According to the American Cancer Society, one in eight women develops breast cancer in her lifetime. Being the second leading cause of death in women, breast cancer takes away one life out of thirty-nine. Biomedical researchers and healthcare professionals have been working tirelessly over the past century to formulate cures and promote preventative measures for breast cancer. In recent studies, researchers discovered that soy, which was once thought to be carcinogenic, helps reduce breast cancer risk in women.
Soy, also known as Glycine Max, is rich in plant estrogen called isoflavone. People often think that consuming isoflavone will disrupt the hormone level in the body. Isoflavone is, indeed, similar to human estrogen in terms of its molecular structure but both of them elicit different responses in the female body. To elaborate, there are two types of estrogen receptor (ER) in the female body that are responsible for binding with the circulating estrogen in the bloodstream— ER α and ER β. The human estrogen has no preference on one ER over the other and therefore binds to both ERs with equal tendency. On the other hand, isoflavone tends to bind to ER β more frequently compared to ER α. Studies have shown that the binding of estrogen, both human’s and plant’s, to ER β slow down or halt the process of cell proliferation, indicating a reduced risk of cancer development—a disease caused by uncontrollable cell division (Greger 2020).
The misconception of soy causing breast cancer might stem from the study conducted in 2001 by researchers from the Department of Food Science and Human Nutrition and the Division of Nutritional Science, UIUC that showed isoflavone stimulates the growth of breast cancer through laboratory mice that were implanted with the estrogen-dependent tumor. However, it was later clarified that rodents and humans metabolize isoflavone differently and many studies have continuously supported that isoflavone does not cause adverse effects in humans (Simon 2019).
Researchers from Vanderbilt University School of Medicine and Shanghai Cancer Institute carried out an observational study which lasted a median of 13.2 years to investigate the effect of soy intake on the risk of breast cancer development in women with a special focus on the menopausal status of women and the subtypes of breast cancer. Breast cancers are classified into several subtypes depending on their hormone receptors status, for example, a patient who is diagnosed with an estrogen receptor (ER)+/progesterone receptor (PR)+ breast cancer has breast cancer cells that display both estrogen receptors and progesterone receptors on their surfaces while a patient with ER+/PR- breast cancer has breast cancer cells that only display estrogen receptors (ACS 2019).
In the study, 74,942 Chinese women whose ages range from forty to seventy years old from seven urban areas in Shanghai were recruited between 1996 to 2000; out of which, 4,364 women who reported a previous cancer diagnosis, an extreme daily calories intake—less than 500 kcal per day or more than 3,500 kcal per day, and the usage of hormone replacement therapy were dismissed from the study to eliminate cofounding factors, resulting in a sample size of 70,578. The group of qualified women were directed to answer a detailed survey regarding their demographic, lifestyle factors, menopausal status, medical history, etc. through questionnaires and interviews. Next, they were prompted to record their dietary intake of each meal using a food frequency questionnaire (FFQ) routinely for the researchers to evaluate the level of isoflavones intake. Follow-up interviews were also held every two to three years for the researchers to assess the group of women. In the case of a breast cancer occurrence, the information regarding its hormone receptor status was obtained from their medical reports.
In the statistical analysis process, the data of the sample was being arranged into three tertiles. The first tertile represented women with the lowest soy intake (median of 4.5g per day), the second tertile represented women with intermediate soy intake (median of 8.2g per day), and the third tertile represented women with the highest soy intake (median of 13.5g per day). Next, the occurrence of breast cancer among the sample was compared between the three tertiles to analyze the relationship between soy intake and the risk of different breast cancers.
By the end of the study, the researchers concluded that high soy intake during adulthood decreases the risk of ER+/PR+ breast cancer significantly in postmenopausal women and the risk of ER-/PR- breast cancer in premenopausal women. The researchers also pointed out that premenopausal women who had high soy intake during both adolescence and adulthood showed a significant reduced risk of breast cancer while postmenopausal women only achieved that when their soy intake during adolescence was low and during adulthood was high.
