High doses of vitamins C and E may actually reduce the beneficial training effects of exercise.
In a Norwegian study of 54 people in their twenties, half took 1,000 milligrams of vitamin C and about 350 IU of vitamin E a day while doing an endurance training program; the other half took a placebo. After 11 weeks, testing revealed that the supplement group experienced reductions in beneficial cellular adaptations to exercise compared to the placebo group.
In particular, they had much smaller increases in markers for the production of new muscle mitochondria (the powerhouses of cells). Previous studies have also found that the vitamins can blunt some of the benefits of exercise—for instance, its ability to improve insulin sensitivity and boost the body’s natural antioxidant system.