This is from Chris Carmichael (former coach of USPS, now Lance's trainer). Nothing too profound, but I had been curious:
With the temperatures rising in southern France , hydration is going to become a serious issue in the next few days of racing. The best preparation in the world is no match for the damage that can be caused by dehydration. If there’s one thing that can stop a potential Tour de France champion in his tracks, it’s running low on fluids. What you drink, however, is just as important as how much you consume.
The science of sports drinks has come a long way in the past twenty years. Instead of simply mixing sugar and salt into water and hoping for a good result, modern sports drinks are carefully formulated to deliver carbohydrate and electrolytes in the most efficient manner possible.
When you mix a sports drink too strong, or the carbohydrate content of the drink is too high, it takes longer for your body to move the drink out of your stomach and into the small intestine, where the majority of digestion and absorption occur. The same is true if the concentration of sodium and other electrolytes is either too high or too low. Most sports scientists and drink manufacturers agree that a 6-8% carbohydrate solution leads to fast absorption of carbohydrate and fluid. If you follow the directions when mixing a sports drink, the carbohydrate concentration will fall within that range. Incorrect mixing, or adding more powder to get a little more carbohydrate, may actually slow the absorption of both fluids and energy.
Taste is another reason to mix sports drinks correctly. During the process of developing the formula for PowerBar Endurance, CTS coaches and I found that when drinks are too sweet or the taste is too strong, athletes stop drinking earlier than when the taste is lighter. This has important implications for riders at the Tour de France because they need to consume at least two bottles per hour while on the bike. If the taste of the drink is just right, they will consume more fluid each time they grab the bottle, which leads to a greater total fluid consumption over the course of the stage. When they’re losing as much as 1.5 liters of fluid per hour due to sweat, getting someone to drink more during a stage can drastically improve their performance.
What about plain water? About half the fluids a Tour rider consumes on the bike is plain water, and the other half is sports drink. The sports drink, besides delivering energy, replenishes electrolytes lost in sweat. Were riders to drink only water for the duration of a stage, they would risk running low on electrolytes. Since the nervous system relies on electrolytes like sodium and potassium to conduct nerve impulses, running low on electrolytes can have significant effects on performance. However, even if you consume only water, the risks of hyponatremia (water intoxication) are relatively small, especially considering the amount of food the riders eat during the stage. Besides the physiological impact of consuming a mixture of sports drinks and water while racing, riders just don’t want to drink sports drink in every bottle they take.
The riders in the Tour de France, even riders on the same team, drink a variety of different sports drinks because they have each found the one that works best for them. That said, there’s often no time to be picky when you’re getting bottles from the car or from a teammate. The right drink for you is the one that tastes good to you when it is mixed correctly. If you have to dilute your sports drink in order to tolerate it when you work out, you’re not getting the best performance from the drink, and it’s not helping you achieve your best performance.