Caffeine preserves muscle sugar. The limiting factor in racing in any sport is the time that it takes to get enough oxygen into your muscles to burn food for energy, so anything that requires less oxygen allows you to race faster. Sugar stored in muscles, called glycogen, requires less oxygen than fat or protein. Anything that helps you keep sugar in muscles longer gives you greater endurance.
Since caffeine is abundant in our food supply (coffee, tea, colas, chocolate and so forth), most people consider it to be very safe. However, Italian researchers report two bicyclists who took massive overdoses of caffeine and developed severe low blood levels of potassium that can cause irregular heart beats and sudden death (Clinical Journal of Sport Medicine, March 2010).
Very small amounts of caffeine help to preserve muscle sugar and increase endurance. You can increase endurance with as little as a third of a cup of most caffeinated soft drinks. No data exists to show that taking large amounts increases benefit. Up to five cups of coffee a day should not damage healthy people. A cup of coffee contains about 100 mg of caffeine, equal to two cups of tea, three cups of Coca Cola or five ounces of dark chocolate.
Caffeine is a diuretic, but not during exercise. It raises blood pressure only temporarily so this should be of concern only to people with high blood pressure. It can cause irregular heart beats, but is not likely to do so in people with healthy hearts. Caffeine appears to lower risk for diabetes .