There are a couple of mega-trials in this week's JAMA. Both looked at whether selenium or vitamin E could prevent cancer. It all started with a trial of whether selenium prevents skin cancer - the answer was no, but it was noticed that prostate cancer was reduced (by a massive 65%) in the intervention arm. Prostate cancer was also down 35% in another trial - of vitamin E supplements. Hence the current trials.
The actual trials reports are solid and worthy but the covering editorial is a good place to start. Various points of interest - these were very big trials so (a) statistically they put the issue beyond doubt and (b) no-one will ever want to fund a 'selenium and cancer' trial again. The editorial points out that the subjects were 'health well-motivated volunteers who had good access to care' - which may have reduced the scope for selenium to make any difference in this group with a startlingly low incidence, as it turned out, of prostate cancer in both the intervention and the control groups.
But it all reminds us to be wary of observation only evidence and of secondary analyses in trials designed for other purposes.