Supplemental vitamin C doses of 1-2 grams per day for 1-2 weeks did not improve immune function before or after prolonged exercise in most studies, but results were more promising in high-endurance athletes when vitamin C was combined with other antioxidants.

“generally they found that supplemental vitamin C doses of one to two grams per day for one to two weeks didn't cause any improvements in immune function before after prolonged exercise but the results among those participating in high endurance training were a little more promising especially if vitamin C was given along with additional antioxidants such as vitamin E and beta carotene”