The Citric Acid Cycle is the favoured high energy yield metabolic pathway of the marathon runner. The cycle only operates under aerobic conditions (with oxygen). The three carbon atom molecule pyruvate losses a carbon atom in its conversion to acetyl CoA in the mitochondria membrane matrix. One molecule of carbon dioxide is produced. The two remaining carbon atoms enter ther cycle and leave as CO₂. NADH and FADH₂ formed are oxidised by the electron transport chain to form ATP - the energy currency of the athletes body.