The statistics are ominous when it comes to women’s risk for developing breast cancer in the United States. This risk is five-fold higher than Asian women’s risks in Japan and China. When these women migrate to the United States and adopt our eating and lifestyle habits, their risk increases to 60% higher than their counterparts who remained in Asia. Studies provide strong evidence that only 5% to 10% of breast cancer cases are attributable to inherited factors, which means up to 95% of breast cancer might be prevented with changes in lifestyle. Diet is associated with up to one in every two newly diagnosed breast cancer cases. This link between lifestyle and breast cancer has led researchers to investigate the possible benefits of soy for the prevention of breast cancer. But the link is not a clear one.
Both epidemiological and clinical data show that exposure to estrogen increases breast cancer risk by inducing malignant breast cell growth. Asian women living in Asia have a 40% lower blood estrogen levels compared to Caucasian women living in the U.S., and they consume more phytoestrogens, such as genistein, in soy products. Some studies show that adding soy to the diet reduces blood estrogen levels in premenopausal women. Genistein inhibits the blood supply to cancer cells and curbs metastasis of breast cancer. It also has a structure similar to estrogen, but its binding affinity is several-fold weaker than that of a woman’s normal estrogen, so it could curb estrogen’s effects on initiating cancer. Finally, genistein has beneficial effects on bone, the cardiovascular system, and blood cholesterol levels, thus helping prevent osteoporosis and heart disease.
The confusion arises from the limited evidence that soy also might increase breast cancer risk. Whether soy increases or decreases breast cancer risk might be dose, tissue, and time dependent. Estrogen production varies dramatically throughout a woman’s life span. It is possible that genistein has varying effects on the breast in the presence of high estrogen (during pregnancy), moderate estrogen levels (during pre-menopause), and low estrogen (during childhood and post-menopause). One reason why Asian women living in Asia have low breast cancer risk is that a lifetime exposure to soy provides the protective effect. It also is possible that soy contains other factors other than the phytoestrogens that oppose the estrogenic effects of genistein and actually reduce breast cancer risk. Or, perhaps other environmental factors might be related more to low cancer rates in Asian women than increased soy consumption.
The bottom line? Moderate intake of minimally-processed soy products, such as tofu and soymilk, are safe and possibly beneficial in the prevention of numerous diseases, including breast cancer. But, until more is known, excessive intake of highly-processed soy constituents, such as soy powders or phytoestrogen pills, especially in women with a family or personal history of breast cancer, should be viewed with a cautionary eye. Photo credit: Mr. Wang via Compfight