Links to all of the studies, news stories and overviews discussed below may be found on the soy page of the LATESTBreastCancer.com website. If the link does not take you directly to the soy page, please click the "Treatments" tab, then search for "soy" in the search box in the top right corner.
A little biology
The issue for breast cancer survivors involves the isoflavones in soy. Isoflavones are phytoestrogens, or an estrogen-like compounds found in plants. They act like weak estrogens and are able to bind to estrogen receptors in cells. For women with estrogen receptor positive breast cancer, there is some concern about the estrogen-like behavior of isoflavones in soy.
April 2011 study: Dietary soy is safe after breast cancer
An April 5 presentation to the American Association for Cancer Research caused quite a media stir, with headlines in the Los Angeles Times ("Soy: Breast cancer survivors can eat soy foods without risk, study finds") and Consumer Reports Health ("No need to avoid soy after breast cancer").
The study analyzed soy consumption data from 18,000 breast cancer survivors in China and America followed for an average of more than 9 years. According to the study conclusion, "Soy food consumption was not associated with an increased risk of mortality or cancer recurrence among breast cancer survivors."
On the positive side, this data represented a large number of women over a long period of time. On the other hand, the study has not yet been published in a medical journal or subject to critical peer review. Also, the study conclusions relate to dietary soy not soy supplements.
August 2011 study: Soy supplements may affect aromatase inhibitor activity
An August 10 study from The Netherlands published in Toxicology examined the effects of the soy isoflavone genistein on estrogen dependent breast cancer cells. In the study, genistein induced tumor cell growth and increased aromatase expression and activity. The data suggested that soy-based supplements "might affect the efficacy of breast cancer treatment with aromatase inhibitors." The authors caution,
Considering the high number of breast cancer patients using soy supplements to treat menopausal symptoms, the increasing risk for adverse interactions with breast cancer treatment is of major concern and should be considered with care.NAMS 2011 Isoflavones Report: More studies are needed
The North American Menopause Society (NAMS) issued a comprehensive report on soy isoflavones for menopausal symptoms (link to full report). It provides a thorough review of the biology of soy and the recent research on hot flashes, breast safety and bone health. With respect to use after a breast cancer diagnosis, the report notes,
Specific recommendations regarding soy or isoflavoneInterestingly, the report discusses human studies from both China and the United States which support the safety of soy after breast cancer. (p. 740) Nevertheless, it does not recommend soy due to safety concerns in cells and animals.
consumption by breast cancer survivors cannot be made at
this time; studies in human subjects indicate a null or
protective effect, whereas cell culture and rodent studies
indicate potential for risk. Further studies are needed. (p. 739)
NAMS is not alone in recommending caution when using soy after breast cancer. The National Institutes of Health National Center for Complementary and Alternative Medicine webpage on soy notes,
Soy's possible role in breast cancer risk is uncertain. Until more is known about soy's effect on estrogen levels, women who have or who are at increased risk of developing breast cancer or other hormone-sensitive conditions (such as ovarian or uterine cancer) should be particularly careful about using soy and should discuss it with their health care providers.The American Cancer Society publishes a similar warning,
The isoflavones in soy have weak estrogen-like activity, and it is unclear how they might affect the growth of estrogen receptor-positive breast cancers. Some researchers suggest they may act as anti-estrogens and reduce cancer growth, while others suggest their estrogen-like activity could cause cancer to grow faster. Until this issue is resolved, many oncologists recommend that people taking tamoxifen or aromatase inhibitors and people with estrogen-sensitive breast tumors should avoid adding large amounts of soy, including soy supplements or isoflavones, to their diets.At LATESTBreastCancer.com, we'll continue to follow and share the latest research on soy for breast cancer survivors. Any new developments will be added to the LATESTBreastCancer.com website and highlighted here. Please stay tuned.