Reactive oxygen species (ROS) are produced at an increasing rate when metabolism is elevated such as during exercise training. Athletes usually consume antioxidants in an effort to reduce ROS, reduce muscle damage and thus minimize fatigue. A recent study in the Medicine and Science in Sports & Exercise (Strobel et al., 2011) examined the effect of long-term antioxidant supplementation with vitamin E and α-lipoic acid on changes in markers of mitochondrial biogenesis in the skeletal muscle of rats. Their results showed that antioxidant supplementation suppresses skeletal muscle mitochondrial biogenesis.
If this is also true in humans, it means that antioxidant supplementation reduces the magnitude of adaptation and benefits gained by aerobic exercise training.