Dennis Mangan points to a fascinating study that shows that vitamin C hampers adaptation to endurance training:
Briefly, the authors took a group of young men and had them train on stationary bicycles. One group took one gram of vitamin C daily, the other did not. A parallel study was done on rats using the same general idea – only with rats, one can exercise them to exhaustion. The result: vitamin C put a major dent in the training effect due to exercise.The figure above shows gene expression of two of the most important internal antioxidant-enhancing enzymes, superoxide dismutase and glutathione peroxidase. The first bar shows levels without training, the second with training, and the third with training plus vitamin C, which shows that the vitamin practically abolished the training effect. The reason seems to be that reactive oxygen species (ROS) formed during exercise are important signals for the synthesis of more mitochondria, the cell’s energy factories. Vitamin C quenches the ROS and thus the signals.