In favor of soy are the following:
- Prostate cancer is lower in Asian populations that consume higher levels of soy. [PMID: 10332923] [PMID: 12869409] [PMID: 15945102]
- Isoflavones (genistein, daidzen, equol) in soy can inhibit prostate cancer cells. [PMID: 17297558]
- One human industry-funded study recorded a reduced risk of prostate cancer due to soy (although a second analysis found inconsistencies and little support for the assertion). [PMID: 15945102] [PMID: 15916986]
- Soy is a good source of low fat protein allowing one to replace fatty red meats with a lower fat protein source. "Many soy products should be beneficial to cardiovascular and overall health because of their high content of polyunsaturated fats, fiber, vitamins, and minerals and low content of saturated fat." [PMID: 16418439]
On the other hand, soy:
- is estrogenic, goitrogenic and may promote vascular dementia, thyroid cancer and other hormone cancers [PMID: 12060828] [full article] [Letter to FDA]
- benefits in terms of reducing PSA rise are likely small in magnitude [PMID: 6682871]
- "The efficacy and safety of soy isoflavones for preventing or treating cancer of the breast, endometrium, and prostate are not established; evidence from clinical trials is meager and cautionary with regard to a possible adverse effect. For this reason, use of isoflavone supplements in food or pills is not recommended." [PMID: 16418439]
- soy may have drug interactions with 5AR2 inhibitors [internet discussion] which can also be found [here]. To understand the discussion there the reader may wish to review the model of prostate cancer discussed in [this post]
- Even if it were beneficial for preventing prostate cancer or even if it were beneficial in earlier stage disease there remains the possibility that it could be detrimental for later stage disease. Although not a formal study, clinical observations by Dr. Leibowitz, a medical oncologist, were that he was able to reverse PSA rises in some patients by simply removing soy and phytoestrogens from their diet. See Dr. Leibowitz on soy. Also see Willet Divides Prostate Cancer into Four.
The Australian Cancer Council has reviewed the evidence and concluded that soy consumption as food as part of the diet is safe and may be beneficial for prostate cancer (although if it is the effect is small) but that in light of the potential dangers that supplements should be avoided due to the possibility that high dosages could be more likely to exhibit the negative aspects. Their report is not long, is easy to read and recommended. [link]
For a good overview read the above position paper by the Australian Cancer Council plus the sections on soy in the next two links.
Peter H. Gann & Edward L. Giovannucci, Nutrition & Prostate Cancer, Prostate Cancer Foundation, 2005. [link]
Natalie Ledesma, Nutrition and Prostate Cancer, 2004. [link]
FDA position on allowing health claims for soy: [link]
The following February 2008 paper reviews the benefits and research on soy: [PMID: 17923857] [Full text].