Athletes have long used caffeine to improve performance. Prior to 2004, caffeine was banned by the International Olympic Committee. It was in the late 1970’s the focus brought on studies on the improved effect on endurance from small amounts of caffeine.
At high intensity bouts or exercise, 10% to 20% improvements in endurance have been recorded. A study in 1999 measured the effect of caffeine when added to an energy bar. The study showed positive cognitive functions during exercise, and improved response of speed and performance of complex moves. No evidence has been found that habitual caffeine use effects these studies, so more is not necessarily better.
There have also been studies on the absorption of carbohydrate and the role caffeine plays. The results of these studies are not conclusive; there are signs caffeine may help carbohydrate absorption during exercise, but the quantity ratio of caffeine to carbohydrates is not clear.
And one of the old standby negatives of caffeine, that caffeine is a diuretic and should not be consumed prior to exercise, now shown in recent studies, not to be the case. No effect on sweat rates have been shown when ingested in moderation. And even though side effects from caffeine still exist, most of the negative side effects are associated with extreme quantities.