There are widely varying opinions towards both sides of the subject that you can find on a search here at HWPC. There are also plenty of studies on both sides of the issue, often showing conflicting results. So we don’t really know from a proof perspective, at least not in regard to most vitamins/herbals etc..
The large scale randomized control trials (RCT) which would provide enough proof to satisfy all seem to be lacking, and likely will remain so. For one thing, there is little money to be made. I'm not saying plenty of money is not made by those who sell us various vitamin/herbal/nutritional concoctions. But if one of them could prove conclusively tomorrow, with very expensive FDA drug quality trials, that vitamin ABC cured cancer, how are they then going increase their earnings with these non-patentable substances? It might increase demand for Vit ABC, but every other manufacturer could still produce that product, so altruism or scientific curiosity will be the only incentive to spend that kind of money. The all powerful financial incentive to really look into this is probably somewhat lacking. In fact, if anything, there may be a financial incentive to try to prove them worthless or better yet harmful.
I am waiting for the results from a “randomized, controlled clinical trial,” out of U. of SC MC where men were given vitamin D 4000 iu/day for 60 days prior to their RPs, and compared to a group who were given Placebo, then the prostates examined as usual post RP and compared with the pre-op Bx results. (This study was by a group which had previously studied VitD/4000iu/day for 1 year in men who were on AS, then compared at the follow up Bx, with good results. That 1st study is here:
www.ncbi.nlm.nih.gov/pmc/articles/PMC3387395/The preliminary report on the study I am waiting on (where the men were treated 60 days prior to RP) is here:
www.acs.org/content/acs/en/pressroom/newsreleases/2015/march/vitamin-d-may-keep-low-grade-prostate-cancer-from-becoming-aggressive.htmlThe preliminary results have been made public and are encouraging, but we won’t know if it is actually helpful or maybe even harmful until the final report is released. (EDIT: and even then, it is still a small study so more will be needed)In the meantime there are plenty of studies, both pro and con, on various vitamins and such. You can find many searching here at HWPC, and of course at Google or at Pubmed. Happy hunting!
(EDIT: I am biased in that I HOPE the results are very favorable. But why would we not all hope that something that simple, cheap and apparently with low to no SEs could be proven to be helpful?)