Hyponatremia essentially is water intoxication caused by low sodium levels in the blood. It is the opposite of dehydration and generally occurs among individuals who have become supersaturated with fluids, usually water or sports drinks. Runners, especially marathoners and ultramarathoners, are susceptible when they imbibe great quantities of fluids over the course of many hours.

Hyponatremia caused a death at both the 2002 Marine Corps and 2003 Boston marathons.

For decades, marathoners were admonished to drink copiously while racing. Dehydration was deemed the enemy of marathoners, and water stations sprouted along nearly every mile of every racecourse. But as marathons and ultramarathons gained popularity and more runners participated, times slowed and the hazards of hyponatremia became apparent.

In today's New York City Marathon, runners have been advised for the first time to drink no more than eight ounces of water every 20 minutes. For the average participant who completes the 26.2 miles in 41/2 hours, that's more than three quarts during the race. At the last two Boston Marathons, race organizers deployed fewer water stations along the course in order to discourage excessive hydration.

"The pendulum has swung away from dehydration and toward hyponatremia," said Dave Watt, executive director of the American Running Association in Bethesda. "We're trying to swing it back."

"There are certain at-risk groups, including women more than men, runners with small [body mass index], and those running marathons in longer than four hours," Watt said. "Hyponatremia is 100 percent preventable, and we need to educate runners and first responders about how to prevent it and how to recognize and treat it." Without a blood sample, however, hyponatremia and dehydration appear similar.

While hyponatremia has become a cause celebre, particularly during the fall marathon season, prevention should not come at the expense of the more common malady of dehydration. At last June's Lawyers Have Heart 10K in Georgetown, morning temperatures climbed into the 80s, and humidity levels were already high. And many of the several thousand runners were unprepared for one of the first hot days of the summer. After the race, more than a dozen runners required medical assistance and at least six were transported to the hospital with various degrees of heat exhaustion.

One experienced runner, Helen Beven, from Kensington, running her first race as a masters (40 and over) competitor, collapsed within sight of the finish line. After two nights in the hospital, one in intensive care, and four months of recovery, Beven still feels the effect of her trauma. "I didn't know if I'd ever be able to push myself again," she said. Beven has been racing this fall but not yet to the level before the incident.

Local Women Make Good: New York City race director Mary Wittenberg, who won the 1987 Marine Corps Marathon as Mary Robertson, was inducted into the Marine Corps Marathon Hall of Fame last weekend. . . .

Tracey Gold, a 1992 All-Met tennis player from Churchill, won the Nike Women's Marathon in San Francisco on Oct. 23 in 2 hours 59 minutes 32 seconds.

-- Jim Hage