat. At the hospital, Ms. Vescio was given an intravenous saline solution, which made her feel better almost immediately.

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Ms. Vescio has since continued competing in endurance races, but is now careful to avoid overhydrating again. "It's been a learning experience," she says.


Running a risk

Jim Winger, assistant professor of family medicine at Loyola University Health System in Maywood, Ill., says a landmark 2005 study on Boston Marathon runners found that up to 13 per cent had biochemical evidence of exercise-associated hyponatremia after the race. That means, while they may not have shown symptoms, the sodium levels in their blood were lower than normal.

Normally, the amount of sodium is between 135 mg and 145 mg per decalitre, he says. Biochemical hyponatremia is any amount lower than 135 mg per decalitre.

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