As thousands of runners prepare to brave the 26-mile course of the Boston Marathon today, researchers at the Harvard Medical School (HMS) have found that in addition to the well known hazards of dehydration, over-hydration can prove dangerous for a small minority of the population as well.

As runners trained for the marathon, Clinical Fellow in Pediatrics Christopher S. Almond and a team of HMS researchers published their findings last week, based on a sample of Boston Marathon runners from the 2002 race.

The study, published April 14 in the New England Journal of Medicine, monitored the sodium levels of 488 runners in the 2002 marathon by analyzing their blood samples from before and after the race.

The researchers found that 13 percent suffered from hyponatremia, a condition characterized by excess water in the blood, which causes sodium levels to fall. Water then moves to areas of higher sodium content in the body, causing cells to swell.

Most of the time, swelling is merely an annoyance, Almond said. But in the brain, where the skull gives cells little room to expand, it can become a problem. Almond noted that in “very rare instances,” increased pressure in the brain caused by swelling can lead to confusion, altered mental status, seizures, coma, or death.

Critical cases of hyponatremia occurred in .6 percent of the sample, or three runners. The most common risk factors were significant weight gain during the race, longer race times, and a lower body-mass index, Almond said.

Almond stressed that it is important for runners not to under-hydrate themselves because of worries about over-hydration, adding that the issue is largely one of awareness.

“Most runners—more often elite runners—do not need to make major changes,” he said. “Non-elite runners should be aware that it is possible to over-drink—especially when you’re out on the course for a long time—and that fluid balance merits careful attention.”

To prevent hyponatremia and dehydration, Almond advised that runners should weigh themselves before and after training periods to check for fluctuations in weight. Gaining significant amounts of weight may signal excessive water intake, while losing too much weight after a workout might point to inadequate hydration, he said.

“We think it is important for runners to begin thinking about their hydration strategy well before race day,” Almond said.

Almond and his fellow researchers did not create specific hydration strategies for all runners, listing the USA Track & Field website (http://www.usatf.org) as a source for further information.

Almond said the researchers first became interested in hyponatremia after treating a teenager who collapsed after a race in 2001.

“This study was a huge team effort,” he said.

—Staff writer Allison A. Frost can be reached at afrost@fas.harvard.edu.