“This condition, hyponatremia, occurs when you have low sodium in your body,” said Dr. James Muntz, internal medicine service chief with The Methodist Hospital in Houston. “When sodium levels drop in the fluids outside the cells, water will get in there and attempt to balance the concentration of salt outside the cells.”

The abundance of water will cause the cells to swell. Most cells can adapt to change, however, the brain cannot. When this occurs in less than 48 hours, it can be fatal if not treated immediately.

Symptoms of hyponatremia include:

• Vomiting.
• Loss of appetite.
• Headache.
• Restlessness/fatigue.
• Abnormal mental status (hallucinations, confusion, change in personality, etc.)
• Muscle weakness.
• Convulsions.

A few days before the race you can take steps to try and prevent hyponatremia by using sports drinks during training and increasing your salt intake, as long as you don’t have high blood pressure.

“During the marathon a good rule of thumb is to drink about one cup of fluid every 20 minutes,” Muntz said. “Drinking any more than that over the course of the race can get you into trouble.”

A recent study of runners in the 2002 Boston Marathon found that 13 percent of those who finished the race developed hyponatremia. The majority of these runners reported feeling “fine” after the race. However, if someone who feels “fine” continues to drink water because they believe the nausea and weakness they are feeling is due to dehydration, they could easily end up having a seizure and falling into a coma.

“You don’t want to drink too much during the race, but if you do, sports drinks like Gatorade that contain salt, would be better than a lot of water,” Muntz said. “If you experience any symptoms, see a physician immediately.”

Note: If no author is given, the source is cited instead.

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