Plain language summary
Studies focusing on the effect of soy on risk for breast cancer are extensive, however there is very little research assessing its affects in men with prostate cancer. This post-hoc analysis of individuals enrolled in a randomised control trial looking at individuals on a soy protein isolate or milk protein placebo aimed to determine if soy had any effect on prostate cancer. The results showed that both circulating testosterone and its carrier molecule, sex hormone binding globulin (SHBG), were both decreased in individuals consuming the soy protein. All other hormones and measures related to cancer cell death and growth remained unaffected. It was concluded that 18 months of consumption of soy protein isolate reduced circulating testosterone and SHBG but had little effect on other measures related to cancer development. This study could be used by healthcare professionals that a diet high in soy may have limited effect on prostate cancer.
Abstract
Many studies have addressed the effects of dietary supplementation with soy protein on cancer risk and mortality, but there are only few randomized studies with soy in males. We used serum samples from a two-year trial of soy protein isolate supplementation in middle-aged to older males at risk of recurrence of prostate cancer after radical prostatectomy to determine soy effects on steroid hormones involved in prostate cancer (testosterone, SHBG, and estradiol) and explore the effects on biomarkers of the growth hormone/IGF-1 axis, apoptosis, and angiogenesis. Compared with a casein-based placebo, 18 mo, of consumption of 19.2 g/day of whole soy protein isolate containing 24 mg genistein-reduced circulating testosterone and SHBG, but not free testosterone, and did not affect serum concentrations of estradiol, VEGF, IGF-1, IGFBP-3, IGF-1/IGFBP-3 ratio, soluble Fas, Fas-ligand, and sFas/Fas-ligand ratio. Thus, soy protein supplementation for 18 mo, affected the androgen axis, but the effects on other cancer biomarkers remain to be more definitively determined. The study was registered at clinicaltrials.gov (NCT00765479).
Methodological quality
Jadad score
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Allocation concealment
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