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Q: Should I Eat Soy? Part 3
A: Since there is so much confusion about the effect of soy phytoestrogens on breast health, some further explanation of the physiology of how phytoestrogens work might be helpful.
Studies consistently show that isoflavones in soy are weakly estrogenic by binding to estrogen receptors in mammals. The mechanism by which isoflavones inhibit cancer growth is still being studied, but here is what we know: isoflavones have antioxidant properties, they inhibit enzyme systems involved in signal transduction, they inhibit angiogenesis, and they inhibit estrogenic activity.
There is one disturbing study in which genistein administered as an isolated nutrient appeared to stimulate cell division and replication of breast cancer cells, but remember, we are not suggesting that people consume isolated genistein. We are suggesting that they consume soy foods.
Soy has also been studied for its effects on hormone levels. According to an article published in the American Journal of Clinical Nutrition, consumption of isoflavone-rich soy foods resulted in the length of the follicular phase of the cycle increasing and the menstrual cycle being lengthened, as well as a decrease in follicle-stimulating hormone and luteinizing hormone. Breast cancer patients tend to have shorter menstrual cycles than controls. Asian women have longer cycles than Western women, and the incidence of breast cancer in Asia is significantly lower. Women with shorter menstrual cycles will spend more time in the luteal phase of the cycle, where the risk for cancer development is almost 4 times greater than during the follicular phase. If soy consumption increases the length of the cycle, this may have a protective effect.
Isoflavones also can increase the metabolism of endogenous estrogens to anti-carcinogenic metabolite. Higher levels of 4 and 16 alpha-hydroxyestrogen (carcinogenic) and lower amounts of 2-hydroxyestrogen (anti-carcinogenic) have been associated with higher risks of breast cancer. When women consumed a diet with an isoflavone rich soy milk, higher levels of the anti-carcinogenic 2-hydroxyestrogens were found in the urine. The ratio of 2-hydroxyestrogen to 16-alpha-hydroxtestrogen was higher in the isoflavone rich diet than the controls.
My advice will remain the same – soy is a wonderful plant source of protein that appears to have some cancer-protective qualities.