Soy-based foods are safe if you have had breast cancer

You may have heard that it’s best to avoid soy-based products if you have (or have had) breast cancer. That makes sense: soybeans—the main ingredient in products like tofu, tempeh, and soy beverages—contain isoflavones, compounds that are chemically related to the female hormone estrogen. We know that long-term exposure to estrogen can increase the risk of breast cancer and may also increase the risk of recurrence after breast cancer. That is why women who have had (hormone-sensitive) breast cancer should not use hormone replacement therapy to treat menopausal symptoms. However, plant estrogens—as isoflavones are also called—are not the same as the estrogen produced by our bodies or used in medications. You can safely consume foods made from soybeans.

Scientifically proven

Large meta-analyses provide no evidence that soy increases the risk of breast cancer. In an analysis of 23 long-term studies, involving a total of more than 330,000 women, there was no association between soy consumption and cancer mortality. On the contrary, when women replaced animal proteins—such as those found in meat—with plant-based proteins, such as those found in tofu or tempeh, they were less likely to die from cancer or cardiovascular disease. Research also shows that women who eat or drink soy products every day do not have a higher risk of recurrence after breast cancer. So if you have had breast cancer, you do not need to avoid soy products.

Soy supplements

What applies to soy-based food products does not automatically apply to soy-based dietary supplements. Soy supplements contain concentrated doses of isoflavones, much higher than those found in food. Laboratory and animal studies show that high-dose isoflavone supplements may stimulate the growth of breast cancer cells. Even though this has not been confirmed in humans, as a precaution, these supplements are not recommended for women who have had breast cancer. There is also a theoretical risk that such supplements may interfere with tamoxifen. Tamoxifen is a medication often prescribed as hormone therapy after breast cancer to reduce the risk of recurrence. Tamoxifen is an anti-estrogen. There is some concern that high doses of phytoestrogens in supplements could reduce the effectiveness of tamoxifen.

Remember: Soy-based foods are perfectly safe if you’ve had breast cancer, but as a precaution, you should avoid soy-based supplements. In supplements, the dose of phytoestrogens is many times higher and more concentrated. A soy food product, such as tofu or a soy beverage, contains roughly 7 to 25 mg of phytoestrogens per serving, while 1 capsule of a soy supplement typically provides 40 to 300 mg of phytoestrogens. That’s a much higher amount.