Epidemiologic studies indicate that soy intake is inversely associated with risk of prostate cancer although most data come from case-control rather than cohort studies. In contrast to the case-control results clinical studies evaluating the effects of soy on prostate specific antigen (PSA) levels have produced very mixed results.
PSA has lost some credibility over the past several years as an indicator of prostate cancer risk but it is still used experimentally to evaluate potential chemopreventive agents. The latest study to do just that enrolled 79 untreated prostate cancer patients. Men were randomized to receive for three weeks either 1) tomato products containing 30 mg lycopene per day; 2) tomato products plus selenium, omega-3 fatty acids, soy isoflavones, grape/pomegranate juice, and green/black tea (tomato-plus); or 3) control diet.
The main analysis, which included patients in all risk categories, did not reveal differences in changes of PSA-values between the intervention and control groups. However, post-hoc, exploratory analyses within intermediate risk (n = 41) patients based on tumor classification and Gleason score post-surgery, revealed that median PSA decreased significantly in the tomato group as compared to controls (-2.9% and +6.5% respectively, p = 0.016). In separate post-hoc analyses, the median PSA-values decreased by 1% in patients with the highest increases in plasma lycopene, selenium and eicosapentaenoic acid compared to an 8.5% increase in the patients with the lowest increase in lycopene, selenium and eicosapentaenoic. There was no indication that isoflavones affected PSA levels.
Paur I, Lilleby W, Bohn SK, Hulander E, Klein W, Vlatkovic L, Axcrona K, Bolstad N, Bjoro T, Laake P, Tasken KA, Svindland A, Eri LM, Brennhovd B, Carlsen MH, Fossa SD, Smeland SS, Karlsen AS, and Blomhoff R. Tomato-based randomized controlled trial in prostate cancer patients: Effect on PSA. Clin Nutr 2016.