This new population study, which included 5,042 breast cancer survivors in the Shanghai Breast Cancer Survival Study who were followed for approximately four years until June 2009, has found that soy food intake is inversely associated with death and recurrence. Risks of breast cancer-related recurrence and death both were found to be approximately 30% lower for women with the highest fourth of soy protein consumption compared to the lowest quartile. The results were found for both estrogen receptor positive (ER+) and estrogen receptor negative (ER-) breast cancer and among both users and nonusers of tamoxifen. The authors concluded that soy food consumption was significantly associated with decreased risk of death and recurrence among women with breast cancer.

Study extends results of numerous other studies

The study results confirm the findings of most other studies that soy consumption is associated with reduced risk of breast cancer in Asian populations. It extends these results to show that soy can also protect against breast cancer recurrence. While the study is encouraging with respect to soy and survival, the follow-up period of approximately four years is relatively short, and the following factors may affect the results:

• There is evidence that the protective affects of soy consumption are strongest for Asians who consume soy during childhood, so that the results may reflect higher soy consumption in childhood as well as after breast cancer diagnosis
• There is a great deal of variation among individuals in the metabolism of the soy phytoestrogens genistein and daidzein, which appears to be dependent partly on environmental factors, including other components of the diet, and partly on genetic factors. Only 25% to 35% of the U.S. Caucasian population is capable of digesting soy in a way that may produce certain cancer-protective metabolites, whereas people in high soy consumption areas of Asia have rates closer to 40% to 60%. Soybeans are less digestible than most other commonly-consumed foods. It makes sense that Asian populations, which have had many thousands of years to develop their ability to digest and extract nutrients from soybeans, would have a different related genetic profile than populations that are relatively new to soy consumption.

Not all soy products are protective

Asians who consume soy tend to consume a great deal of it. However, this is mostly in the form of less processed types of soy food such as tofu and edamame. The study did not include soybean oil or focus on soy protein isolate, which are the most common forms of soy consumed in the U.S. and which may increase breast cancer risk. Nor did it include soy isoflavone supplements, such as genistein, which are also suspected of increasing breast cancer risk. Miso and soybean paste are also not desirable because of their high salt content. The most beneficial soy foods for women appear to be the least processed, including tofu and edamame.