The soy isoflavones genistein and daidzein are compounds with estrogenic properties (phytoestrogens) with a variety of effects in human cells. Soy supplements have been proposed as safe substitutes for hormone replacement therapy (HRT) to relieve menopausal symptoms such as hot flashes and bone loss. Soy supplements have also been suggested as a strategy to reduce breast cancer risk since population studies that have found reduced breast cancer risk among women with high intake of soy foods. However, studies for the most part have reported that soy isoflavones are not effective in reducing menopausal symptoms and have raised the possibility that their use by some women could increase risk of breast cancer recurrence. Now a new study has reported that soy isoflavone supplements do not reduce breast cancer risk markers in a group of healthy, but high risk women. In addition, such supplements might increase breast cancer risk among premenopausal women.

Soy supplements do not relieve menopausal symptoms

A number of studies have reported that genistein and daidzein supplements are not successful in relieving menopausal symptoms. Not all studies are in agreement, but such supplements appear to be ineffective in reducing hot flashes and vaginal dryness, or preserving bone mineral density, mental acuity or thyroid function.

Soy isoflavones appear to increase risk of recurrence in some women

Soy isoflavone supplements appear to introduce increased risk of breast cancer recurrence in some women. High intake of soy isoflavones have been reported to increase the risk of cancer recurrence in HER2/neu overexpressing (HER2+) breast cancer patients. Several studies have found that genistein interferes with the effectiveness of tamoxifen. One study also reported that soy-based supplements might reduce the efficacy of breast cancer treatment with aromatase inhibitors.

Latest research finds soy isoflavones do not reduce markers of breast cancer risk

The new study referenced at the beginning of this news article was designed to investigate whether soy isoflavone supplements could protect against breast cancer in healthy, but high risk, women. To examine this question, the authors conducted a double-blind trial of soy isoflavone supplementation and measured its effects on breast cell proliferation and other breast cancer markers. The study included 98 women considered at high risk for developing breast cancer who were randomly assigned to receive either a soy isoflavone supplement or a placebo for six months. Breast cancer cells retrieved before and after treatment were tested for proliferation and other signs of abnormality. The expression of 28 genes involved in proliferation, apoptosis, and estrogenic effect were also measured. In addition, hormone and protein levels were measured in nipple aspirate fluid.

The proliferation index (Ki-67) was not reduced by isoflavone treatment. In fact, the Ki-67 index increased for premenopausal women treated with isoflavones. In addition, there were significant increases in the expression of 14 of the 28 genes examined in the isoflavone group, but not the control group. No treatment effects were found for the other parameters measured. The authors concluded that six months of treatment with soy isoflavones did not reduce breast cell proliferation in healthy, high-risk women, suggesting a lack of efficacy for breast cancer prevention. The authors also observed a possible adverse effect in premenopausal women. In a separate interview, lead researcher Seema A. Khan stated, “Simply put, supplements are not food. Although soy-based foods appear to have a protective effect, we are not seeing the same effect with supplementation using isolated components of soy.”

Please see our article on genistein and daidzein for more information soy isoflavones and breast cancer.