Article: You don't know beans... Soy Beans.
Soy beans may be one of the most misunderstood beans. I can't mention soy in a conversation without someone asking, "what about the phytoestrogens?" or "is soy really safe?" What's the deal and should we be concerned?
Soy has played a prominent role throughout history with many countries relying on it as their main protein source. For example, centenarians living in the Blue Zone of Okinawa, Japan heavily rely on soy and sweet potato for calories. And increasingly, I am seeing more and more soy products on the shelf at my local grocery store here in Maine.
What about the phytoestrogens in soy? When we hear the term phytoestrogen, most of us only hear the "estrogen" part and worry about our hormones. Reasonable, sure. But is eating soy the same as taking an estrogen pill? If that was the case, I can only imagine the labels on edamame beans for the treatment of hormone therapy in menopause (formerly, HRT)! But that's not the case, even though soy does appear to improve menopausal hot flashes (10).
Phytoestrogens are isoflavones that function in the body as selective estrogen receptor modulators. What does that mean? It means that phytoestrogens can increase the effects of estrogen in certain places of the body, while decreasing the effects of estrogen in other areas. It acts differently depending on where the estrogen receptor is located. A good thing! Having higher estrogen levels in the body increases the risk for breast cancer and soy appears to do the opposite, lowering breast cancer risk (1)(3)(4).
When we take estrogen in the form of a pill (for example, for the treatment of hot flashes), this estrogen increases our risk of breast cancer, uterine cancer and stroke. It is not selective. While our bodies all have some level of circulating estrogen, higher levels are not better for our health. Soy appears to lower estrogen levels while selectively improving symptoms of menopausal hot flashes (acting in two opposing ways on estrogen receptors located in different areas of the body).
Okay, but what if we actually have breast cancer? How does soy affect this? The isoflavone in soy called genistein appears to inhibit the effects of estradiol (the estrogen our body makes) on the growth of cancer cells by essentially blocking the receptor, reducing cancer growth (1)(2)(3). Now this does not make eating soy an effective treatment for reversing breast cancer, but it can help to potentially slow breast cancer growth.
As we get older, the chances of being diagnosed with breast cancer increase year after year until about age 80, because at that age, we often succumb to other chronic illness like heart attack or stroke. The average chance of being diagnosed with breast cancer at age 40 compared to age 70 is, 1-2 in one-thousand and 3-4 in one-thousand, respectively (5). And this is despite lower levels of circulating estrogen that occurs naturally following menopause.
In 1990, researchers compared the circulating estrogen levels in rural post-menopausal North Eastern Japanese women (eating plenty of soy) and White American women living in Southern California around the ages of 60-70 (4). As we know, Japanese women tend to be thinner, and excess body weight increases circulating estrogen levels and breast cancer risk. After correcting for body weight, the Japanese women eating a traditional dietary pattern, had significantly lower estrogen levels (table below)(4). The authors note that other studies comparing Japanese women living in Tokyo (eating a modernized dietary pattern) to British women did not show any significant difference (4). There appears to be a protective quality when we chose to eat a dietary pattern focused on minimally processed foods and soy.
"As incomes steadily increase globally, traditional diets have been displaced by diets that are usually animal-based with a high content of “empty calories” or refined sugars, refined fats, and alcohol...The Japanese traditional diet (Washoku)... is characterized by high consumption of fish and soybean products and low consumption of animal fat and meat..." (6). Any movement towards a more plant-predominant dietary pattern can lower our disease risk. Okay, but how much soy do we need to consumer?
Adding soy to the diet, in the form of soy milk, has been shown to reduce the levels of circulating estrogen in both men and premenopausal women and does not appear to lower testosterone levels (7)(8). How much? Just 400 ml (a little over 1.5 cups) of soy milk per day for 3 months reduced circulating estrogen levels by around 25%, but the results were of borderline statistical significance as the study only included 60 premenopausal Japanese women who were already consuming soy at baseline (7).
When we look at studies on American women with low baseline soy intake, we see a highly significant reduction in circulating estrogen levels (9). Women in Texas where brought into a metabolic ward (a fancy research hospital where researchers have complete control over the dietary pattern) and were asked to drink 12 oz (about 355 ml) of soy milk three time a day for 1 month (9). Estrogen levels significantly lowered in the soy-drinking group, with effects lasting even 3 months after the intervention ended (9).
What do I think? We know that women who carry excess body fat have higher levels of circulating estrogen, higher risk of breast cancer, and tend to experience more significant perimenopausal symptoms like hot flashes, to name just a few. An intervention aimed at swapping out dairy for soy in the diet is a reasonable way to lower circulating estrogen levels and reduce our risk. When patients ask me about soy, my answer is yes. Eat it. From edamame to tempeh, tofu, soy milk and miso... soy appears to be beneficial, especially when we focus on the big picture of eating more whole plant foods.
References
Inhibition of proliferation of estrogen receptor-positive MCF-7 human breast cancer cells by flavonoids in the presence and absence of excess estrogen. So et al. Cancer Letters. (1997) https://doi.org/10.1016/s0304-3835(96)04557-0
Phytoestrogens and their low dose combinations inhibit mRNA expression and activity of aromatase in human granulosa-luteal cells. Rice et al. Journal of Steroid Biochemistry and Molecular Biology. (2006) https://doi.org/10.1016/j.jsbmb.2006.06.021
Role of steroid sulfatase in local formation of estrogen in post-menopausal breast cancer patients. Nakata et al. Journal of Biochemistry and Molecular Biology. (2003) https://doi.org/10.1016/s0960-0760(03)00357-1
Serum oestrogen levels in postmenopausal women: comparison of American whites and Japanese in Japan. Shimizu et al. British Journal Cancer. (1990) https://doi.org/10.1038/bjc.1990.316
U.S. Cancer Statistics Working Group. U.S. Cancer Statistics Data Visualizations Tool, based on 2022 submission data (1999-2020): U.S. Department of Health and Human Services, Centers for Disease Control and Prevention and National Cancer Institute. (released November 2023) https://www.cdc.gov/cancer/dataviz
The Role of the Japanese Traditional Diet in Healthy and Sustainable Dietary Patterns around the World. Gabriel et al. Journal Nutrients. (2018) https://doi.org/10.3390%2Fnu10020173
Effect of soymilk consumption on serum estrogen concentrations in premenopausal Japanese women. Nagata et al. Journal of the International Cancer Institute. (1998) https://doi.org/10.1093/jnci/90.23.1830
Effect of soymilk consumption on serum estrogen and androgen concentrations in Japanese men. Nagata et al. Journal of Cancer, Epidemiology, Biomarkers and Prevention. (2001) https://pubmed.ncbi.nlm.nih.gov/11303585/
Effects of soya consumption for one month on steroid hormones in premenopausal women: implications for breast cancer risk reduction. Lu et al. Journal of Cancer, Epidemiology, Biomarkers and Prevention. (1996) https://www.researchgate.net/profile/Manubai-Nagamani/publication/14431259_Effects_of_Soya_Consumption_for_One_Month_on_Steroid_Hormones_in_Premenopausal_Women_Implications_for_Breast_Cancer_Risk_Reductio/links/02e7e5319c2bf06e73000000/Effects-of-Soya-Consumption-for-One-Month-on-Steroid-Hormones-in-Premenopausal-Women-Implications-for-Breast-Cancer-Risk-Reductio.pdf
Quantitative efficacy of soy isoflavones on menopausal hot flashes. Li et al. British Journal of Clinical Pharmacology. (2014) https://doi.org/10.1111/bcp.12533
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