Is tofu healthy? The truth about soy isoflavones. Soy is everywhere-tofu, soy milk, and more- and is surrounded by myths. Does it affect hormones, cancer risk, thyroid health, or men differently? In this post, a Registered Dietitian separates fact from fiction with the science on soy. (Soy Myths PDF linked below.)
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🐁 Where the Soy Myths Came From
Much of the fear around soy started decades ago with early animal research1. In those studies, mice given high doses of isolated soy compounds showed hormonal changes, which led to concern that soy might act like estrogen in the human body. But mice metabolize isoflavones very differently from humans, making the results misleading.
When researchers turned to human clinical trials and large population studies, those concerns didn't hold up. Over and over, the evidence has shown that soy foods are safe for both men and women-and in many cases, they're linked to health benefits like improved cholesterol, reduced inflammation, and even better cancer outcomes.
🤷🏽♀️ Soy & Breast Cancer
Myth: Soy increases the risk of breast cancer, especially estrogen receptor (ER)-positive cancers.
Truth: Randomized controlled trials show soy isoflavones do not affect breast cancer risk markers such as hormone levels, breast cell growth, or breast density. No human studies show soy increases breast cancer risk. Soy intake is linked with greater survival and lower recurrence in survivors, and eating soy in childhood or adolescence may offer protective effects later in life. 2,3,4
👨🏽🦰 Men & Hormones
Myth: Soy causes breast growth in men.
Truth: Temporary breast development in boys during puberty (gynecomastia) is a normal part of growth and resolves on its own-it is not caused by soy. A meta-analysis of 41 clinical trials found no effect of soy or isoflavones on male reproductive hormones. Two placebo-controlled trials confirmed that soy does not cause gynecomastia in men. Research also shows that soy does not affect fertility or alter hormone levels in children. 5,6,7,8,9,10,11
🩺 Soy & Thyroid Health
Myth: Soy harms thyroid function.
Truth: Research consistently shows soy is safe for thyroid health. Clinical trials and a 2019 meta-analysis of 18 studies found no effect of soy or isoflavones on thyroid hormone levels. Concerns about soy combined with low iodine have not been confirmed in later research. While one 2011 study suggested a possible effect in people with subclinical hypothyroidism, follow-up trials found no adverse outcomes. Hashimoto's and Graves' disease have not yet been directly studied concerning soy. 12,13,14
Recommendations for people on thyroid medication:
If you take thyroid medication, here are a few tips to help manage soy intake safely.
- Take your thyroid medication on an empty stomach and wait 1-3 hours before eating soy foods or taking supplements.
- Be aware that soy protein, like calcium and iron, can reduce the absorption of thyroid medication.
- If you consume soy regularly and at consistent times, your doctor can adjust your dosage so you may not always need to separate soy and medication.
🍘 Whole Soy Foods vs. Isolates
Myth: Soy protein isolates are stronger sources of isoflavones than whole soy foods.
Truth: Whole soy foods-like tofu, tempeh, edamame, and soy milk-contain higher concentrations of isoflavones plus other nutrients. Isolates usually contain less (only 10-20% of what's in whole soy). Clinical studies show that even concentrated soy proteins consistently improve cholesterol, and foods like textured vegetable protein (TVP) remain rich in isoflavones.15,16,17,18
🫛 Anti-Nutrients in Soy
- Myth: Soy's phytates and lectins make it unhealthy because they block nutrient absorption.
- Truth: Proper preparation-soaking, cooking, fermenting-dramatically reduces these compounds. Fermented soy foods like tempeh, miso, and natto are especially beneficial. When soy is eaten as part of a varied diet, anti-nutrients are not harmful and can even play beneficial roles.19,20,21,22,23,24
🍽️ Soy Intake: How Much Is Safe?
Myth: Eating soy every day is unsafe.
Truth: In many Asian countries, people regularly consume 35-40 mg of isoflavones per day, and some eat up to 100 mg (about four servings) with no evidence of harm.
Myth: Only fermented soy foods are healthy.
Truth: Most soy eaten in Asia is unfermented-over 95% in China, around 70% in Korea, and about 50% in Japan-yet populations still see health benefits.25
⚕️ Bottom Line
Isoflavones are not estrogens. They do not raise hormone levels in men or women.
