Soy and Breast Cancer

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 Soy foods have a lot of isoflavones, which is classified as phytoestrogens (weak estrogen-like compounds found in plants). Laboratory data show that isoflavones have a wide range of biological actions. They have an affinity for estrogen receptors in vitro, therefore potentially competing for the binding sites of estrogen receptors. In addition, they have been shown to be anti-proliferative, anti-angiogenic, anti-oxidative, and anti-inflammatory. These properties of soy have suggested many potential benefits in breast cancer. Nevertheless, there is some epidemiological evidence of an inverse association. Thus, no clear consensus has emerged regarding isoflavones and breast cancer.

 A number of reports have studied the preventive role of soy in breast cancer. We have consistently observed that country with high soy diet like Japan has historically much lower incidence of breast cancer than western countries. The incidence rate among Japanese immigrants rises as the length of time in the host country increases. This may possibly be due to both prepubertal or pubertal exposure as well as greater level of soy consumption among the Japanese as compared to the west. The Shanghai Women’s Health Study reported a cohort of more that 70,000 Chinese women using a validated food-frequency questionnaire showed strong evidence of a protective effect of soy food intake against premenopausal breast cancer (relative risk reduction of 43%). A similar study conducted in Singapore on 34,000 postmenopausal women reported a diet characterized by vegetables, fruit and soy has an early acting protective effect on breast cancer (relative risk of 30% and 43% with > 5 years follow up).

 Other studies on soy and breast cancer survivors were reported both in US and China.

A US study on a cohort of 1,954 breast cancer patients were prospectively followed up for >6 years. Increasing quintiles of soy intakes was associated with lower breast cancer recurrence compared to no intake among postmenopausal women and among Tamoxifen users. They concluded that soy isoflavones consumed at levels comparable to those in Asian populations may reduce the risk of cancer recurrence in survivors, even with Tamoxifen therapy. Moreover, soy does not appear to interfere with Tamoxifen efficacy.

 The most recent Shanghai Breast Cancer Survival Study reported a cohort of 5,042 breast cancer survivors in China in Oct 2009, aged 20 to 75 years old. With 4 years follow up, they reported the findings of an inverse correlation on mortality and recurrence among highest and lowest quartile of soy intake. (Relative risk of 29% for mortality and 32% for recurrence). The inverse association was evident among women with either estrogen receptor positive or negative breast cancer as well as both users and nonusers of Tamoxifen, in death and recurrence.  They also observed that women not taking Tamoxifen who ate the most soy seemed to have a better prognosis than women who took Tamoxifen and ate the least soy.  The associations did not vary by menopausal status, cancer stage, estrogen receptors of the breast cancer or intake of Tamoxifen. Of significance, the high soy protein intakes in this study were more than 15.3 mg of soy protein or more than 62.3 mg of isoflavones.

The relationship between soy and breast cancer is complicated by other factors. Most women living in Asia depend on soy as their main source of protein. They consume only small amounts of beef, chicken and pork. Also, compared to the average women in the US, the average Asian woman eats more fresh vegetables, is closer to her ideal body weight and is more physically active as well as less alcohol consumption. All these other factors may contribute to better breast cancer prognosis seen in the Chinese women in this study who ate the most soy.

  So far, these studies support the benefits of dietary soy in higher content of Asian diet while the probable benefit of supplemental soy in the western diet remains undetermined.

According to these population studies, soy consumption appears to be safe, and potentially  protective for women on breast cancer occurrence  as well as survival despite fears about estrogen-like effects.

 References

 1) Soy food intake and breast cancer survival. JAMA 2009 Dec 9; 302(22); 2437-43.

2) Soy isoflavones and risk of cancer recurrence in a cohort of breast cancer survivors: the Life After Cancer Epidemiology Study. Breast Cancer Research Treat 2009 Nov ;118(2) 395-405.

 3) A vegetable-fruit soy dietary pattern protects against breast cancer among postmenopausal Singapore Chinese women.  Am J of Clin Nutr 2010 91; 1013-1019.

 4) Adolescent and Adult soy food intake and breast cancer risk: results from the Shanghai Women’s Health Study.  Am J of Clin Nutr 2009; 89 (6); 1920-6.