 The vast majority of breast cancer start out hormone dependent, meaning the primary human estrogen, called estradiol, plays a crucial role in breast cancer development and progression. That's one of the reasons why soy food consumption appears so protective against breast cancer, because soy phytoestrogens like Genestine act as estrogen blockers. They block the binding of estrogens like estradiol to breast cancer cells. But wait a second. The majority of breast cancers occur after menopause when the ovaries have stopped producing estrogen. What's the point of eating estrogen blockers if there's no estrogen to block? It turns out the breast cancer tumors themselves produce their own estrogen from scratch to fuel their own growth. Estrogens may be formed in breast tumors by multiple pathways. The breast cancer takes cholesterol, and using the romatase enzyme, or two hydroxy-steroid dehydrogenase enzymes, produces its own estrogen. So there's two ways to stop breast cancer. One is to use anti-estrogens, estrogen blockers like the soy phytoestrogens or the anti-estrogen drug Tamoxifen. However, another way to block estradiol is by using anti-enzymes to prevent the breast cancer from making all the estrogen in the first place, and indeed there are a variety of anti-eromatase drugs in current use. In fact, inhibiting the estrogen production has been shown to be more effective than just trying to block the effects of the estrogen, suggesting that the inhibition of estrogen synthesis is clinically very important for the treatment of estrogen-dependent breast cancer. Turns out soy phytoestrogens can do both. Using ovary cells taken from women undergoing in vitro fertilization, soy phytoestrogens were found to reduce the expression of the aromatase enzyme. What about in breast cancer cells, though? Breast cancer cells, too. Not only suppressing aromatase activity, but the other estrogen producing enzyme, too. But this is in a Petri dish, does soy suppress estrogen production in people, too? Well, circulating estrogen levels appear significantly lower in Japanese women than in American white women, and Japan does have the highest per capita soy food consumption, but you don't know it's the soy until you put it to the test. Japanese women were randomized to add soy milk to their diet or not for a few months. Estrogen levels did seem to drop about a quarter in the soy milk supplemented group. Interestingly, when they tried the same experiment in men, they got similar results, a significant drop in female hormone levels with no change in testosterone levels. These results, though, are in Japanese men and women that were already consuming soy in their baseline diet, so it's really just looking at higher versus lower soy intake. What happens if you give soy milk to women in Texas? Circulating estrogen levels cut in half. Since increased estrogen levels are a marker for high risk for breast cancer, the effectiveness of soy to reduce estrogen levels may help explain why Chinese and Japanese women have such low rates of breast cancer. And what was truly remarkable is that estrogen levels stayed down a month or two even after they stopped drinking it. This suggests you don't have to consume soy every day to have the cancer protective benefit.