There have been a number of new research reports about potential adverse effects of supplements, the most recent one being an apparent 17% increase in prostate cancer in men taking 400 IU or more of Vitamin E a day. Here's the link: Vitamin E This study is at odds with 4 other studies in the last ten years which have either shown a preventive benefit of Vitamin E in prostate cancer, or no difference one way or the other. Probably more than anything else, these studies illustrate how difficult it is to really know anything conclusively through epidemiologic research. Ultimately, it is the bulk of the evidence over time that is the most convincing evidence.
In the meantime, this study looks pretty substantial, at least at first read. It involved 35,000 men and the research design looks valid, so I'd probably limit Vitamin E to 200 IU a day for now if you are a man.
The fallacy that if a little is good, then more must be better is one that is prevalent in some people's approach to nutrition, but it may not be valid. At the same time, studies like this shouldn't lead us to think that supplementation is not a good idea - I think we just need to get more personalized and accurate with our supplementation.
That's why I like to use laboratory testing and a careful nutritional history to determine what supplements and doses are optimal for my patients.