Soy isoflavones are a group of compounds called phytoestrogens, or plant-derived chemicals that acts like estrogen in the body. Phytoestrogens are believed to be the component of soy that causes both the benefits and drawbacks, although research is till ongoing. The fact that these chemicals mimic estrogen may make some men nervous about them, but soy has been a part of millions of men's diets for many years with no associated adverse effects. The potential problems arise when concentrated isoflavones are taken in pill form or in a concentrated soy protein isolate.
Prostate Cancer
Prostate cancer prevention is a major reason for soy isoflavone supplementation in men. The phytoestrogens are believed to reduce the risk of developing prostate cancer, but according to the Linus Pauling Institute at Oregon State University, the jury is still out. Studies haven't shown soy isoflavones to decrease the levels of prostate-specific antigens in men without cancer, but several smaller studies have shown it to slow PSA increases in men with prostate tumors. Further research is needed to confirm these findings. The results of a clinical trial for a soy isoflavone-based prostate cancer treatment reported in a 2004 issue of "Nutrition and Cancer" that men experienced minor side effects like hot flashes and breast changes while taking very high doses.
Fertility
A 2008 study in the journal "Human Reproduction" found that consuming large quantities of soy isoflavones may impair fertility. The study assessed the sperm quality and quantity of 99 men whose wives were having difficulty conceiving. After controlling for other fertility-related factors like age and smoking, researchers found that men who ate high amounts of soy had 41 million fewer sperm per milliliter of semen than men who did not eat soy. The difference was even greater in men who were overweight or obese. Analysis revealed the sperm quality to be unaffected, but sperm quantity definitely was. If you plan to conceive a child, consult your doctor before taking soy isoflavones.
Other Health Effects
Soy can lower total cholesterol and LDL cholesterol and the isoflavones, specifically genistein and daidzein, may be the compounds responsible for this effect. It is still unclear whether soy isoflavone supplements and soy foods demonstrate the same effects -- eating soy foods does not appear to affect mortality. Still, its cholesterol-lowering effects may help prevent cardiovascular problems. Soy has been linked to reduced diabetes risk and lowering blood pressure, but no conclusive evidence is available as of July 2011.
Safety
Soy has been a part of the human diet for millenia, but soy isoflavone supplements are newer, more concentrated forms of soy whose long-term safety record has not yet been established. Short-term effects may include nausea, constipation and bloating, and migraine headaches have been specifically linked to soy isoflavone supplements. Soy may react with some medications, including blood thinners, antitumor medication and thyroid medication, so consult your doctor before using a soy isoflavone supplement.
References
- Linus Pauling Institute at Oregon State University; Soy Isoflavones; Jane Higdon; 2006
- MayoClinic.com: Soy (Glycine Max)
- "Human Reproduction"; Soy Food and Isoflavone Intake in Relation to Semen QUality Parameters Among Men From an Infertility Clinic; Jorge Chavarro, et al.; November 2008
- "Nutrition and Cancer"; Clinical Characteristics and Pharmacokinetics of Purified Soy Isoflavones: Multiple-Dose Administration to Men With Prostate Neoplasia; L. Fischer, et al.; 2004
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