Soy contains a group of phytochemicals called isoflavones. Genistin, daidzin and glycitin are the best characterised and most thoroughly researched of these isoflavones. These isoflavones are bonded to sugar molecules in plants, forming glycosides that are named genistein, daidzein and glycitein. Most legumes contain isoflavones, but soy is particularly rich in them, with genistein being the most abundant isoflavone in soy. Animal and human experiments show that soy isoflavones have oestrogenic activity in animals and this may influence the hormonal milieu. Increasing awareness of soy in the West, based largely on clever marketing and the funding of pro-soy academic research, has caused an increase in consumption of soybeans and soybean products. Despite claims that soy consumption is safe, experimental evidence shows that the consumption of phytoestrogens such as genistein can have significant effects on reproductive development and may influence cancer initiation, progression and propagation.
Researchers have assessed the strength with which phytoestrogens can bind to the oestrogen receptor in mammals and in this regard designate oestradiol a receptor binding affinity of 100. The receptor binding affinity for genistein has been estimated to be around 0.45 based on cell culture studies. Equol, a metabolite of the isoflavone daidzein formed in the colon through the action of bacteria, has a binding affinity around 33 % of that of genistein. Administration of equol to rats during pregnancy has been shown to cause developmental abnormalities. Genistein has also been shown to cause uterine cancers in adult rats that are treated when pups. Administration of 30 grams of soy flour per day to healthy human women has been shown to cause hormonal changes including a lengthening of the menstrual cycle. Infant humans on soy formula are exposed to isoflavone concentrations 5-fold higher than these women. It is estimated that 20 % of infants in America consume soy formula milk.
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