Can soy reduce breast cancer risk, recurrence and mortality?

This article investigated the safety and efficacy of soy and red clover (Promensil) on improving the symptoms, risk and recurrence of breast cancer.

Soy and red clover are plants that contain a class of organic compounds called isoflavones. Owing to their structure similarities with estrogen (the main female sex hormone), isoflavones can bind to the same receptor (a protein found on the surface of cells) as estrogen and cause estrogenic and/or anti-estrogenic effects on breast cells. Basically, estrogen stimulates the growth of cells in the breast, including breast cancer cells, thus may contribute to breast cancer development. Though still under clinical investigation, it is highly believed that soy and red clover can lower the risk and recurrence of breast cancer and also relieve the side effects of breast cancer treatments, such as hot flashes. This article reviewed the current medical literature on the effects of soy and red clover in patients with breast cancer. 

This review summarized 131 articles concerning the safety and efficacy of soy and red clover isoflavones in patients diagnosed with breast cancer or in those at high risk of developing the disease.

32 articles showed that a higher consumption of soy foods or isoflavones was associated with a lower risk of developing primary breast cancer, with lower breast cancer mortality and improved prognosis in breast cancer patients. While 5 articles suggested that soy intake among breast cancer patients had no significant reduction in hot flashes compared to placebo (a substance with no medical effects used as a control when testing new drugs), one article presented a positive association between soy consumption and reduction of hot flashes. Also, several articles found that soy consumption decreased the risk of breast cancer recurrence. There was no difference in effect between the different types of soy consumed (soy foods, isoflavones as tablets or soy proteins as powder).

Studies conducted in Asian countries revealed a higher protective effect of soy consumption on breast cancer development than in Western countries. This may be due to higher soy consumption in the Japanese diet (containing between 6-11 g of soy protein and 25-50 mg isoflavones). Evidence on the effects of red clover, however, was limited. One study reported that Promensil did not significantly reduce hot flashes in patients with breast cancer. 

In summary, the authors surmised that there is no evidence from this research that soy is harmful; however the arguments for the protective effects of soy are not strong. Until further studies can confirm this relationship, patients should be cautious with supplementing their diet with isoflavones.