Two recent reports in prominant medical journals tell us that high doses of antioxidant supplements including Beta-carotene, vitamin A, vitamin E, and selenium, used separately or in combination are not helpful for chronic medical problems, and indeed may cause harm.
General Studies: The first comes from the American College of Physicians [Bjelakovic G 2007]. The authors reviewed and combined the results of 68 quality clinical trials that had studied the effect of antioxidant vitamins. Results on 232,606 people from different patient groups that included healthy adults and variety of chronic disorders (neurologic, cardiovacular, endocrine, gastrointestinal, rheumatoid, etc.) were analyzed. What did they find? People who took supplements including Beta-carotine, vitamin A, or vitamin E alone or in combination increased their risk of earlier death than those who did not. Further, the larger the dose of any of these vitamins, the greater the risk. The use of vitamin C or selenium showed neither positive or negative effect in this review, but another recent quality clinical trial suggests that people who take selenium supplements have a greater risk of developing diabetes [Stranges S 2007].
Neurologic Studies: The majority of neurologic trials have focused on vitamin E. Though there are further clinical trials in progress for Alzheimers, the authoritative 2007 Cochrane review finds no evidence of vitamin E benefit for any stage of Alzheimer's disease. Likewise, studies in Parkinson's have shown no vitamin E benefit.
Comments: The best evidence suggests that not only is there no benefit derived from large doses of vitamins contained in many supplements, there may be harm. My recommendation is to stay with doses near to daily requirements which five servings of vegetables and fruits will supply.
If you take a multivitamin, check the label to make sure it does not greatly exceed the daily requirement.
Bjelakovic G, Gluud C. Surviving antioxidant supplements. J Natl Cancer Inst. 2007 May 16;99(10):742-3. PubMed abstract
Stranges S, Marshall JR, Natarajan R, Donahue RP, Trevisan M, Combs GF, Cappuccio FP, Ceriello A, Reid ME. Effects of long-term selenium supplementation on the incidence of type 2 diabetes: a randomized trial. Ann Intern Med. 2007 Aug 21;147(4):217-23. Epub 2007 Jul 9. PubMed abstract