Numerous studies discuss the positive and negative effects of soy on breast cancer. Some studies indicate that consuming soy can prevent cancer, while others argue it can exacerbate or even cause breast cancer. The plant-based, estrogen-like chemicals found in soy can either be harmful or beneficial in breast cancer.
Identification
Soy contains phytoestrogens that can block estrogen from reaching its receptors. This has the potential to protect women whose natural estrogen levels are high from developing breast cancer. However, soy also contains plant-based estrogen that chemically resembles human estrogen. Isoflavones are a class of plant estrogen that are present in high concentrations in soy. Genistein is the most active isoflavone in soy because it can activate the estrogen receptor present on the surface of some cells.
Benefits/Controversy
Some studies indicate that soy can offer a protective effect against breast cancer. Other studies indicate that the estrogen-like effects of isoflavones can be harmful for women who are postmenopausal, or who have breast cancer. "The resolution of this paradox may lie in the timing of estrogen administration," says University of Illinois food science and human nutrition professor William Helferich. If a woman is exposed to genistein before puberty, it can cause mammary gland differentiation. However, if other sources of estrogen are administered after an estrogen-responsive tumor has developed, then the growth of that tumor is stimulated.
Premenopausal Studies
Studies have shown that feeding genistein to female rats prior to puberty reduces the number of mammary tumors. Studies have also shown that Asian women who consume a diet high in soy products have a low incidence of breast cancer.
Breast Cancer
Some forms of breast cancer have high levels of the estrogen receptor on the surface of the cells. This form of breast cancer is termed ER+ since it is positive for estrogen receptors. Genistein can stimulate the growth of ER+ tumors and interferes with treatments such as tamoxifen, which targets estrogen receptors. Studies indicate genistein speeds the growth of breast cancer in laboratory mice.
Supplements
Genistein and other isoflavones sold as natural alternatives to hormone-replacement therapy are very risky, especially in women ages 50 and older. These women are at a greater risk for developing breast cancer. According to professor Helferich, although purified genistein is not as potent as hormone replacement therapy, it still poses a risk to women who are near menopause or are postmenopausal.
