bicycling magazine sited a study that said those who drink a 4:1 carb to protein mix instead of a pure carb mix(like gatorade) could sustain their 80% VO2 max for 40% longer than those drinking the pure carb stuff. the protein also helps build lean muscle mass if you take it during excersise and helps recovery if you drink it afterwards. i use cytomax and i like it a lot, but i've heard a few flavors are pretty crappy. check in the trianing and nutrition section, there have been several threads about it.