1. Recent, well-controlled studies have established that moderate doses of caffeine ingested 1 h prior to exercise enhance the performance of certain types of endurance exercise in the laboratory. Moderate caffeine doses produce urinary caffeine levels well below the allowable limit, as determined by the International Olympic Committee. The results are specific to well-trained elite or recreational athletes. It is not known if these findings will improve performance in competitions because controlled field studies of the effects of caffeine are lacking. 2. The mechanisms responsible for improved exercise endurance in prolonged exercise remain elusive. A metabolic mechanism appears to contribute early in exercise, when caffeine ingestion increases plasma free-fatty acid concentrations and muscle triglyceride use and spares muscle glycogen. However, it is not clear if increased fat oxidation causes the glycogen sparing in muscle. Increases in plasma epinephrine concentrations usually occur following caffeine ingestion but are not essential for the accompanying metabolic changes. When studying caffeine effects in the human it is difficult to identify the primary source of the "stimulus" because caffeine usually increases epinephrine secretion and is also rapidly metabolized in the liver to three dimethylxanthines (paraxanthine, theophylline and theobromine). The dimethylxanthines can remain in the circulation longer than caffeine and may be metabolic signals in their own right. 3. Caffeine appears to enhance performance during short-term, intense cycling lasting ~5 min in the laboratory and in simulated 1500 m race time. However, positive ergogenic effects of caffeine are much less frequent during sprint exercise lasting less than 90 s and in incremental exercise tests lasting 8-20 min. 4. Potential mechanisms for improving performance during intense exercise lasting 5-20 min include direct effects of caffeine on the central nervous system and/or excitation-contraction coupling and increased anaerobic energy provision in skeletal muscle.
