I was brought up in the old school thinking that one must drink water as much as possible. My mother hammered me the adage of drinking more than eight glasses of water a day. It seemed like sound advice. Besides, whatever excess water you drink will eventually be expelled once you urinate, so I figured there was no harm in over-hydrating.
It comes out that, during moments of intense exercise, there is a danger of over-hydrating. The condition is called hyponatremia and it is described as “a metabolic condition in which there is not enough sodium (salt) in the body fluids outside the cells .”
Sodium is found mostly in the body fluids outside the cells. It is very important for maintaining blood pressure. Sodium is also needed for nerves and muscles to work properly.
When the amount of sodium in fluids outside cells drops, water moves into the cells to balance the levels. This causes the cells to swell with too much water. Although most cells can handle this swelling, brain cells cannot, because the skull bones confine them. Brain swelling causes most of the symptoms of hyponatremia.
I found more worrisome info courtesy of Runaddicts:
Hyponatremia is a condition where the body over hydrates itself and believe it or not it is more dangerous, and in some cases fatal, than dehydration. Since the fluid intake is more than one has lost during the event, the body is low on sodium level. The symptoms are similar to dehydration, and cause vomiting, perplexity or muscle weakness. Adding more liquids to hyponatremic runners can only worsen the situation. It will dilute the blood salt level further and result in coma and also death
Think this is an exaggeration? Read the story of Michele Burr. On July 20, after finishing an ultra-marathon, she was disoriented, did not recognize her husband, was vomiting uncontrollably, had a seizure, and went into a coma. Ok, you may think that hyponatremia will only hit ultra-marathoners, but consider that the symptoms hit Michele at mile 18 (or after 29 kilomters).
The last thing I clearly remember is going into the mile 18-aid station. I can’t remember anything at all between miles 18 and 68.
Then in the next breath the friend watched as the walk turned to a wobble and she collapsed. Going into seizure, worried spectators called for an ambulance but by the time she arrived at a hospital Cynthia Lucero was in a coma. Two days later she died.
So how do you prevent water-intoxication? There may be no one answer for all types of people. Obviously one should not be guzzling liters and liters of water. Thirst may in fact be the best indicator on whether one should begin drinking water and it may be a myth that one should drink copious amounts to ward off dehydration. The belief that dehydration begins even before one feels the pangs of thirst may also be just a myth. And since hyponatremia is a result of excess body water diluting the serum sodium, it may also be adviseable to consume sports drinks during high-intensity runs. I checked a few brands like Pepsi 100 and Gatorade and these drinks do have sodium.