Rethinking Soy: Its Impact on Breast Cancer Risk and Survivorship

Amidst a sea of dietary dos and don'ts, new evidence emerges on soy - not as a villain, but as a potential ally in breast cancer prevention. Learn how this common food is making a scientific comeback

Breast cancer (BC) is the second most commonly diagnosed cancer globally and the most frequently diagnosed cancer in women, with approximately 2.3 million new cases and over 680,000 deaths annually.¹ While overdiagnosis and overtreatment is part of the growing breast cancer epidemic, it does constitute a serious and significant health threat for women (and increasingly men) which should be supported with all available modalities, natural, integrative, and conventional.

A recent study aims to reconcile the varying global recommendations on soy consumption, particularly for BC survivors, by analyzing the relationship between soy isoflavones and BC recurrence and mortality.²

Historical Context of Soy Use in the East

Soy consumption is significantly higher in East and Southeast Asia compared to Western countries. This difference in diet, particularly the high intake of phytoestrogens from soy, is hypothesized to contribute to the lower incidence rates of BC in these regions.³ Phytoestrogens, due to their structural similarity with 17β-estradiol, can bind to estrogen receptors and exert estrogenic effects. They are selective estrogen receptor modulators (SERM), showing a preferential selectivity for ERβ over ERα. 

Differential SERM Activity and Breast Cancer

Selective estrogen receptor modulators (SERMs) are compounds that can either block or activate estrogen receptors (ER) in different tissues. In the context of breast cancer, the role of SERMs is particularly significant due to the involvement of estrogen in the development and progression of some breast cancers. The estrogen receptors, ERα and ERβ, play different roles in breast tissue.⁴

ERα is primarily associated with the proliferation of breast cancer cells. Activation of this receptor can promote the growth of estrogen-responsive breast cancer cells. In contrast, ERβ has been found to have a protective role. It can inhibit the proliferative effects of ERα and is associated with a better prognosis in breast cancer. Therefore, phytoestrogens that selectively bind to and activate ERβ over ERα, like those found in soy, may provide a protective effect against breast cancer development and progression. This preferential selectivity reduces the risk of stimulating estrogen-responsive breast cancer cells while potentially offering the protective benefits associated with ERβ activation. 

Study Review and Findings

The systematic literature review focused on prospective cohort studies published since 2009, targeting the association between soy isoflavone intake and BC recurrence or mortality.⁵ Seven studies were included in this analysis.

Key findings include: 

• None of the studies found statistically significant adverse effects of soy consumption on BC recurrence or mortality. 
• Two studies reported a decrease in BC recurrence associated with higher isoflavone intake, specifically in post-menopausal women.⁶ 
• Four studies found positive associations regardless of menopausal status. 
• Better results were observed in women with hormone-sensitive cancer and/or patients receiving hormonal treatment. 
• One study indicated a stronger association for patients with estrogen receptor-negative BC. 

Meta-Analysis and Implications 

A meta-analysis conducted by Boutas et al. in 2022 concluded that soy isoflavone consumption could reduce the risk of BC in both pre- and post-menopausal women.⁷ This aligns with other studies suggesting soy isoflavones may exert beneficial effects and are safe for BC survivors. 

Limitations and Recommendations 

The studies in the review had limitations, including variable cohort sizes and potential reporting bias in dietary studies. The mean duration of follow-ups was 5.42 years, which might limit the evaluation of long-term effects of soy on BC outcomes. 

Conclusion 

The review found no significant adverse effects of soy consumption on BC recurrence or mortality. Almost all studies indicated beneficial effects of soy isoflavone intake on BC recurrence and mortality, aligning with recent research suggesting the safety of soy intake for BC survivors. 

Qualification Regarding Soy Types 

While the review establishes the potential protective role of soy against BC, it is crucial to differentiate between organic, non-GMO, and chemical-free soy, and GMO and Roundup-contaminated soy. The latter, due to its potential contamination, might have different health implications and could be carcinogenic, whereas organic, non-GMO soy is considered safe and potentially beneficial, especially when consumed in quantities and in forms (e.g. fermented) used traditionally for hundreds, and in some cases, thousands of years.

For more information on the researched health benefits of soy, visit our database on the subject here.

For more information on natural approaches to breast cancer prevention and treatment, visit our database on the subject here.