From the NY Times:
In a study published in The New England Journal of Medicine in 1994, 29,000 Finnish men, all smokers, had been given daily vitamin E, beta carotene, both or a placebo. The study found that those who had taken beta carotene for five to eight years were more likely to die from lung cancer or heart disease.That's from a NY Times article a friend sent me and asked for my opinion about. First, you should probably weight the findings of a peer-reviewed study much more heavily than the opinion of a fourth year medical student; read the article and follow up the individual studies if you're curious. It's also worth pointing out this nice summary is written by Paul Offitt, the pediatrician who co-invented the rotavirus vaccine and wrote "Autism's False Prophets" (for his efforts he receives frequent death threats from anti-vaxxers).
Two years later the same journal published another study on vitamin supplements. In it, 18,000 people who were at an increased risk of lung cancer because of asbestos exposure or smoking received a combination of vitamin A and beta carotene, or a placebo. Investigators stopped the study when they found that the risk of death from lung cancer for those who took the vitamins was 46 percent higher.
Then, in 2004, a review of 14 randomized trials for the Cochrane Database found that the supplemental vitamins A, C, E and beta carotene, and a mineral, selenium, taken to prevent intestinal cancers, actually increased mortality.
All that said, the findings are not so surprising. Why is that people think an animal which existed where nutrients, predation, and infectious disease were always limiting, is then freed from those constraints, and is suddenly immortal and perfect? If a car is designed to last for 100,000 miles, and you take really good care of it (more than the engineers expected) you might get 200,000 miles before a major breakdown, but you won't get a million. Less metaphorically: megadoses of fat-soluble vitamins are already known to be bad, so consuming anything in dramatically different quantities than nature has accounted for is at best, not going to do anything. In any event I'm betting the effect sizes are still pretty small. It's much more likely air pollution or a bad gene will get you than too many supplements. But more supplements won't make you live forever. (If you're interested in transhumanist/immortality issues, here's a post mostly about cryonics on my science fiction/fact blog.)
For anyone who has any protective feelings for the supplement industry and harbors any suspicions that the big bad doctors and pharmaceutical companies are attacking a poor little defenseless business - the supplement industry is a $32 BILLION dollar industry, complete with lobbyists. They're maximizing profits, period, like everyone else.
To expand on this: to my transhumanist comrades in the rationalist movement, I say your desire for long life and/or immortality is a worthy goal! But there's no hack for it, any more than using a certain kind of motor oil will make your Ford get to a billion miles. Probably not coincidentally, I find it's often programmers who don't understand the limitations of non-symbolic moving parts, unlike those in software, but very like those in cars or organs. To that end the impulse to become code via the still complete-science-fiction of uploading is understandable, though it makes some pretty big assumptions about the nature of consciousness that I don't think people are ready to literally bet their lives on.
If anyone is aware of backlash against these studies and this article I would be very interested to read it; please post in comments.