BOSTON -- Doctors said a runner who collapsed and died in the Boston Marathon developed a rare condition caused by drinking too much.

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Watch Heather Unruh's Report HYPONATREMIA Symptoms include bloating and puffiness in the face and fingers, nausea, vomiting, muscle weakness, headache and disorientation. Women appear more prone to hyponatremia than men. A racer can calculate their "sweat rate" ahead of a race by weighing before and after exercising for half an hour at the pace and under the conditions anticipated in the race. For every pound lost through sweating, the racer should drink a pint per hour during the race. A sports beverage that contains carbohydrates and electrolytes may help prolong an athlete's peak performance and replace sodium lost in sweat. Source: MedTerms.com

NewsCenter 5's Heather Unruh reported that Cynthia Lucero, 28, suffered from hyponatremia, or water intoxication, a condition complicated by stress hormones.

"You can get too much of a good thing," said Dr. Arthur Siegel, of McLean Hospital. "With prolonged, strenuous exercise, the combination of too much water plus the muscle injury that shuts down the ability of the kidney to excrete the water produces this dangerous, life-threatening problem."

The longer the event, the greater the risk, whether athletes drink water or sodium-laden sports drinks. Siegel said that Lucero's tragedy was preventable if scales were used to monitor marathoners' weight.

"The way to prevent this is for a runner to weigh themselves before the race and write the weight down on a bib," Siegel said.

Siegel said runners who feel bad could be weighed again, and if the weight is down, they could be treated for dehydration.

"If their weight is up, however, as Cynthia's would have been shown to be, then the problem is water intoxication," Siegel said. "And the important approach then is no fluids."

Siegel said anyone out for a 30- to 45-minute workout can drink without risk. But athletes in long-distance sports, such as marathoners, should be aware of the dangers of drinking too much.