Fifty-eight men at high risk of prostate cancer or with low-grade prostate cancer were randomly assigned to consume 1 of 3 protein isolates containing 40 g protein: 1) soy protein (SPI+, 107 mg isoflavones/d); 2) alcohol-washed soy protein (SPI-, <6 mg isoflavones/d); or 3) milk protein (MPI). Proliferating cell nuclear antigen (PCNA), epidermal growth factor receptor, B-cell non-Hodgkin lymphoma-2 (Bcl-2), and Bcl-2-associated X protein (Bax) were assessed in baseline and ending prostate biopsy cores. Serum collected at 0, 3, and 6 mo was analyzed for total and free prostate specific antigen (PSA). Consumption of SPI+ did not alter any of the prostate cancer tumor markers. Bax expression decreased from baseline in the SPI- group, resulting in lower Bax expression than the MPI group. PCNA expression also decreased from baseline in the SPI- group, but this was not different from the other 2 groups. PSA did not differ among the groups at 3 or 6 mo. Interestingly, a lower rate of prostate cancer developed in the soy groups compared to the milk group (P = 0.01). These data suggest that 6-mo SPI+ consumption does not alter prostate tissue biomarkers, SPI- consumption exerts mixed effects, and less prostate cancer is detected after 6 mo of soy consumption regardless of isoflavone content.
Bibliographical noteFunding Information:
Immunohistochemistry was performed by Kenji Takamura. The authors thank Kayla Vettling and Ellie Wiener for tissue scoring and Lori Sorensen, Nicole Nelson, and Mary McMullen for assistance with clinic visits and data entry. We are also quite grateful to the study participants for their time and dedication. The Soy and Prostate Cancer Prevention (SoyCaP) trial was supported by Grant DAMD 17–02–1–0101 (M. S. Kurzer) and W81XWH–06–1–0075 (J. M. Hamilton-Reeves) from the United States Army Department of Defense Prostate Cancer Research Program. The protein isolates were donated by The Solae Company, St. Louis, MO. Neither sponsor was involved in writing this report.