Inside the bright-green meat of the soybean lie two powerful phytoestrogens, the isoflavones genistein and daidzein. Because they are similar in structure to naturally occurring estrogen, these compounds can affect estrogen levels in a woman's body in two ways. For example, if you're suffering from excess estrogen, soy food phytoestrogens may block certain negative estrogenic effects including fibroid growth. However, in some cases, isoflavones have also been shown to bind to vacant estrogen receptors and exert their own estrogenic effect. For example, in a study of genistein's effect on breast cancer cells, researchers found that, at low doses, genistein is capable of slightly stimulating human breast cancer cells to enter the cell cycle (Environmental Health Perspectives, 1997, vol. 105). In other words, it appears that phytoestrogens might be capable of encouraging already-existing hormone-dependent cancers. Therefore, before treating any hormonal imbalance through diet, you should check with a knowledgeable health practitioner. Also, by paying attention to whether hormonal symptoms such as moodiness, cramping or bloating increase or decrease after adding soy to your diet, you'll have a better idea whether soy's isoflavones are exerting a beneficial or detrimental effect on your body.