These studies employed well-trained, elite or serious,recreational athletes. Studies with untrained individuals cannot be performed due to their inability to reliably exercise to exhaustion. The mechanism to explain these endurance improvements is unclear. Muscle glycogen is spared early during submaximal exercise following caffeine ingestion (5-9 mg/kg). It is unknown whether glycogen sparing occurs as a result of caffeine’s ability to increase fat availability for skeletal muscle use. Furthermore, there is no evidence supporting ametabolic component for enhancing performance at a low caffeine dose (3 mg/kg).Therefore, it appears that alterations in muscle metabolism alone cannot fully explain the ergogenic effect of caffeine during endurance exercise.