For more than a decade uncertainty has surrounded the issue of whether women with a history of breast cancer can safely consume soyfoods. Concern is based on the estrogen-like effects of soybean isoflavones which are phytoestrogens. While isoflavones stimulate the growth of mammary (breast) tumors in one type of mouse study, the findings from Chinese and U.S. epidemiologic studies convey a different picture. In these studies, the prognosis of thousands of breast cancer patients is tracked for several years and the results analyzed according to the amount of soy consumed by the participants. The findings indicate that post-diagnosis soy consumption actually reduces tumor recurrence and improves survival. This is true for women with tumors that respond to estrogen as well as those that don’t.
The results from the most recent study to examine this issue are a bit mixed. Among Korean HER2- breast cancer patients, with a type of breast cancer referred to as HE2-, higher soy intake was associated with about a 75% reduction in tumor recurrence. These findings support previously published epidemiologic research. However, among HER2+ patients, soy intake was associated with about a fourfold increased risk of recurrence. Approximately three-fourths of all breast cancer patients are HER2-, so the Korean results suggest that soy consumption is beneficial for the vast majority of women with a history of breast cancer. On the other hand, because in general, women with HER2+ breast cancer have a poorer prognosis, it’s especially important to identify factors that affect outcome.
Perhaps even more importantly, the small size of the Korean study raises doubts as to whether the findings in HER2- or HER2+ patients are worthy of consideration. There were only 339 women in this most recent study, whereas there were over 5,000 participants in the largest Chinese study and nearly 3,000 in the largest US study. In fact, in the Korean study, only 25 women reported having a recurrence whereas in the Chinese and US studies, there were approximately 500 recurrences. The total number of women and the number of women with recurrences are just too small for the Korean results to impact current understanding of the relationship between soy intake and breast cancer prognosis. However, they do serve to illustrate the need to understand how soy intake affects different types of breast cancers. More information about soy and HER2+ breast cancer is forthcoming.
Woo HD, Park KS, Ro J, et al. (2012) Differential influence of dietary soy intake on the risk of breast cancer recurrence related to HER2 status. Nutr Cancer.