The Link Between Soy Intake and Breast Cancer Risk

Studies show that women who regularly consume soy products have a lower incidence of breast cancer. One study found that women who drank an average of one cup of soymilk or ate about half a cup of tofu daily had a 30% lower risk of developing breast cancer than women who rarely or never consumed soy products. This may be partly due to a protective substance in soy foods called 'isoflavones'.

Significant Protective Effects in Asian Women

In a 2013 meta-analysis, researchers analyzed data from 22 studies and found that among Asian women, the group with the highest intake of soy isoflavones had a 32% lower risk of developing breast cancer compared to the group with the lowest intake. This protective effect was observed in both premenopausal and postmenopausal cancers. Another meta-analysis in 2014 reached similar conclusions. Because Western women typically do not consume soy frequently, it is more difficult to compare the differences between high and low intake levels. However, consuming soy foods around puberty, when breast tissue is developing, may have a particularly protective effect.

Reducing the Risk of Cancer Recurrence

What about women who have already been diagnosed with breast cancer? Researchers found that among women diagnosed with estrogen-negative breast cancer, patients with the highest intake of soy isoflavones had a 21% lower risk of dying from cancer than those with the lowest intake.

The Women’s Healthy Eating and Living (WHEL) Study also indicated that soy helps protect breast cancer survivors. The study found that women who consumed the most soy had their risk of cancer recurrence or death from cancer reduced by half. Another study followed 5,042 women who had been diagnosed with breast cancer for four years. The results showed that women who regularly consumed soy products like soymilk, tofu, or edamame had about a one-third lower chance of cancer recurrence and a 29% lower chance of dying from cancer. Many other studies have confirmed these findings: there is no benefit for women who avoid soy, while women who consume soy have a lower likelihood of cancer recurrence.

The Mechanism of Action of Soy Isoflavones

How does soy help? Soy isoflavones may help maintain estrogen signaling at healthy levels. Estrogen is a hormone produced in the bodies of women (and in small amounts in men) that works by binding to 'receptors' on cells. When estrogen binds to its receptors, it can trigger the growth of certain cancer cells. However, human cells have two different types of estrogen receptors: alpha (α) and beta (β). The main estrogen in postmenopausal women, estrone, primarily binds to alpha receptors. Soy isoflavones, on the other hand, preferentially bind to beta receptors, and this binding appears to help block cell growth associated with the cancer process.