Because of these uncertainties, it is not surprising that regular multivitamin use was found not to be associated with the risk of early or localized prostate cancer. However that taking high levels of supplements increases prostate cancer risk does not necessarily follow from their study.
An alternative explanation of their results is that men who are aware of being at higher than average risk of prostate cancer are likely to consume larger amounts of vitamins and trace elements in an effort to reduce their risk of developing the disease.
The authors of the article actually state this much themselves:
(Quote) "The increased risk of advanced prostate cancer and prostate cancer mortality with heavy use of multivitamins among men with a positive family history of prostate cancer could be due to men with a positive family history taking additional, unspecified supplements as part of a 'prostate health' package to prevent the future development of prostate cancer...
Thus, confounding associations between individual agents that we were unable to assess and the risk of prostate cancer among men with a positive family history in our study were possible." (End quote)
Millions of Americans have been taking multivitamin- and mineral supplements as a general health maintenance measure for decades, and more recently, to reduced the cancer risk. If this had any negative effects it should have resulted in an increase of all cancer, but this did not occur.
The prostate cancer deaths, in fact, declined. by 17 % since 1994! This decline has been attributed to better diagnostic procedures, early detection and improved therapy. However, it could also be because more American men are taking supplemental selenium, an element which has been shown in a placebo-controlled study to reduce prostate cancer incidence by 63%.
One conclusion reached by the authors, however, may be justified; It concerns zinc.
They report that men taking a zinc supplement with multivitamin use at more than seven versus seven or fewer times per week exhibited an increased risk of fatal prostate cancer (RR = 4.36, 95% CI = 1.83 to 10.39), whereas no association with multivitamins was observed for men not taking a zinc supplement (RR = 1.13, 95%CI = 0.46 to 2.80; P value for test of interaction = .042).
This effect may be real because zinc is needed for tumor growth and acts as a selenium antagonist. But this is not a new observation -it was demonstrated by Schrauzer and coworkers 30 years ago.