Breast cancer is the most diagnosed cancer for women over 20, and is the second leading cause of cancer deaths in women, second to lung cancer (CCDP, 2005-2010). It is thought that 24,000 Canadians will be diagnosed with breast cancer this year and 5000 will die from it, as stated by the Canadian Breast Cancer Foundation (Coles 2013). Given these alarming statistics, along with the side effects of conventional treatment (i.e. fatigue, nausea, lymphedema, nerve damage and more), there has been an increasing interest in the use of natural supplements to prevent and treat cancer alone or alongside conventional options. This growing interest has included the controversial but promising use of phytoestrogens found in soy and red clover. The study of soy as a therapeutic ingredient has increased over the last decade. It has been researched for its use in alleviating menopausal hot flashes, diminishing bone loss, reducing cholesterol levels and in cancer prevention (Messina 2010).   There was a perceived notion that soy phytoestrogens were weak estrogen agonists/antagonists with molecular characteristics similar to synthetic endocrine disruptors that could theoretically disrupt hormones and promote cancer, however, this claim has not been confirmed in human studies. In fact, soy may have a real role in the prevention and treatment of breast cancer.

Soy is a complete protein as it contains all the essential amino acids including histidine. It is known to be low in cysteine, which is required for infants, and therefore not recommended for use in infancy. Food sources of soybeans include soy milk and tofu. Fermented foods with soy are natto and fermented bean paste. The form and type of soy consumed should also be noted, as this has an impact on absorption after consumption. For instance, it is known that majority of the isoflavones in soy milk are glycosides, but they are converted to aglycones by fermentation (Takagi 2015). Isoflavone absorption is increased in fermented soy milk beverage (in which the isoflavones are largely converted to aglycones) in comparison to regular soy milk. Some consideration should be given to the fact that 93 per cent of soy is now a genetically modified organism (GMO), and that it can cause allergic reactions in some, however, further studies are needed to assess the full effects of this (Kelly, 2015). Until this can be done, it is best to use non-GMO soy and to avoid usage if allergic.

It should be noted that people metabolize soy phytoestrogens differently. Soy contains isoflavones, such as genistein and daidzein. It also contains glycitein, an O-methylated isoflavone that makes up 5–10% of the total isoflavones in soy food (Song 1999). Equol is a gut bacterial metabolite of the isoflavone daidzein, and has been associated with beneficial health effects (Törmälä, 2007). Those with the intestinal capacity to convert daidzein to equol can change steroid metabolism and estrogen bioavailability, and thus have an advantage using soy.

Red clover is a plant that has been used in traditional medicine for centuries. It has phytoestrogens coumestrol (CM) and formononetin (FM). It has been known for its use in asthma, cholesterol reduction, hot flashes, indigestion and cancer prevention. Recently, more research has been done on the use of red clover in female conditions, including menopause and cancer, alongside soy. The value of the use of phytoestrogens (found in soy and red clover) to combat cancer is now being assessed. For instance, one source suggests a therapeutic dose of 40-50mg of isoflavones (biochanin A, daidzein, formononetin and genistein) as daily dose for cancer prevention and additional health benefits, and this is derived from the traditional Japanese diet (Beck 2005).

Molecular mechanisms of soy phytoestrogens

Soy phytoestrogens are thought to be involved in breast cancer prevention through certain mechanisms of action. Genistein, for example, is believed to be involved in the modulation of different signaling pathways associated with the growth of cancer cells through pleiotropic actions including inhibition of tyrosine kinases, DNA topoisomerase II, 5α-reductase, galectin-induced G2/M arrest, protein histidine kinase, and cyclin-dependent kinases (Varinska 2015). Another in vitro and in vivo study showed that genistein reduced MCF-7 cell proliferation and induced the inactivation of IGF-1R and p-Akt and downregulated the Bcl-2/Bax protein ratio (Chen 2014). Overall, these mechanisms have shown potential in breast cancer prevention.

