The most common symptoms are headache and fatigue. The headache may begin as soon as 18 hours following the last dose of caffeine and may worsen with exercise. It is not known why some people experience caffeine withdrawal and others do not. Despite considerable research in this area, the role of caffeine as a performance enhancing drug is still controversial. Some of the data are conflicting, which is in part due to how the experimental studies were designed and what methods were used.
Caffeine can enhance performance in endurance sports.
Glycogen is the principal fuel for muscles and exhaustion occurs when it is depleted. A secondary fuel, which is much more abundant, is fat. As long as there is still glycogen available, working muscles can utilize fat. Caffeine mobilizes fat stores and encourages working muscles to use fat as a fuel. This delays the depletion of muscle glycogen and allows for a prolongation of exercise.