Reports of soy’s health benefits have made soy products increasingly popular. You may have heard that “soy helps prevent breast cancer,” but you may have also heard the opposite: “soy is bad for women at high risk of breast cancer. It’s still unclear how. Soy is a major source of isoflavones, which are phytoestrogens, plant compounds that act like estrogens in the body, but are much less potent than natural estrogens. Like estrogen, phytoestrogens bind to estrogen receptors in cells, but phytoestrogens have a two-way regulatory effect, and the effect they have depends on the level of natural estrogen in the body. For example, when the body has sufficient natural estrogen (estradiol), phytoestrogens can replace it and thus reduce the effects of estrogen, but when there is little natural estrogen, such as after menopause, phytoestrogens act like estrogen and thus reduce menopausal symptoms. The idea that soy products can prevent breast cancer is mostly based on the fact that Asian women who like to eat soy products have a lower incidence of breast cancer, and that Asian women who immigrate to the United States have an increased risk of breast cancer due to changes in their lifestyle and eating habits. But there are too many differences between Eastern and Western lifestyles for soy products to be the only factor. Hundreds of studies have tried to confirm that eating soy products does help prevent breast cancer, but the results are contradictory, with most studies not finding that consuming large amounts of soy products can block the risk of breast cancer, and several studies showing that consuming too much soy products in middle age increases the risk of breast cancer. There is no definitive answer to the question of what role soy products play, so how do we approach this issue? The right approach is “in moderation.” If you like soy products, it makes sense to eat them in moderation, regardless of whether you are at risk for breast cancer or already have breast cancer. However, experts do not recommend eating soy products as the only way to reduce the risk of breast cancer.