OBJECTIVES: ● The objective of this systematic review was to evaluate the evidence from randomised controlled trials (RCTs) for the efficacy and safety of soy/isoflavones in men with prostate cancer or clinically-identified risk of prostate cancer. PATIENTS AND METHODS: ● MEDLINE, EMBASE, AMED (Allied and complementary medicine), CINAHL(Cumulative Index to Nursing and Allied Health Literature) and the Cochrane Library databases were searched. ● RCTs investigating soy/soy isoflavones as dietary supplements or dietary components for secondary prevention or treatment of prostate cancer in men with prostate cancer or clinically identified risk of developing prostate cancer. ● 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 has been followed. RESULTS: ● Of the eight RCTs that met the inclusion criteria, six restricted recruitment to men diagnosed with prostate cancer, while two included men with clinically-identified risk of prostate cancer. ● 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. ● Risk of bias was assessed as low in all assessed studies except for one, for which risk of bias was unclear. Meta-analyses of the two studies including men with identified risk of prostate cancer found a significant reduction in prostate cancer diagnosis following administration of soy/soy isoflavones [RR = 0.49, 95%CI 0.26, 0.95]. ● Meta-analyses indicated no statistically significant differences between groups for PSA levels or sex steroid endpoints (SHBG, testosterone, free testosterone, estradiol and DHT). CONCLUSIONS: ● The result of a meta-analysis of two studies suggests there may be support for epidemiological findings of a potential role for soy/soy isoflavones in prostate cancer 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, prostate cancer could not be derived from these data, given the limitations of sample size and study duration in individual trials. ● A good safety profile is demonstrated for soy/soy isoflavones supplementation.
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