Athletes drinking too much water during exercise can be very risky. It can cause fatal water intoxication.
Hyponatremia, also called water intoxication, is generally the result of drinking excessive amounts of plain water which causes a low concentration of sodium in the blood needed for the organ to function. There is much more fluid in the body than the kidney can excrete. Once a rare occurrence at sporting events, it is becoming more prevalent as participation increases and more novice exercisers are entering endurance events.
One recent study found that 13 percent of Boston marathon runners suffered from Exercise induced Hyponatremia (EAH), though most cases were mild. But since 1985, there have been at least eight documented fatalities from EAH related to long-distance events.
"The public's impression of the amount of water that is necessary to drink for good health is not based on real factual data," said Dr. Joseph Verbalis at Georgetown University Medical Center. "Many in our society have promoted the idea that you need to continually drink a large amount of fluid, such as 8 ounces of water eight times a day. But most people don't really need that much."
When the body is inundated with water, hyponatremia (abnormally low sodium) can result, and in extreme cases, the brain swells like a balloon inside the skull, producing confusion, dizziness, nausea, coma, and in extreme cases, death; a constellation of symptoms that together are known as "water intoxication."
Under normal conditions, the body keeps a good balance of water and salt.
Water can move relatively freely between the insides and outsides of cells; typically about two thirds of the water in your body is located inside cells and the other third is located in the circulating fluids outside of the body's tissues. Sodium, on the other hand, is highly concentrated in the extracellular fluid, ensuring that intracellular and extracellular fluids have about the same amount of "stuff" dissolved in them, and keeping the net movement of water across cell membranes to a minimum. In the case when you have, for example, maybe ingested a pound of rock salt or five gallons of tap water, it's not hard to imagine this delicate balance being thrown entirely out of balance. In the latter case, the extracellular fluid can become more diluted than the intracellular fluid, causing the cells in organs like the brain to quickly soak up excess water like sponges, expanding in the process.
If identified quickly, even severe cases of acute water intoxication can be treated relatively easily. As you might expect, the condition can be rapidly reversed by the intravenous administration of hypertonic saline.
The issue remains controversial, however. Some trainers and sports physiologists contend that by the time you're actually thirsty, you have lost enough fluid to already be dehydrated, so they say you need to drink in anticipation of becoming dehydrated, Verbalis explained. "We dispute that notion," He said, "and contend that thirst is a good indicator of your body's need for fluids, and that there is a window of time over which you can rehydrate safely."