Soy is GOOD for Breast Cancer Patients

Many breast cancer patients were advised by their oncologist to avoid soy foods and products. This has severely restricted the sources of protein for anyone who is considering a plant-based diet. A barrier towards a wholesome health promoting diet. Is such advice valid?

Actually, it is not. soy-beans

When my wife was undergoing her cancer treatment five years ago, she too was told by her oncologist to restrict her intake of soy. However, I was not convinced! My reading into the published research has led me to conclude that, quite the contrary, soy is beneficial for breast cancer patients. As such, soy remains a main component of our plant-based diet. My wife has stayed cancer free for several years now.

Why do oncologists advise against taking soy?

Soybean contains isoflavones, a class of compounds that is classified as phytoestrogen. Isoflavones share a chemical structure that is similar to the oestrogen, i.e. the female sex hormone that is responsible for the development and regulation of the female reproductive system and secondary sex characteristics. Isoflavones can bind to the oestrogen receptor (ER) on cells and exert oestrogen-like effects under experimental conditions [1]. Using a specific mouse model, several laboratory studies showed that a specific type of isoflavones, namely genistein, can stimulate growth of oestrogen-sensitive breast tumours [2]. Since genistein represents more than 50% of the total soybean isoflavones content [3], this has sparked the fear that soy foods may cause growth of tumours in breast cancer patients whose cancer tumours are tested positive to oestrogen (ER+). As a result, doctors start to caution breast cancer patients against regular consumption of soy foods.

Phytoestrogens2

Chemical structure of Isoflavones compare to animal oestrogen

Does soy stimulate cancer growth?

Further research has soon discovered that this is not really the case, with the following evidence:

The result cannot be generalised to minimally processed soy foods

The effect of growth stimulation is not the same across different processed soy foods. Isolated genistein stimulates tumours at greater extent compared to mixed isoflavones. Whereas soy flour, being the least processed soy foods tested by the researchers, does not stimulate tumours at all [4]. This casts doubt on whether soybeans itself and minimally processed soy foods, such as tofu, can be harmful to breast cancer patients.

Mouse model is not relevant to human

It has also been shown that mice and humans differ significantly in their isoflavones metabolism. Humans, both infants and adults, are able to metabolise isoflavones more efficient than mice, resulting in significantly lower concentrations of biologically active genistein in blood after ingesting pure genistein [5]. Therefore, it is doubtful that the research results obtained using mouse model is applicable to human.

Isoflavones did not affect breast tissues in clinical research with human subjects

There have been many clinical studies that evaluated the effects of isoflavones exposure on markers of breast cancer risk. These markers include reproductive hormone levels, mammographic density, nipple aspirate fluid contents, and breast cell proliferation. For instance, in a meta-analysis of forty-seven studies, Hooper et al [6] concluded that soy or isoflavone consumption did not affect primary reproductive hormone levels. In the most up-to-date review of the current clinical evidence, Messina et al [1] concluded that “the totality of evidence indicates that regardless of the source, isoflavone exposure does not adversely affect breast tissue.”    

Therefore, we can rest assure that soy foods are not harmful to breast cancer patients.

Why is soy good for breast cancer patients?

Another type research reveals that soy consumption is positively correlated with lower mortality and less recurrence of breast cancer patients. This has been reported in many prospective epidemiologic studies. A prospective epidemiologic study is a type of population study that tracks the occurrences of a disease or disease patterns within different groups of people over time.

Here are a few major prospective epidemiologic studies on soy intake and breast cancer:

  1. The Shanghai Breast Cancer Survival Study (SBCSS) [7] – this is a large study of 5042 breast cancer survivors recruited between March 2002 and April 2006 and followed through June 2009. It was found, over the median follow-up time of 3.9 years, that the group of breast cancer survivors who consumed the highest level of soy protein (> 15.72 g/day) has approximately 30% less recurrence rate and death due to breast cancer, when compared to the group that consumed the least level of soy protein. Therefore, the study concluded that soy food intake is not only safe, but also associated with lower mortality and recurrence among breast cancer patients.
  2. The Life After Cancer Epidemiology (LACE) Study [8] – this is an American study that followed a cohort of 1954 female breast cancer survivors, predominantly Caucasian women diagnosed during 1997-2000, for 6.31 years. The results from this study suggest that there are trends of reduced risk of cancer recurrence among those who consumed high soy isoflavones compared to those with no intake. This trend is especially significant among postmenopausal women treated with tamoxifen, a 60% reduction in breast cancer recurrence rate was observed.
  3. The Women’s Health Eating and Living (WHEL) Study [9] – In this study, 3088 breast cancer survivors, diagnosed between 1991 and 2000 with early-stage breast cancer, were followed for a median of 7.3 years. It was found that as soy isoflavones intake increased, the risk of death decreased. Women who consumed the highest levels of isoflavones (>16.3 mg/d) had a 54% reduction in risk of death, albeit nonsignificant in statistical terms. Taking the results from SBCSS, LACE, and WHEL studies in totality, the authors advised that “clinicians no longer need to advise against soy consumption for women with a diagnosis of breast cancer“.
  4. The Singapore Chinese Health Study [10] – This is a study among 35,303 Chinese Women in Singapore during April 1993 to December 1998 in the Singapore Chinese Health Study. The researcher found that breast cancer risk was reduced significantly in association with high soy intake. With a mean intake of soy intake of 10.6 mg isoflavone per 1000 Kcal, women with above median intake showed a significant 18% risk reduction in breast cancer risk when compared to those taking lower level of soy. The effect is especially strong for postmenopausal women with higher body weight. As such, the authors commented that “approximately 10 mg of isoflavones per day, obtained in a standard serving of tofu, may have lasting beneficial effects against breast cancer development.”
Consume just a serving of tofu everyday is associated with lower recurrence of breast cancer

