A study from the Netherlands shows that sugar in sports drinks slows absorption and increases stomach cramping in running races shorter than 12 miles. Fluids pass through your stomach and are absorbed almost immediately in your intestines. Exercise slows fluid passage from the stomach but does not affect intestinal absorption. Sugar added to drinks can delay stomach emptying to increase risk for cramps.
Another study from the University of Utah, reported in the same journal, shows that taking a salty drink just before exercise increases endurance. Dehydration is the most common cause of fatigue during exercise in fit men and women. This study used salted drinks or placebo (unsalted) drinks with two groups of cyclists, and demonstrated a significant improvement in an endurance time trial as well as better maintenance of blood volume in the group that had the salted drinks.
Anything that increases blood volume should increase endurance. Taking in fluid before exercising increases blood volume, and using salty drinks increases blood volume more than pure water. But a major problem with salty drinks is that they usually taste awful. You can accomplish the same results by drinking water, soda or any other beverage you like and eat a handful of salted peanuts or other salty food before and during your endurance events.
When you're not exercising, don't get in the habit of using sports drinks or any other sugared drinks to quench thirst. They'll add up to a lot of calories with little other nutritional value. Use plain water or a calorie-free beverage instead.
International Journal of Sports Medicine, Volume 26, 2005