Although this study is acclaimed to be the largest and the most thorough study on this topic (Straight… 2019), there is a flaw that was not addressed by the researchers in the publication. The researchers collected information about the soy intake of the participants during their adolescence through interviews. It is highly questionable that the researchers would be able to collect accurate data regarding this from the participants who were sixty years old on average and tried to recall their soy intake forty years ago. Therefore, future research and replication studies are recommended to keep track of the soy intake of the participants from adolescence to adulthood in order to improve the accuracy and the reliability of the experimental result.
Besides observational studies, many experimental studies were conducted to explore the protective effect of soy against breast cancers from the molecular level. Although the molecular mechanism of soybean in reducing breast cancer development still remains unclear, researchers from the affiliated hospital of Bei Hua University and China‑Japan Union Hospital of Jilin University showed that soy reduces the expression of JMJD5, a type of enzyme called histone demethylase that regulates cellular transcription activity (Kooistra and Helin 2012), which in turn inhibits breast cancer progression.
Overall, soy does not cause breast cancers and actually has a positive effect in preventing them in women. As the vegan diet increasingly gaining popularity and with lactose intolerance widely affecting the human population, it is crucial for people to possess the correct knowledge about soy so that they can enjoy the delicious soy products in their coffee or salad without worrying about developing breast cancers from them. While it is supported that soy protects women from breast cancers, it is important to remember that various factors contribute to breast cancer, and one should live a healthy lifestyle on top of consuming soy products to effectively prevent breast cancers.
References
[ACS] American Cancer Society. 2019. Breast Cancer Hormone Receptor Status | Estrogen Receptor. [accessed 2021 Feb 25]. https://www.cancer.org/cancer/breast-cancer/understanding-a-breast-cancer-diagnosis/breast-cancer-hormone-receptor-status.html.
Allred CD, Allred KF, Ju YH, Virant SM, Helferich WG. 2001. Soy Diets Containing Varying Amounts of Genistein Stimulate Growth of Estrogen-dependent (MCF-7) Tumors in a Dose-dependent Manner. Cancer Res. 61(13):5045–5050.
American Cancer Society. 2021. How Common Is Breast Cancer? | Breast Cancer Statistics. [accessed 2021 Feb 25]. https://www.cancer.org/cancer/breast-cancer/about/how-common-is-breast-cancer.html.
Baglia ML, Zheng W, Li H, Yang G, Gao J, Gao Y-T, Shu X-O. 2016. The association of soy food consumption with the risk of subtype of breast cancers defined by hormone receptor and HER2 status. Int J Cancer. 139(4):742–748. doi:10.1002/ijc.30117.
Greger Michael. 2020. The Difference Between Alpha and Beta Receptors Explain Soy’s Benefits. NutritionFacts.org. [accessed 2021 Feb 25]. https://nutritionfacts.org/2020/03/24/the-difference-between-alpha-and-beta-receptors-explain-soys-benefits/.
Kooistra SM, Helin K. 2012. Molecular mechanisms and potential functions of histone demethylases. Nature Reviews Molecular Cell Biology. 13(5):297–311. doi:10.1038/nrm3327.
Simon Stacy. 2019. Soy and Cancer Risk: Our Expert’s Advice. [accessed 2021 Feb 25]. https://www.cancer.org/latest-news/soy-and-cancer-risk-our-experts-advice.html.
Straight Talk About Soy. 2019. The Nutrition Source. [accessed 2021 Feb 11]. https://www.hsph.harvard.edu/nutritionsource/soy/.
Wang Y, Liu L, Ji F, Jiang J, Yu Y, Sheng S, Li H. 2018. Soybean (Glycine max) prevents the progression of breast cancer cells by downregulating the level of histone demethylase JMJD5. J Can Res Ther. 14(10):609. doi:10.4103/0973-1482.187292.
Featured Image Source
Google Images, Creative Commons license