Soy foods are rich in isoflavones, a major group of phytoestrogens that have been hypothesized to reduce the risk of breast cancer. However, the estrogen-like effect of isoflavones and the potential interaction between isoflavones and tamoxifen have led to concern about soy food consumption among breast cancer patients.
In the largest study conducted to date on breast cancer survival, researchers from the Vanderbilt University Medical Center, Nashville, Tennessee examined the safety of soy food consumption among 5,042 breast cancer survivors, aged 20 to 75, in China participating in the Shanghai Breast Cancer Survival Study. Women were recruited into the study six months after breast cancer diagnosis.
After four years, women with the highest soy intake - measured as soy protein intake - had a 29 percent lower risk of death and were 32 percent less likely to have their breast cancer recur compared to women with the lowest intake of soy protein.
The protective effect of soy was observed for women with either early or late stage breast cancer and in women with estrogen-receptor (ER) positive and ER-negative breast cancer. (Doctors test breast cancer cells to see if they have hormone receptors. If breast cancer cells have estrogen receptors, the cancer is said to be ER-positive. If cancer cells do not have estrogen receptors, the cancer is called ER-negative.)
Soy food consumption was associated with improved survival regardless of tamoxifen use. Among women who consumed the most soy, both users and non users of tamoxifen were 35 percent less likely to have their breast cancer recur.
Interestingly, women who consumed the most soy and did not take tamoxifen had a lower risk of cancer recurrence and death than tamoxifen users with low soy intakes. This suggests that high soy intake and tamoxifen use may have a comparable effect on breast cancer survival.
This study found that soy food intake is safe and was associated with lower mortality and recurrence among breast cancer patients. A daily intake of 11 grams of soy protein offered the most benefit, an amount found in about 1.5 cups (375 ml) of soy beverage, 1 soy burger, ½ cup (125 ml) of edamame (young green soy beans), or ½ cup (125 ml) of tofu. Higher intakes did not offer greater protection
These findings suggest that moderate soy food intake is safe and potentially beneficial for women with breast cancer.
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