After years of telling athletes to drink as much liquid as possible to avoid dehydration, some doctors are now saying that drinking too much during intense exercise poses a far greater health risk.

An increasing number of athletes -- marathon runners, triathletes and even hikers -- are severely diluting their blood by drinking too much water or too many sports drinks, with some falling gravely ill and even dying, the doctors say.

New research on runners in the Boston Marathon confirms the problem and shows just how serious it is. The research, involved 488 runners who participated in the 2002 marathon. The runners gave blood samples before and after the race. While most were fine, 13 percent -- or 62 of them -- drank so much that they had hyponatremia, or abnormally low blood sodium levels. Three had levels so low that they were in danger of dying.

The runners who developed the problem tended to be slower, taking more than four hours to finish the course. That gave them plenty of time to drink copious amounts of liquid. And drink they did -- an average of three liters, or about 13 cups of water or a sports drink -- so much that they actually gained weight during the race.

As more slow runners entered long races, doctors began seeing athletes stumbling into medical tents, nauseated, groggy, barely coherent and with their blood severely diluted. Some died on the spot or in the hospital.

In 2003, USA Track & Field, the national governing body for track and field, long-distance running and race walking, changed its guidelines to warn against excessive intake of liquids.

Marathon doctors say the new study offers the first documentation of the problem.

"Before this study we suspected there was a problem," said Dr. Marvin Adner, the medical director of the Boston Marathon, which is Monday. "But this proves it."

Hyponatremia is entirely preventable, Adner and others said. During intense exercise the kidneys cannot excrete excess water. But as people keep drinking, the extra water moves into their cells, including brain cells. The engorged brain cells, with no room to expand, press against the skull and can compress the brain stem, which controls vital functions like breathing. The result can be fatal.

But the marathon runners were simply following what has long been conventional advice: avoid dehydration at all costs.

Doctors and sports drink companies "made dehydration a medical illness that was to be feared," said Dr. Tim Noakes, a hyponatremia expert at the University of Cape Town in South Africa.

"Everyone becomes dehydrated when they race," Noakes said. "But I have not found one death in an athlete from dehydration in a competitive race in the whole history of running. Not one. Not even a case of illness."

On the other hand, he said, he knows of people who have become sick and died from drinking too much.

Hyponatremia can be treated, Noakes said. A small volume of a highly concentrated salt solution is given intravenously and can save a patient's life by pulling water out of swollen brain cells.

However, he said, doctors and emergency workers often assume the problem is dehydration and give intravenous fluids instead, sometimes killing the patient. Noakes and others advise testing the salt concentration of a sick athlete's blood before starting treatment.

For their part, runners can estimate how much they should drink by weighing themselves before and after long training runs to see how much they lose -- and thus how much water they should replace.

Adner said athletes also should be careful when a race is over. He advised runners to wait until they start to urinate before drinking any more fluids.

The paper's lead author, Dr. Christopher S.D. Almond, a cardiology fellow at Children's Hospital, said he had first heard of hyponatremia in 2001 when a cyclist drank so much on a ride from New York to Boston that she had a seizure. She eventually recovered.

Until recent years, the condition was all but unheard of because endurance events like marathons and triathlons were populated almost entirely by fast athletes who did not have time to drink too much.

In a letter, also published today in the journal, doctors describe 14 runners in the 2003 London Marathon with hyponatremia who waited more than four hours on average before going to a hospital. Some were lucid after the race, but none remembered completing it.

Source: Diabetes In Control.com: New England Journal of Medicine, April, 2005.