It is important to keep hydrated when running a marathon but did you know that too much water can be a bad thing?

A good rule of thumb is to drink about one cup of fluid every 20 minutes during a marathon [1], anything more than that could be dangerous and could cause hyponatremia [2] — when you have low amounts of sodium in your body. It can be fatal if not treated immediately. To prevent it, be sure to drink sports drinks during training and, as long as you don't have high blood pressure, increase your salt intake a few days before the race.

For symptoms of hyponatremia just .
Symptoms of hyponatremia include:

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

Fit's Tip: If you experience any symptoms, you should see your doctor.

Source [3]