Phytoestrogens and risk of prostate cancer: a meta-analysis of observational studies

BACKGROUND Epidemiologic studies have reported various results relating phytoestrogens to prostate cancer (PCa). The aim of this study was to provide a comprehensive meta-analysis on the extent of the possible association between phytoestrogens (including consumption and serum concentration) and the risk of PCa. METHODS Eligible studies were retrieved via both computer searches and review of references. The summary relative risk ratio (RR) or odds ratio (OR) and 95% confidence interval (CI) were calculated with random effects models. RESULTS A total of 11 studies (2 cohort and 9 case-control studies) on phytoestrogen intake and 8 studies on serum concentration were included in the meta-analysis. The pooled odds ratio (OR) showed a significant influence of the highest phytoestrogens consumption (OR 0.80, 95% CI 0.70-0.91) and serum concentration (OR 0.83, 95% CI 0.70-0.99) on the risk of PCa. In stratified analysis, high genistein and daidzein intake and increased serum concentration of enterolactone were associated with a significant reduced risk of PCa. However, no significant associations were observed for isoflavone intake, lignans intake, or serum concentrations of genistein, daidzein, or equol. CONCLUSIONS The overall current literature suggests that phytoestrogen intake is associated with a decreased risk of PCa, especially genistein and daidzein intake. Increased serum concentration of enterolactone was also associated with a significant reduced risk of PCa. Further efforts should be made to clarify the underlying biological mechanisms.