Will a new study end vitamin E as a dietary supplement for men?
The study in the Journal of the American Medical Association reports that “dietary supplementation with vitamin E significantly increased the risk of prostate cancer among healthy men.”
The study would seem to have some merit as it is clinical trial-ish, involved 35,533 men who were followed for 7-12 years and produced statistically significant results. The researchers estimate that vitamin E supplementation increases prostate cancer risk by 1.6 extra cancers per 1000 person-years.
So what’s the problem?
Though statistically significant, the results were weak (i.e., a relative risk of 1.17) and only marginally (if not suspiciously) statistically significant (i.e., the lower bound of the 95% confidence interval was 1.004).
While clinical trials can conceivably get away with weaker statistics — i.e., they tend to be more highly-controlled than, say, case-control epidemiology — this one fails the mark for two reasons.
Although the study subjects were provided with vitamin supplements, no one knows how well study subjects adhered to the vitamin regiment. So intake of vitamin E is somewhat uncertain, a problem for weak statistical results.
Next and perhaps more importantly, it’s not at all clear that data were collected on potential confounding risk factors for prostate cancer. The study, if its omission of potential confounders from mention is any indication, dubiously pretends that vitamin E supplementation is the only risk factor for prostate cancer. [Note: The lead study author assures me that family history and other confounders were considered but we await confirmation of this.]
Finally, the researchers acknowledge that they have no idea how vitamin E supplementation could possibly alter prostate cancer risk:
A biological explanation for the observed increased risk of prostate cancer in the vitamin E arm [of the trial] is not apparent from these data.
So while it’s not clear that vitamin E supplementation does any good, this study does not constitute evidence that it causes any harm.