Soy and Cancer Survivorship

Soy foods contain several key nutrients and phytochemicals studied for their cancer prevention properties. Early animal studies once suggested that soy might be harmful for breast cancer survivors. Soy foods contain isoflavones, compounds that in some ways mimic the action of estrogen, and high levels of estrogen spur the growth of some breast cancers. Yet stronger human studies and a consistent body of research show soy foods are safe for those diagnosed with cancer and those without.

What Research Shows

Breast Cancer Survivors: Scientists now conclude that soy is safe for women with breast cancer. Overall, population studies examining soy consumption among breast cancer survivors show that consuming moderate amounts of soy foods does not increase a woman’s risk for poorer outcomes. Some of the studies even point to a potential benefit among women receiving certain treatments or with certain tumor characteristics.

In addition, population studies do not show any harmful interactions between soy foods and anti-estrogen medications. A small number of studies even suggest soy foods may be most protective for women who take tamoxifen or an aromatase inhibitor, but more research is needed.

Although more studies are needed to find out whether eating soy foods prevents recurrence, simply eating moderate amounts of soy foods does not increase a breast cancer survivor's risk of recurrence or death.

What is a Moderate Amount of Soy?

Moderate consumption is 1 to 2 standard servings daily of whole soy foods, such as tofu, soy milk, edamame and soy nuts. (One serving averages about 7 grams of protein and 25 mg isoflavones.) Studies have demonstrated up to 3 servings/day – up to 100 mg/day of isoflavones – consumed in Asian populations long-term does not link to increased breast cancer risk.

Prostate Cancer Survivors: Increasing levels of PSA (prostate-specific antigens) may indicate signs of prostate cancer development. Overall, trials have hinted that soy foods may lower PSA levels and may benefit prostate cancer survivors. No studies have shown any harm from soy foods to prostate cancer survivors.