In women who have been diagnosed with ER-positive breast tumors, tamoxifen and raloxifene are often used to block estrogen receptors so that estrogen can't reach them and stimulate breast tumor growth.
Animal, human studies show different results
Ok. Now back to soy. Most of the concern about soy has come from studies in laboratory animals. Rats injected with ER-positive tumor cells were given varying doses of genestein or daidzen. Those given more of the isoflavones had a greater growth of the breast tumors compared to rats given little or no isoflavones. But not all animal studies have shown harmful effects.
Furthermore, while isoflavones may act like estrogen, they also have anti-estrogen properties. That is, they can block the more potent natural estrogens from binding to the estrogen receptor. In addition, they stop the formation of estrogens in fat tissue and stimulate production of a protein that binds estrogen in the blood (to make it less able to bind to the receptor). They also have anti-oxidant and anti-inflammatory properties and work in other ways to reduce cancer growth.
Epidemiological studies, in which large populations of healthy women have reported details about their usual diet and were followed for many years, have either shown no association between soy and breast cancer, or a protective association, meaning that people who ate more soy had less breast cancer.
In general, studies in Asian women have found a lower risk of breast cancer with eating more soy, whereas studies in the U.S. have tended to not find any association between how much soy a woman consumes and her risk of breast cancer. Indeed, a recent study combined data from 14 epidemiologic studies on this topic and found that in Asian countries, women who ate the most (compared to the least) soy isoflavones had a 24% lower risk of developing breast cancer, while there was no association in Western countries such as the U.S.
Part of the challenge in studying this topic in the U.S. and other Western countries is that women eat much less soy food than in Asia, so evaluating the risk of breast cancer with high levels of soy consumption is difficult. For example, women who ate and drank the most soy in a study in Shanghai consumed about 40 mg isoflavones/day (roughly equal to 4 servings daily) and those who ate and drank the least ate 5 mg/day (or half a serving). But in the U.S., the highest category of consumption was 1-2 mg/day (less than half a serving per day), and the lowest category was 'none.' In other words, women in the highest categories in the U.S. would still fall into the lowest categories in China.
Keep in mind also that women in Asia likely have other cultural dietary patterns that may not be assumed in the U.S., such as lifelong consumption of soy and other foods that may have influenced breast development during adolescence in ways not yet fully understood. A study in Shanghai found that women who ate a high amount of soy protein consistently during adolescence and adulthood had a significantly reduced risk of breast cancer before menopause, but not after menopause.
Soy for cancer survivors
What about women who have had breast cancer, especially ER-positive breast cancer? A recent study looked at soy consumption in the diets of more than 9,000 breast cancer survivors who were participating in 3 studies of eating habits and other lifestyle factors after breast cancer. Two of the studies were from the U.S. and 1 was from China. Women from both the U.S. and China who consumed 10 mg/day or more of soy had a 25% lower risk of breast cancer recurrence. These protective associations were slightly stronger in women with ER-negative tumors. In women with ER-positive tumors, the associations also seemed protective (though not strongly so) for women regardless of whether they were taking tamoxifen or not.
But to find out for sure whether we should recommend soy foods to women, researchers would need to repeat these findings, ideally through a controlled study (considered the gold standard in research). At the very least, the evidence from the studies in women reassures scientists that moderate consumption is likely to be safe.