Caffeine is a legal stimulant which can be an endurance aid for activities of > 30 minutes duration. It is a member of a group of compounds called methylxanthines found naturally in coffee beans, tea leaves, chocolate, cocoa beans, guarana, and cola (kola) nuts and similar to the asthma medication theophylline.
During prolonged exercise, the onset of fatigue correlates closely with the depletion of muscle glycogen stores (and is delayed if glycogen is spared). The metabolism of free fatty acids (FFA) as an alternative energy source can lead to decreased use of muscle glycogen. Caffeine can increase blood FFAs, and it is felt that this is its major method of action. In one study, caffeine produced a 50% increase in FFA at 3 to 4 hours. This effect was seen after 300 mg of caffeine (an average 6 ounce cup of brewed coffee contains 100 - 150 mg of caffeine but a Starbuck's 8 oz cup contrains 250 mg!).
There is speculation that some of its benefits may also be related to its central nervous system effect as a stimulant, and a recent study has demonstrated a direct positive effect on the muscle fiber itself via a mobilization of calcium from the sarcoplasmic reticulum with a reported 7% increase in power output over a 6 second cycle exercise task.
In one controlled study, subjects were able to perform for 90 minutes to fatigue as compared to 75 minutes in controls (a 20% increase) after the drinking the equivalent of 3 cups of coffee or 6 caffeinated colas 1 hour before, even though values for heart rate and oxygen uptake were similar in both groups. Another study, looking at performance with acute altitude change (4300 meters), demonstrated a 50% increase in performance with caffeine supplements. How this would help at lesser elevations, riding in the Rocky Mountains for example, is not clear.
The suggested dose of caffeine for the recreational rider is 5 mg per kg of body weight (range 3 - 9 mg/kg) taken 1 hour before the ride although some riders take smaller doses periodically throughout the ride itself.
But there are potential side effects. Caffeine can cause headaches, insomnia, and nervous irritability. In addition it is a diuretic (can cause an increase in urinary water loss) and can lead to dehydration. However the biggest negative is that in high concentrations it is considered a drug and is banned by the US Olympic Committee and US Cycling Federation (to exceed the US cycling Federation's legal limit for caffeine - urine concentration of 12 micrograms/ml - one would have to ingest 600 mg of caffeine (6 cups of coffee) and have a urine test within 2 to 3 hours). Fortunately the ergogenic effects can be achieved well below these limits.
Habitual use will induce tolerance so a period of abstinence is recommended for several weeks before the event. The bottom line is that most endurance athletes consider caffeine useful if used correctly.
