Soy or Soya is high in antioxidants. It has a group of flavanoids called isoflavones. Genistein is the main phytoestrogen present in soya beans. It can be considered both a phytoestrogen and an antioxidant.
According to a study conducted in 2006 genistein has been implicated as a factor causing infertility in mice. But the effect on human egg cells is unclear. If you have been diagnosed to have an estrogen related cancer it is better that you avoid genistein.
Alternatively studies conducted by Finnish researchers have shown that the Japanese have lower rates of breast, ovarian and prostate cancers due to their Genistein content.
Perhaps the best conclusions we can reach is that an approximate intake of 15 grams of Soy Protein and 50 mg of Isoflavones per day may be considered as safe and even have potential health benefits. This can roughly correlate to about two servings of the traditional soya foods.
However the effects of soya foods on those on the drug Tamoxifen are unclear.