Background:It has long been discussed whether supplementation of antioxidants promotes or inhibites adaptation to exercise. Regular exercise leads to increased oxidative stress in muscles through increased production of reactive oxygen species (ROS), and antioxidants have therefore been seen as a mean to counteract the potentially harmful effects of ROS. However, recent studies show that ROS regulate processes that are central in the adaptive responses of muscle cells. It is therefore possible that supplementation of antioxidants through inhibition of ROS may have a negative effect on adaptation to exercise and physical performance.Objective:Our aim was to investigate the effects of combined vitamin C and E supplementation on the adaptation to endurance exercise. More concrete, we have investigated the relationship between supplementation of 1000 mg vitamin C and 235 mg vitamin E per day and the change in capillary density of the vastus lateralis muscle.Design: The study was double-blinded and randomized. 46 young, healthy subjects were randomized into two groups; one intervention group who received supplementation of vitamin C and E, and one placebogroup. Muscle biopsies were obtained before and after 12 weeks of endurance training. The biopsies were analysed for capillaries using a CD31 antibody, and muscle fiber type was decided using the SC71 antibody. VO2max was mesured before and after the training period. Results:The capillary denisty did not change significantly during the training period.No significant differences between the two groups were observed concerning changes in capillary density or change in VO2max. VO2max increased significantly in both groups during the training period. Conclusions:Based on our results, supplementation of vitamin C and E to young, healthy individulas has no effect on adaptation to endurance training.