High use of multivitamin supplements associated with increased fatal prostate cancer risk.

Analysis of data from a large US cohort suggests that regular use of high dose multivitamin supplements is associated with an increased risk of advanced and fatal prostate cancer. The authors of the analysis note that some previous epidemiological studies have linked high intake of vitamin and mineral supplements with an increase in risk of fatal prostate cancer. In this analysis they aimed to study the potential link further and clarify whether an effect on earlier prostate cancer was present.

They used data from a large prospective cohort study, the US National Institutes of Health (NIH)–AARP Diet and Health Study. Participants in this study were selected from 3.5 million residents of six US states, aged 50 to 71 years: at enrolment, they completed questionnaires on usual dietary intake, vitamin supplement use, demographic factors, and health-related behaviours. Major exclusions for the current analysis were women and participants not free from cancer at enrolment. Outcomes were relative risk of total, localised, advanced, and fatal prostate cancer according to level of multivitamin supplement use over five years follow-up, or six years for fatal cancers.

The initial cohort included 567,169 individuals, of whom 295,344 were eligible men. About a third (36%) reported consistent daily use of some type of multivitamin, and 5% were heavy users (more than seven times per week). Analysis of potential confounding factors found that multivitamin use was associated with a range of healthy lifestyle factors (e.g. less current smoking, greater frequency of exercise, healthy diet choices etc.). During the five-year non-fatal cancer follow-up period, 10,241 participants were diagnosed with incident prostate cancer, including 8,765 localized and 1,476 advanced cancer. Over the extended six-year follow-up, there were 179 fatal prostate cancers in the cohort.

There was no significant association between multivitamin use and risk of localised prostate cancer, with relative risks (RR) for all levels of use being similar to never-use (the reference). Heavy use, however, was associated with significantly increased risks of advanced (RR = 1.32, 95% CI = 1.04 to 1.67) and fatal (RR = 1.98, 95% CI = 1.07 to 3.66) prostate cancer. Subgroup analysis found that the positive associations with excessive use were strongest in men with a family history of prostate cancer or who took individual micronutrient supplements, including selenium, beta-carotene, or zinc; however numbers in some of these groups were small and thus confidence intervals wide.

The authors conclude that their analysis shows that multivitamin use does not protect against prostate cancer. It supports previous work, however, in indicating an association between heavy use of multivitamin supplements and increased risk of advanced and fatal prostate cancer. They caution that the association with heavy use may be related to confounding factors, such as an increased likelihood of screening in heavy users, or increased intake as a preventive attempt in those with a positive family history, nevertheless it is of potential concern and should be investigated further.

J Nat Cancer Inst 2007; 99: 754-64 (link to abstract);
BBC News report