Effect of soy isoflavones on breast cancer recurrence and death for patients receiving adjuvant endocrine therapy

Author: Kang X, Zhang Q, Wang S, Huang X, Jin S
Publication: CMAJ. 2010 Oct 18

In recent years the relationship between isoflavones and breast cancer has become controversial because of concerns they may stimulate the growth of existing estrogen-sensitive breast tumors. Isoflavones have been shown to have weak estrogenic effects and some scientists and doctors fear that the intake of isoflavones may be a concern for woman with hormone-sensitive breast cancer. These concerns are mainly based on in vitro and rodent studies. But there is little clinical evidence about the safety of isoflavones for patients with breast cancer.

Worldwide, breast cancer comprises about 10% of all cancer incidence among women, making it the fifth most common cause of cancer death. Some breast cancers are sensitive to hormones such as estrogen and progesterone which makes it possible to treat these cancers by blocking the effects of these hormones. Tamoxifen and anastrozole are used as therapy for hormone-sensitive breast cancers, resulting in prevention of recurrence and prolonging survival.

Kang and co-workers at the Cancer Hospital of Harbin Medical University in China conducted a clinical study on 524 patients who underwent surgery for breast cancer about 5 years earlier and who were receiving adjuvant endocrine therapy. They examined the associations between dietary intake of isoflavones and recurrence of breast cancer and death. The patients completed a food frequency questionnaire, including questions about consumption of soy foods such as soymilk, soy flour, dry soybeans, fresh soybeans, soybean sprouts and tofu. Soy milk accounted for about 25% of isoflavones intake, followed by tofu and soy flour.

They found that the overall death rate was not related to intake of isoflavones but recurrence for postmenopausal patients was significantly lower when their isoflavones intake was higher. Women with estrogen- and progesterone-positive breast cancer in the highest quartile of isoflavones intake had 13% lower rates of recurrence than those in the lowest quartile. Posmenopausal women receiving anastrozole therapy showed 19% lower recurrence rates. In premenopausal women, there was no association between isoflavones intake and recurrence and death.

The scientists concluded that intake of isoflavones was associated with lower risk of recurrence among postmenopausal patients with breast cancer, positive for estrogen and progesterone receptor, and those patients who received anastrozole as endocrine therapy. But they also mentioned some limitations of their study. One limitation is that other components in soy, such as soy protein, may have an effect on breast tissue. They also cautioned that this study could not be generalized to other populations with much lower soy consumption. Large clinical trials and other epidemiological studies are required to confirm the obtained results.