Consume just a serving of tofu everyday is associated with lower recurrence of breast cancer

To summarize the results of all these studies, a recent meta-analysis study pooled together results from 35 studies that reported results between soy isoflavone intake and breast cancer risk concluded that soy isoflavones intake could lower the risk of breast cancer for both pre- and post-menopausal women in Asian countries. However, in Western countries¸ soy isoflavones intake has only a marginally significant protective effect [11]. It was suggested that exposure to soy isoflavones in early life and high intake level may be important contributing factors to the protective effects of soy among Asian women [11]. So, we are not talking about an isolated case or a single study, the current evidence now clearly shown that high soy consumption can reduce breast cancer risk!

Conclusion

The extensive research over the past 20 years has proven that soy consumption is clearly not harmful to break cancer patient. The laboratory results that show genistein can stimulate growth of oestrogen-sensitive breast tumours obtained from a specific mouse-model cannot be simply extrapolated to human. In fact, the growing body of evidence is indicating that consumption of soy and soy products is associated with reduced risk of breast cancer incidence, recurrence, and mortality. Hence, for cancer patients, don’t stop taking soy even if you are told by your oncologists. Many oncologists simply have not kept up to date with the current evidence. So, please enjoy soy beans, tofu, edamame, tempeh, soy milk, etc., every day. They are truly GOOD for you!

By viviandnguyen_/Flickr.com By Ruocaled/Flickr.com By Alpha/Flikr.com By Jefferey Loo/Flickr.com By dharmalily /Flickr.com

References

[1]         M. Messina, B.J. Caan, D.I. Abrams, M. Hardy, G. Maskarinec, Its time for clinicians to reconsider their proscription against the use of soyfoods by breast cancer patients, Oncol. (United States). 27 (2013) 1–12. http://www.cancernetwork.com/oncology-journal/it’s-time-clinicians-reconsider-their-proscription-against-use-soyfoods-breast-cancer-patients.

[2]         W.G. Helferich, J.E. Andrade, M.S. Hoagland, Phytoestrogens and breast cancer: A complex story, Inflammopharmacology. 16 (2008) 219–226. doi:10.1007/s10787-008-8020-0.

[3]         isoflavones-content @ www.isoflavones.info, (n.d.). http://www.isoflavones.info/isoflavones-content.php (accessed May 13, 2016).

[4]         C.D. Allred, K.F. Allred, Y.H. Ju, T.S. Goeppinger, D.R. Doerge, W.G. Helferich, Soy processing influences growth of estrogen-dependent breast cancer tumors., Carcinogenesis. 25 (2004) 1649–57. doi:10.1093/carcin/bgh178.

[5]         K.D.R. Setchell, N.M. Brown, X. Zhao, S.L. Lindley, J.E. Heubi, E.C. King, M.J. Messina, Soy isoflavone phase II metabolism differs between rodents and humans: implications for the effect on breast cancer risk., Am. J. Clin. Nutr. 94 (2011) 1284–94. doi:10.3945/ajcn.111.019638.

[6]         L. Hooper, J.J. Ryder, M.S. Kurzer, J.W. Lampe, M.J. Messina, W.R. Phipps, A. Cassidy, Effects of soy protein and isoflavones on circulating hormone concentrations in pre- and post-menopausal women: a systematic review and meta-analysis., Hum. Reprod. Update. 15 (2009) 423–40. doi:10.1093/humupd/dmp010.

[7]         X.O. Shu, Y. Zheng, H. Cai, K. Gu, Z. Chen, W. Zheng, W. Lu, Soy food intake and breast cancer survival., JAMA. 302 (2009) 2437–43. doi:10.1001/jama.2009.1783.

[8]         N. Guha, M.L. Kwan, C.P. Quesenberry Jr, E.K. Weltzien,  a. L. Castillo, B.J. Caan, Soy Isoflavones and Risk of Cancer Recurrence in a Cohort of Breast Cancer Survivors: Life After Cancer Epidemiology (LACE) Study, Breast Cancer Res. Treat. 118 (2009) 395–405. doi:10.1007/s10549-009-0321-5.Soy.

[9]         B.J. Caan, L. Natarajan, B. Parker, E.B. Gold, C. Thomson, V. Newman, C.L. Rock, M. Pu, W. Al-Delaimy, J.P. Pierce, Soy Food Consumption and Breast Cancer Prognosis, Cancer Epidemiol. Biomarkers Prev. 20 (2011) 854–858. doi:10.1158/1055-9965.EPI-10-1041.

[10]      A.H. Wu, W.-P. Koh, R. Wang, H.-P. Lee, M.C. Yu, Soy intake and breast cancer risk in Singapore Chinese Health Study., Br. J. Cancer. 99 (2008) 196–200. doi:10.1038/sj.bjc.6604448.

[11]      M. Chen, Y. Rao, Y. Zheng, S. Wei, Y. Li, T. Guo, P. Yin, Association between soy isoflavone intake and breast cancer risk for pre- and post-menopausal women: a meta-analysis of epidemiological studies., PLoS One. 9 (2014) e89288. doi:10.1371/journal.pone.0089288.

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