Soy-based foods like tofu and soy milk contain chemicals known as isoflavones, a type of phytoestrogen. They are compounds only present in plants, and they perform functions in the human body similar to the hormone estrogen. Some men may avoid soy foods out of worry that too many phytoestrogens can have a negative effect on the hormone balance of the male body. As of August 2011, current scientific research indicates that phytoestrogens do not have a negative impact on male hormone balance.
There are two forms of isoflavones in plant foods: those bonded with the sugar glycone and those without glycone. Isoflavones without glycone may have the greatest estrogenic effects, according to New York University. Neither form of isoflavones appears to raise estrogen levels above normal in men because they are prone to bond with the estrogen receptor sites in cells when estrogen levels are low, and they block estrogen cell receptors when there is already an adequate amount of estrogen in your body.
Consuming high amounts of soy isoflavones in your diet may decrease your chances of developing prostate cancer. A 2006 review in the "Journal of AOAC International" and a 2009 meta-analysis appearing in the "American Journal of Clinical Nutrition" both found that increased soy consumption is associated with a decreased risk of prostate cancer. As of this publication, randomized controlled trials of a much larger size are ongoing to confirm the results of the small trials mentioned in the reviews.
One fear men have with regards to consuming soy-based phytoestrogen is that it could affect testosterone levels and make it difficult to achieve an erection. Columbia University states that the regulation of estrogen levels performed by soy isoflavones likely prevents this from happening. However, a 2011 case study appearing in "Nutrition" outlines the treatment of a 19-year-old vegan male who had low levels of testosterone and a loss of libido. The subject discontinued his vegan diet and experienced an initial drop in testosterone levels, but his hormones and libido normalized after one year of remaining off of the vegan diet.
Effect on Infants
The use of soy formulas for children increases the amount of phytoestrogens males consume significantly. A 2004 study in the "Journal of Pediatric Endocrinology & Metabolism" found that even if an infant receives soy formula for more than six months, it will not affect his growth and development negatively. In 2001, the "Journal of the American Medical Association" published a retrospective study of 811 male and female adults who consumed soy formula as infants. The adult men in the study were fully developed in terms of height and weight, and experienced puberty at the same age as children who consumed cow's milk formulas.