On reading the New Scientist of a couple of weeks back I found an interesting article on dietary supplements, in particular antioxidants:
The conclusion is becoming clear; whatever is behind the health benefits of a diet rich in fruits and vegetables, you cannot reproduce it by taking purified extracts or vitamin supplements.
Although there are plausible ways that these supplements might work, in clinical trials there's no evidence that they do anything, and some disputed evidence that they may actually be detrimental to your health.
For now, the advice is simple. "Stick to flavonoid-rich foods, red wine in moderation, tea, fruits and vegetables," says [biochemist Barry] Halliwell. "Don't start taking high-dose supplements or heavily fortified foods, until we know more."
Although I should add that the article (which isn't available free online) does make it clear that supplements do work where people have a deficiency in that substance. ie. Don't stop taking your vitamin C pills if you have scurvy.