Here's part of an article in the Athleteinme:
SPORTS DRINKS vs. WATER
Amazingly, up until 1969, athletes were discouraged from consuming fluids while running. It was thought that drinking fluids would impair athletic performance. Since then, hundreds of scientific studies have examined every aspect of these beverages on athletic performance. Today we know that athletic performance falls as an athlete loses fluids (Armstrong LE, et al. 1985).
When Robert Cade, MD, invented Gatorade® at the University of Florida, the goal was to find a better way to hydrate college athletes who were perspiring heavily in the hot Florida sun. But, despite their popularity, are sports drinks like Gatorade® any better than plain water?
It turns out that water by itself is not the best rehydration beverage during strenuous activities. Drinking plain water shuts down the sensation of thirst too quickly. Many athletes stop drinking too soon. When Puerto Rican soccer players were left to drink plain water on their own, they were still significantly dehydrated (Leiper JB, et al. 2001). A thirsty person drinks more fluid when the beverage contains sodium (and flavoring) compared to plain water.
Obviously, the easiest way to correct this is to get athletes to drink more water, however this can backfire. Drinking too much plain water can lead to hyponatremia, a serious medical condition where the concentration of sodium in the blood gets too low. This has been documented in marathon runners (Almond CSD, et al. 2005) and in soldiers during training. In rare cases, hyponatremia can be fatal (Ayus JC, et al. 2000).