Soy and soy isoflavones in prostate cancer: a systematic review and meta-analysis of randomized controlled trials

Objective: To evaluate the evidence from randomized controlled trials (RCTs) on the efficacy and safety of soy/isoflavones in men with prostate cancer (PCa) or with a clinically identified risk of PCa.

Patients and methods: MEDLINE, EMBASE, the Allied and Complementary Medicine (AMED), the Cumulative Index to Nursing and Allied Health Literature (CINAHL) and the Cochrane Library databases were searched. We identified RCTs investigating soy/soy isoflavones as dietary supplements or dietary components for the secondary prevention or treatment of PCa in men with PCa or with a clinically identified risk of developing PCa. Studies of multi-component formulations were excluded. Six authors were contacted for further information for the meta-analyses. Methodological quality was assessed using the Cochrane Collaboration's risk-of- bias tool. The PRISMA statement for reporting systematic reviews was followed.

Results: Of the eight RCTs that met the inclusion criteria, six restricted recruitment to men diagnosed with PCa, while two included men with clinically identified risk of PCa. A large degree of heterogeneity was found with respect to dosages and preparations of soy/isoflavones administered. Most studies had small sample sizes and were of short duration. The risk of bias was assessed as low in all assessed studies except for one, for which the risk of bias was unclear. Meta-analyses of the two studies including men with identified risk of PCa found a significant reduction in PCa diagnosis after administration of soy/soy isoflavones (risk ratio = 0.49, 95% CI 0.26, 0.95). Meta-analyses indicated no significant differences between groups for prostate-specific antigen (PSA) levels or sex steroid endpoints (sex hormone-binding globulin [SHBG], testosterone, free testosterone, oestradiol and dihydrotestosterone).

Conclusions: The results of a meta-analysis of two studies suggest there may be support for epidemiological findings of a potential role for soy/soy isoflavones in PCa risk reduction; however, a clear understanding of the impact of soy/isoflavones on PSA, total testosterone, free testosterone and SHBG levels in men with, or at identified risk of, PCa could not be derived from these data, given the limitations of sample size and study duration in individual trials. A good safety profile is shown by this meta-analysis for soy/soy isoflavones supplementation.