Soy contains isoflavone, which is a type of Photoestregen. Here, I got this from Wikipedia, very interesting, I think basically in animals it has shown a negative reaction, and there hasn't been enough study done yet to see what affects it has on humans.
Soybeans contain isoflavones called genistein and daidzein, which are one source of phytoestrogens in the human diet. Because most naturally occurring estrogenic substances show weak activity, normal consumption of foods that contain these phytoestrogens should not provide sufficient amounts to elicit a physiological response in humans.[citation needed]
Plant lignans associated with high fiber foods such as cereal brans and beans are the principal precursor to mammalian lignans which have an ability to bind to human estrogen sites. Soybeans are a significant source of mammalian lignan precursor secoisolariciresinol containing 13–273 µg/100 g dry weight.[45] Another phytoestrogen in the human diet with estrogen activity is coumestans, which are found in beans, split-peas, with the best sources being alfalfa, clover, and soybean sprouts. Coumestrol, an isoflavone coumarin derivative is the only coumestan in foods.[46][47]
Soybeans and processed soy foods do not contain the highest "total phytoestrogen" content of foods. A study in which data were presented on an as-is (wet) basis per 100 g and per serving found that food groups from highest to lowest levels of total phytoestrogens per 100 g are nuts and oilseeds, soy products, cereals and breads, legumes, meat products, various processed foods that may contain soy, vegetables, and fruits.[48]
[edit] Women
A 2001 literature review suggested that women with current or past breast cancer should be aware of the risks of potential tumor growth when taking soy products, based on the effect of phytoestrogens to promote breast cancer cell growth in animals.[49]
A 2006 commentary reviewed the relationship with soy and breast cancer. They stated that soy may prevent breast cancer, but cautioned that the impact of isoflavones on breast tissue needs to be evaluated at the cellular level in women at high risk for breast cancer.[50]
[edit] Men
Because of the phytoestrogen content, some studies, but not all, have suggested that there is an inverse correlation between soybean ingestion and testosterone in men.[51] For this reason, they may protect against the development of prostate cancer.[52] A theoretical decrease in the risk of prostate cancer should, however, be weighed against the possible side-effects of decreased testosterone, which are still unclear. The popular fear that soybeans might cause reduced libido and even feminine characteristics in men has not been indicated by any study; the popularity of the notion seems to be based on the simplistic misapprehension that estrogen and testosterone have a simple, inverse relationship in sexual hormone systems and sex-related behaviour. Their interplay is very complicated and largely still unknown.[53]
Studies published in July 2008 show that Soy products and, more specifically, the phytoestrogen they contain might lower a man's sperm count.[54]
[edit] Infant formula
There are some studies that state that phytoestrogen in soy can lead to alterations in the proliferation and migration of intestinal cells. The effects of these alterations are unknown.[55] However, some studies conclude there are no adverse effects in human growth, development, or reproduction as a result of the consumption of soy-based infant formula.[56] Other reviews agree, but state that more research is needed to answer the question of what effect phytoestrogens have on infants.[57][58] Soy formula has also been linked to autoimmune disorders of the thyroid gland.[59]
