The Truth about Soy, Dietary Phytoestrogens, and Cancer
In the mid-20th century, the modern understanding of plant chemistry and its effects upon the human body was in its infancy. At that time, the fear arose that a group of compounds known as phytoestrogens may cause certain hormone-sensitive cancers. This fear has persisted to this day, and I still hear patients mention that they are avoiding soy because they have breast cancer or because someone in their family once had it. But are soy and the phytoestrogens found in many other foods really that dangerous?
What are Phytoestrogens?
These are a group of naturally-occurring compounds found in plants that have chemical structures similar to human estrogen. Some common dietary phytoestrogens include Dadzein, Genestein, and Coumestrol. They were long thought and feared to have similar effects as estrogen, which is why they were believed to contribute to the growth of certain hormone-sensitive cancers such as breast, endometrial, and prostate cancer. The truth of the matter is that these compounds are not estrogen, and that perhaps the term phytoestrogen is misleading. In some more recent scientific literature, these compounds are occasionally referred to as Phyto-SERMs. A SERM is a Selective-Estrogen-Receptor-Modulator, or a compound that selectively binds to and interacts with the array of estrogen receptors in the human body, without necessarily having the exact properties of estrogen. Some of the medications used to treat breast, ovarian, and endometrial cancers are SERMs, such as Tamoxifen. A Phyto-SERM is a SERM that simply comes from a plant.
What the Science Tells Us
We are now lucky to live in a time where we have access to a reasonable amount of research data on this matter. What the data tells us is not what was feared: not only are dietary phytoestrogens safe to consume if worried about cancer, they also appear to both prevent it and lessen the severity.
A study of over 73,000 Chinese women showed those who ate 13 grams of soy protein a day were 22% less likely to develop breast cancer than those who ate less than 5 grams. The same study also showed a 54% decreased risk of developing hormone receptor negative breast cancer in postmenopausal women - meaning the protective effects even helped the type of breast cancer that does not feed off of estrogen [1]. A study of 5,042 female breast cancer survivors showed that those who continued to eat soy after cancer diagnosis had significantly decreased risk of both death and cancer recurrence. This reduction in risk applied to all breast cancer types, all stages, and in both tamoxifen users and nonusers [2].
There are different kinds of estrogen receptors in the body, and phytoestrogens typically have a greater affinity for Estrogen Receptor Beta or ER-Beta, which is believed to have more of a regulatory anti-cancer role, as opposed to ER-Alpha which is implicated moreso with cancer [3]. Additionally, the phytoestrogen Genestein found in soy is known to block tyrosine kinase, an important enzyme involved in cancer growth. It also blocks the formation of new blood vessels in tumors. Phytoestrogens stimulate autophagy in many kinds of cancer, which is the body’s process of cleaning out damaged cells in order to create healthier ones [4,5]. This includes even colorectal cancer and brain cancers, which are not typically associated with hormone sensitivity.
When to be Careful
There is some research that soy or phytoestrogen supplements, rather than food sources, may increase the risk of hormone sensitive cancers. This is partly because in supplements the compounds are being isolated from their whole-food source where many thousands of other synergistic compounds work in symphony together. Additionally, the levels of phytoestrogens found in supplements can be far higher than the amount that would be consumed in the diet. Whereas phytoestrogens play more of a regulatory role in the body when eaten, as a supplement they can act more like a drug with unwanted side-effects. This is why we must always be skeptical of claims from supplement companies, and make sure supplementation is done under the guidance of a licensed healthcare provider.
Foods with Phytoestrogens
You can incorporate phytoestrogens into your diet by eating more of the following foods: tofu, miso, ground flax seeds, tempeh, sesame seeds, lentils, beans, alfalfa, and edamame.
Works Cited:
[1]Lee SA, Shu XO, Li H, Yang G, Cai H, Wen W, et al. “Adolescent and adult soy food intake and breast cancer risk: results from the Shanghai Women's Health Study.” The American Journal of Clinical Nutrition. 2009; 89(6):1920-6.
[2]Shu XO, Zheng Y, Cai H, Gu K, Chen Z, Zheng W, et al. “Soy food intake and breast cancer survival.” JAMA : The Journal of the American Medical Association. 2009; 302(22):2437-43.
[3]Kuiper, George G. J. M.; Lemmen, Josephine G.; Carlsson, Bo; Corton, J. Christopher; Safe, Stephen H.; van der Saag, Paul T.; van der Burg, Bart; Gustafsson, Jan-Åke (1998). "Interaction of Estrogenic Chemicals and Phytoestrogens with Estrogen Receptor β". Endocrinology. 139
[4]Gossner, G; Choi, M; Tan, L; Fogoros, S; Griffith, K; Kuenker, M; Liu, J (2007). "Genistein-induced apoptosis and autophagocytosis in ovarian cancer cells". Gynecologic Oncology. 105 (1): 23–30.
[5]Nakamura, Yoshitaka; Yogosawa, Shingo; Izutani, Yasuyuki; Watanabe, Hirotsuna; Otsuji, Eigo; Sakai, Tosiyuki (2009). "A combination of indol-3-carbinol and genistein synergistically induces apoptosis in human colon cancer HT-29 cells by inhibiting Akt phosphorylation and progression of autophagy". Molecular Cancer. 8: 100.