DannoXYZ 

If you look carefully at the Tour riders in the last hour or so before the finish, you'll see many of them gulping down small 1/2-size sodas. The 55 calories in the drink in inconsequential, they just burned off 6000-8000 calories that day. The trick is in the "speed" compounds like caffeine, ginseng, etc. that picks up your metabolism. These have a glucagon-inducing effect which causes your body to increase lipid conversion into glycogen. It supplements your energy stores since you can only ingest about 250 calories/hour through eating. The caffeine also makes you pee, good for flushing anything you have laying around in your system you don't want to show up in a drug test...