Soy is a superstar in your arsenal against breast cancer, and has been a food stable in the Asian diet for thousands of years. Asia has far lower rates of cancer than the United States does, and researchers think that eating a lot of soy may be one of the reasons. Japanese men and women eat about ten times more soy than American men and women. According to many studies, if you eat an adequate amount of soy often enough, your risk of breast cancer will drop by 30–50 percent. For instance, a study published in the Journal of the National Cancer Institute in June 2003 found that women who ate three bowls of miso soup a day had a 40% lower risk of breast cancer. . Those who ate two bowls of miso soup a day cut their risk by 26 percent.   

Genistein
There are several substances in soy that are active against breast cancer. A particular type of phytochemical (a naturally occurring plant chemical) called genistein (pronounced gen-is'-teen) appears to be one of the most important protagonists. Genistein is classified as a phytoestrogen, or plant estrogen, because it has a weak estrogenic effect. There is currently a lot of confusion and misinformation about phytoestrogens. It is important to understand that plant estrogens are not the same as the estrogens our body makes or synthetic estrogens found in HRT or BCP. They are very different. Most act more like selective estrogen modulators or SERMS (Tamoxifen is a SERM) and as aromatase (an enzyme used in the production of estrogen) inhibitors like the new anti-cancer drug, Arimidex. In other words, phytoestrogens act more like estrogen blockers rather than estrogen which I explain in more detail below. These types of plant chemicals act in so many complex ways that we may never fully understand them all.  Two other major phytoestrogens in soy are daidzein and glycitein. Genistein is the most abundant and well-researched of the three and is usually the only one that is listed on the label of soy products. Research shows that genistein is extraordinarily effective at reducing the risk of breast cancer. It has been shown to stop tumor growth, prevent metastasis, and shut off new blood vessels in growing tumors.  

One reason why genistein is able to prevent and fight breast cancer is because it blocks the cancer-promoting estrogens from attaching to the estrogen receptors on breast cells. Breast cancer is a hormonal disease—which means a hormone causes the cancer to develop by inciting cells to grow and divide. For breast cancer, that hormone is estrogen. The more estrogen you are exposed to, the higher your risk of getting breast cancer is.  

Estrogens come in different strengths and behave differently. Strong estrogens increase your risk of cancer because they tell cells to grow and divide rapidly. Phytoestrogens and other weak estrogens decrease your risk of cancer because they slow down cell division. Genistein acts like a weak estrogen in the body. It blocks the effects of strong estrogens and slows down cell division. Genistein is very weak—in fact, less than one one-hundredth of the strength of estradiol (the most potent type of natural estrogen). So, if genistein attaches to an estrogen receptor, the rate of cell division is only one one-hundredth of the speed that it is if estradiol attaches to the receptor. The more genistein there is to compete with estradiol, the slower the rate of cell division is and the lower your risk of breast cancer is.