Soybeans contain phytoestrogens, which can bind to estrogen receptors and have an estrogenic effect in humans. Practitioners have erred on the side of caution and advised women who have breast cancer to avoid soy because of the theoretical risk that it may stimulate estrogen receptor-positive breast cancers or prevent estrogen-receptor blocking medications like tamoxifen from working. But now, new evidence suggests that women can enjoy soy without worrying.
Isoflavone, which is the major phytoestrogen in soy, inhibits estrogen production. However, it also binds to estrogen receptors on breast cancer cells, which may interfere with tamoxifen therapy.
Previous studies have shown that in Asian populations, soy consumption is associated with reduced all-cause mortality and recurrence in women with breast cancer. This particular study followed a group of 6235 multi-ethnic women in North America and Australia with breast cancer for about 9 years. There was a 21% decrease in all-cause mortality in women with the highest intake of isoflavone (greater than or equal to 1.5 mg per day) compared to women with the lowest consumption (less than 0.3 mg per day).
This decrease in mortality was statistically significant in women who had estrogen receptor-negative/progesterone receptor-negative breast cancers and in those who weren’t being treated with hormones as part of their treatments. No associations were found in women who had hormone receptor-positive tumors or who were being treated with anti-estrogens like tamoxifen.
Therefore, eating soy can be beneficial for women who have hormone receptor-negative breast cancer or who aren’t being treated with hormone blockers, and has no positive or negative effect in those who have hormone receptor-positive breast cancer or are being treated with hormone blockers.