This caught my eye in the paper this weekend: someone wrote in to the doctor's column asking if they should take vitamin A and E supplements to prevent cancer and heart disease, and the doctor's response was:
Several long-term and large trials have shown that taking extra vitamins A (such as betacarotene) and E does not reduce heart attack risk. In fact, some of the trials were stopped because there were more deaths in the vitamin groups than in those given placebos. As long ago as 14 June 2003 the Lancet reviewed the evidence and strongly discouraged any more research into the long-term use of such vitamin supplements. We get enough for our needs from a normal diet.
Blimey! I already knew that vitamin supplements were pointless (for healthy people) as long as you eat right. But do they actually do harm?
The doctor was referring to this 2004 review in the Lancet, which is a pretty good source. A web search also finds a 2008 Cochrane review of the evidence (another good source, but it's essentially an update of the earlier paper), which concludes:
We found no evidence to support antioxidant supplements for primary or secondary prevention. Vitamin A, beta-carotene, and vitamin E may increase mortality. Future randomised trials could evaluate the potential effects of vitamin C and selenium for primary and secondary prevention. Such trials should be closely monitored for potential harmful effects. Antioxidant supplements need to be considered medicinal products and should undergo sufficient evaluation before marketing.
This is pretty scary. According to these authors, there's no evidence that these supplements prevent cancer but there are hints that they might increase mortality? Such meta-analyses, when done properly, are very good ways to summarise the current state of research, but they're not set in stone - for example, when that review was published in the Lancet, the next issue featured some responses from some of the studies involved, who took issue with the general conclusion. But then, if the possibility of a negative effect looms strongly enough out of a systematic review like this, then it certainly needs to be considered.
Even this year more evidence arrives: this 2009 study finds that supplements of vitamins C or E or beta-carotene have no statistically significant effect on mortality (they don't increase or decrease the risk of death).
A couple of things to note: