Abstract A83: Effects on serum testosterone and indicators of apoptosis and the IGF axis of soy protein isolate consumption in a randomized controlled trial with men after radical prostatectomy
Authors: Huang Jonathan , Schlicht Michael J. , Xie Hui , Gao Weihua , Zeleniuch-Jacquotte Anne , Baumann Nikola , Enk Erika E. , Schmoll Joanne , Bosland Maarten C. 
Published: May 2012

Soy consumption may prevent prostate cancer and be beneficial for men with prostate cancer as suggested by epidemiologic and experimental data. However, the mechanisms by which soy might be protective are not clear, but effects on apoptosis, the insulin growth factor [IGF] axis, and steroid hormones may be involved. We have examined these factors in a randomized controlled clinical trial in which we are testing the hypothesis that soy reduces biochemical recurrence after radical prostatectomy. A standardized soy protein supplement and a casein-based placebo are consumed daily by men at elevated risk for PSA failure during the first 2 years after surgery. The soy protein isolate and placebo (Solae) were made identical in composition, except for the source of protein (20 g protein/day). The soy protein isolate provides 24 to 26 mg genistein and 40 to 43 mg of total isoflavones per day. The placebo is devoid of these and other soy-specific constituents. Self-reported compliance is excellent, serum isoflavones levels are measured as an independent compliance measure, dropout rate is low, and adverse effects minimal to non-existent. In serum samples taken at baseline, and at 2, 4, 8, 12, and 18 months from subsets of subjects we determined indicators of global apoptotic activity (soluble Fas [sFas] and Fas ligand [FasL]) and the IGF axis (IGF-1 and IGF binding protein 3 [IGFBP3]) (7–8 subjects/treatment group), and measures of androgen status, testosterone [T] and sex hormone binding globulin [SHBG] (12 subjects/group) using ELISA and enzyme immuno-assays. Data were analyzed using Generalized Estimating Equations. FasL increased slightly over time (p=0.05), but sFas did not and there were no differences in change over time for sFAs and FasL between the soy and placebo groups. Similarly, IGF-1 increased slightly over time (p=0.06), but IGFBP3 did not and there were no differences in change over time for IGF-1 and IGFBP3 between the soy and placebo groups. In contrast, T levels decreased significantly (p=0.05) from baseline over time by approx. 15% in the soy group compared to placebos, in which T levels did not change over time. SHBG levels were not changed over time or different between the two treatment groups. While data in the literature about soy effects on T and SHBG are mixed, this is the first report of soy effects on circulating T and SHBG with repeated serum sampling over as long as 1.5 years. It is also the first report about soy effects on serum sFas and FasL, while the absence of effects on the IGF axis is in line with other studies that have examined this. Thus, consuming 20 g of soy protein isolate per day for 1.5 years slightly reduced circulating T over time in the absence of changes in SHBG, but did not affect indicators of apoptosis and the IGF axis. (Supported by NIH grants CA27790 & CA166195 and a grant from the Prevent Cancer Foundation.)

Citation Information: Cancer Prev Res 2010;3(1 Suppl):A83.