Will soy disrupt my hormones?
I’ve had client after client lately describing their fears of soy and the reasons they avoid it, even as vegetarians or vegans. Because this fear still seems pretty prevalent, I thought I’d lay out the science here!
Soybeans have been a dietary staple for thousands of years, celebrated for their versatility and nutritional value. But soy has also been the subject of persistent myths. I talk to so many people who are worried about eating soy foods like tofu because they’ve heard claims that soy causes feminization in men, increases breast cancer risk, or is unsafe during pregnancy or menopause. Let’s take a closer look at where these concerns come from, and whether they hold up.
One Origin of Soy Aversion
Much of the fear surrounding soy stems from research in the late ‘90s and early 2000s. In these studies, mice were given high doses of isolated phytoestrogens, which are compounds found in soybeans that have a similar structure to estrogen in our bodies (fun fact—male and female bodies both make estrogen!). The results of these studies seemed alarming: hormonal disruptions and increased cancer risks were observed in the mice fed phytoestrogens from soybeans.
The first problem with these studies is that evidence from animal studies is low quality, because rodents process food and compounds very differently to humans. Findings from animal studies need to be replicated in human trials to be clinically relevant. The second problem is the dose. The doses of phytoestrogens the mice were given far exceed what a human realistically consumes from actual soy foods like tofu or soy milk. Soy foods contain small amounts of phytoestrogens—nowhere near the amount that was isolated and fed to the mice. These research findings were stripped of this context and sensationalized by the media, fueling the misconceptions about soy you might have heard.
Let’s go through each of the soy fears that come up in my practice.
Debunking Common Soy Myths
1. Does soy cause feminization in men?
No. The idea that soy can lead to feminization in men comes mainly from mice studies. Clinical trials of humans have found no effect of soy foods on testosterone levels, sperm, semen parameters or breast size. Humans eat whole soy foods like tofu and soy milk, not isolated components of foods like phytoestrogens in high doses like those given to the mice. As with anything in nutrition, the dose makes the poison.
2. Does soy increase breast cancer risk?
No, soy does not cause breast cancer. Soy contains isoflavones, a type of phytoestrogen, which triggered concern since high levels of estrogen are linked to an increased risk of cancer. However, research consistently shows that soy foods either have no association with breast cancer risk or offer a protective effect. The levels of isoflavones in soy foods are too low to pose a risk. In Japan and China, populations typically eat much higher amounts of soy foods like tofu, and have lower rates of breast cancer, than in North America.
3. Does soy interfere with thyroid function?
Research suggests that soy foods and isoflavones generally do not impair thyroid function, and are safe for people with an overactive thyroid who have adequate iodine intake. However, monitoring iodine intake can help mitigate any potential risks individuals with thyroid conditions may encounter while having soy foods in their diet. People with hyperthyroidism should not take isoflavone supplements (again, the dose is key).
4. Is soy safe during pregnancy or menopause?
Yes! Soy is a nutrient-dense food that can be safely included during both pregnancy and menopause. Isoflavones may even offer benefits, such as alleviating menopausal symptoms like hot flashes and supporting bone health. Additionally, research on soy in pregnancy shows that the consumption of soy foods was actually associated with a decreased risk of gestational diabetes.
My case for the soybean
Soy is one of the few plant-based proteins considered “complete,” meaning it contains all nine essential amino acids. As a meat alternative, it’s such a convenient option to meet your needs. It’s also cholesterol-free, low in saturated fat, and packed with nutrients like iron, potassium, and magnesium. There are also so many delicious ways to enjoy it! Edamame beans, tofu, tempeh, soy nuts, and soy milk can all help reduce your risk of heart disease, osteoporosis, and type 2 diabetes.
The fears about soy stem from outdated, misleading studies that don’t reflect real-world consumption or human biology. Modern research consistently supports soybeans as safe and beneficial.
References
Hsieh, C. Y., Santell, R. C., Haslam, S. Z., & Helferich, W. G. (1998). Estrogenic effects of genistein on the growth of estrogen receptor-positive human breast cancer (MCF-7) cells in vitro and in vivo. Cancer research, 58(17), 3833–3838.
Harvard T.H. Chan School of Public Health. (n.d.). Straight talk about soy. The Nutrition Source. Harvard University. Retrieved June 17, 2024, from https://nutritionsource.hsph.harvard.edu/soy/
Otun, J., Sahebkar, A., Östlundh, L., Atkin, S. L., & Sathyapalan, T. (2019). Systematic Review and Meta-analysis on the Effect of Soy on Thyroid Function. Scientific reports, 9(1), 3964. https://doi.org/10.1038/s41598-019-40647-x
Reed, K. E., Camargo, J., Hamilton-Reeves, J., Kurzer, M., & Messina, M. (2021). Neither soy nor isoflavone intake affects male reproductive hormones: An expanded and updated meta-analysis of clinical studies. Reproductive Toxicology (Elmsford, N.Y.), 100, 60–67. https://doi.org/10.1016/j.reprotox.2020.12.019
Van Die, M. D., Bone, K. M., Visvanathan, K., Kyrø, C., Aune, D., Ee, C., & Paller, C. J. (2024). Phytonutrients and outcomes following breast cancer: a systematic review and meta-analysis of observational studies. JNCI Cancer Spectrum, 8(1). https://doi.org/10.1093/jncics/pkad104
