According to a study from the Norwegian School of Sports Scientists, high doses of vitamin C ( 1000 mg or more per day) and vitamin E ( 235 mg or more per day) can disrupt muscle hypertrophy stimulated by strength training as well as positive adaptations that take place during endurance training.
These vitamins are antioxidants which fight free-radicals. Free-radicals are highly reactive compounds with unstable electrons which cause oxidative stress and can damage healthy cells in the body, however, some processes in the body actually require oxidation to promote adaptions to exercise-induced stress.
One such process is called oxidative phosphorylation, whereby ATP is produced in `energy factories` called mitochondria. Naturally when cell energy cannot be optimally supplied, physical performance may diminish. (Journal of Physiology, 592:1887-1901, 2014)
Many people `load up` on vitamins in an attempt to prevent catching a cold or the flu. It is best to consume the recommended dietary allowances for these vitamins in order to avoid other adverse complications, by getting your daily nutrients from whole foods instead of high-dosed supplements. Healthy, younger and older adults should require no more than 100 mg vitamin C and 15 mg vitamin E per day to maintain a strong immune system.