Molecular mechanisms of red clover phytoestrogens

Mouse models and in vitro evidence have both shown interesting ways in which red clover isoflavones, like formononetin (FM) can be preventative in breast cancer. Cells treated with FM have demonstrated reduced migration and invasion of MDA-MB-231 and 4T1 cells in a wound healing assay, chamber invasion assay, and an in vivo mouse metastasis model. In vitro, FM inhibited breast cancer cell migration and invasion by decreasing the expression of matrix metalloproteinase-2 (MMP-2), MMP-9 and increased the expression of tissue inhibitor of metalloproteinase-1 (TIMP-1) and TIMP-2 (Zhou 2014). This finding, while preliminary, suggests that FM may have anti-invasive properties for breast cancer, and that red clover deserves further evaluation in this context. Molecular evidence from Chen et al found that FM prevented the proliferation of MCF-7 cells and arrested the cell cycle in comparison to controls (Chen 2011). Levels of p-IGF-1 R, p-Akt, cyclin D1 protein expression, and cyclin D1 mRNA expression were also downregulated. FM also prevented tumor growth of human breast cancer cells in nude mouse xenografts in the same study. Overall, these results show that FM halts the cell cycle at the G0/G1 phase by inactivating IGF1/IGF1R-PI3K/Akt pathways, which demonstrate its potential in preventing breast cancer carcinogenesis (Chen 2011).

Human studies of the protective effect of red clover and soy

There are many epidemiological studies that support the effects of soy and, a study for red clover, in either breast cancer prevention or treatment alongside other therapies. A recent systematic review suggested that soy food intake, similar to a traditional Japanese diet (2-3 servings daily, containing 25-50mg isoflavones) may be protective against breast cancer and its recurrence (Fritz 2013). One study, done on WHO-CARDIAC study populations, demonstrated that the lower mortalities of breast cancers were inversely related to the 24-h urinary isoflavone excretion (Yamori 2006). Another interesting, longitudinal, prospective study of breast cancer patients in China noted that soy food intake was safe and associated with a lower mortality and recurrence of breast cancer in patients (Shu 2009). A meta-analysis of breast cancer risk in Chinese women showed that soy was a relevant factor in reducing the risk of breast cancer development; significant differences were also found between those who consumed soy foods, OR = 0.68 (95% CI 0.50-0.93) and those who ate a high-fat diet, OR = 1.15 (95% CI 1.01-1.30) (Wu 2015). One study proposed that soy should be used only in indicated patients and not in receptor positive females. Woo et al. found that high intake of soy isoflavones increased the risk of cancer recurrence in HER2-positive breast cancer patients (Woo, 2012), however, majority of the evidence proposes that soy is still protective.

Other convincing evidence comes from systematic reviews and meta-analyses, such as the one by Liu et al., which concluded that consumption of both soy and fruit was significantly associated with a decreased risk of breast cancer, with summary ORs of 0.65 (95% CIs: 0.43-0.99; I2=88.9%, P<0.001; N=13) and 0.66 (95% CIs: 0.47-0.91; I2=76.7%, P<0.001; N=7), respectively (Liu 2014). A case control study support this finding in Korean women; among soy foods, a high intake of cooked soybeans, including yellow and black soybeans, had an association with diminished risk of breast cancer (OR=0.67; 95% CI=0.45-0.91; P<0.02) (Do 2007). A case control German study showed that high and low consumption of soybeans, as well as of sunflower and pumpkin seeds were associated with significantly reduced breast cancer risk compared to no consumption (OR = 0.83, 95% CI = 0.70-0.97; and OR = 0.66, 95% CI = 0.77-0.97, respectively) (Zaineddin 2012).

Research on red clover is limited, however, it still gives hope that could ease controversial claims that phytoestrogens, such as those found in red clover, may contribute to breast cancer or recurrence. One systematic review suggests that it may not promote breast cancer (Fritz 2013). Further studies for red clover are required to support the dietary use of phytoestrogens from this source and others.

Conclusion

Overall, molecular and human studies support the position that soy based phytoestrogens do not promote nor cause breast cancer; rather, studies have shown that soy may decrease breast cancer risk, as well as protect against recurrence. Red clover is thought to act via similar mechanisms, however, requires further human study. For soy (preferably non GMO), supplementation has been thought to be more beneficial in equol producers. It has also been found that breast cancer mortality was decreased in those with increased excretion of phytoestrogen metabolites (i.e. in the urine). There are some challenges in current research, such as a need to bridge the findings of observational studies to individual patient care (i.e. to address issues of intake and heterogeneity), however, a start would be to utilize doses with significant results in current studies and move towards creating more structured, double-blind, placebo controlled trials whenever possible (Ballard-Barbish, 2009). A careful review of the evidence is required alongside a specific tailored regimen for each case and patient. It is important to consider the study population (Asian or other), estrogen receptor status and the presence or absence of menopause. Although interventional clinical trials are needed to conclusively assess the role of soy and red clover, the existing research suggests that red clover and soy may have an important role to play in the prevention and treatment of breast cancer when used in the right context.