Why Vitamins May Be Bad for Your Workout

Many people take vitamins as part of their daily fitness regimens, having heard that antioxidants aid physical recovery and amplify the impact of workouts. But in another example of science undercutting deeply held assumptions, several new experiments find that antioxidant supplements may actually reduce the benefits of training.

Source: Why Vitamins May Be Bad for Your Workout

TLDR: avoid high dosages of antioxidants (vitamin C, vitamin E) while training.