There is considerable debate regarding the health effects of vitamin C supplementation. This brand new scientific study concludes that it decreases training efficiency because it prevents some key mitochondrial adaptations to exercise.
In my opinion, taking synthetic antioxidants regularly may not be wise. There's lots of hype about this powered by a multibillion-dollar supplement industry, so it's easy to get confused. I believe that research has shown quite clearly that although supplements may correct certain dietary deficiences, in the long run they simply cannot replace real food.