Whole soy foods are safe, beneficial, and linked to improved health outcomes-including lower breast cancer recurrence and better cholesterol levels.26
Free PDF:
📄 References
General
1. Messina, M., et al. (2022). The health effects of soy: A reference guide for health professionals. Frontiers in Nutrition. https://www.frontiersin.org/journals/nutrition/articles/10.3389/fnut.2022.970364/full
Breast Cancer
2. Soy and Breast Cancer: Myths and Misconceptions - American Institute for Cancer Research (AICR) https://www.aicr.org/resources/blog/soy-and-cancer-myths-and-misconceptions/
3. Soy and Breast Cancer: How to Put Headlines in Context of Overall Research - Karen Collins, MS, RDN, CDN, FAND https://karencollinsnutrition.com/soy-and-breast-cancerwhere-are-we-now/
4. Webinar for dietitians: "Empowering Health: Exploring the Nexus of Diet and Soy in Breast Cancer Recurrence" - Lawrence H. Kushi, ScD; Mark Messina, PhD, MS; Karen Collins, MS, RDN, CDN, FAND (Sponsored by SNI Global) https://sniglobal.org/soy-and-breast-cancer/
Men & Hormones
5. Pubertal Gynecomastia - Pediatric Endocrine Society https://pedsendo.org/patient-resource/pubertal-gynecomastia/
6. The health effects of soy: A reference guide for health professionals. Frontiers in Nutrition. 2022 https://www.frontiersin.org/journals/nutrition/articles/10.3389/fnut.2022.970364/full
7. Soy and Child Nutrition Guide - SNI Global
https://sniglobal.org/wp-content/uploads/2022/07/Childrens_Soy-and-Nutrition-Guide.pdf
8. Physical Development in Boys - American Academy of Pediatrics, Healthy Children site https://www.healthychildren.org/English/ages-stages/gradeschool/puberty/Pages/Physical-Development-Boys-What-to-Expect.aspx
9. Nourished Child Podcast: Body Image Issues in Boys https://thenourishedchild.com/body-image-issues-boys/
10. Satter, E. (2014). Kids Thrive at Every Size: How to Nourish Your Big, Small, or In-Between Child for a Lifetime of Health and Happiness. Kelcy Press.
11. Castle, J. L., & Jacobsen, J. (2013). Fearless Feeding: How to Raise Healthy Eaters from High Chair to High School. Jossey-Bass.
Thyroid Health
12. Messina M, Redmond G. Effects of soy protein and soybean isoflavones on thyroid function in healthy adults and hypothyroid patients: a review. Thyroid. 2006;16(3):249-258. https://pubmed.ncbi.nlm.nih.gov/16571087/
13. Sathyapalan T, Manuchehri AM, Thatcher NJ, Rigby AS, Chapman T, Kilpatrick ES, Atkin SL. The effect of soy phytoestrogen supplementation on thyroid status and cardiovascular risk markers in patients with subclinical hypothyroidism: a randomized, double-blind, crossover study. J Clin Endocrinol Metab. 2011;96(5):1442-1449. https://pubmed.ncbi.nlm.nih.gov/21325465/
14. Otun J, Sahebkar A, Shrestha S, et al. Effects of soy isoflavones on thyroid function: a systematic review and meta-analysis of randomized controlled trials. Sci Rep. 2019;9:5406. https://www.nature.com/articles/s41598-019-40647-x
Soy Isolates
15. Messina, M., et al. (2016). Isoflavone content of soy foods and soy protein isolates: implications for nutrition and health. Nutrition and Cancer. https://pmc.ncbi.nlm.nih.gov/articles/PMC5188409/
16. Setchell, K. D. R. (2003). Soy isoflavones-benefits and risks from nature's selective estrogen receptor modulators (SERMs). Journal of the American College of Nutrition. https://pubmed.ncbi.nlm.nih.gov/12822960/
17. Sacks, F. M., et al. (2006). Soy protein, isoflavones, and cardiovascular health: an American Heart Association Science Advisory. Circulation. https://www.ahajournals.org/doi/10.1161/circulationaha.106.171052
18. Messina, M. (2024). Health impact of traditional vs processed soy foods. Nutrients. https://pmc.ncbi.nlm.nih.gov/articles/PMC10992291/
Anti-Nutrients
19. Petroski, W., & Minich, D. M. (2020). Is there such a thing as "anti-nutrients"? A narrative review. PeerJ. https://doi.org/10.7717/peerj.8676
20. Sokrab, A. M., et al. (2012). Effect of fermentation on antinutrients and mineral bioaccessibility of pearl millet and soy flour blends. Food Chemistry. https://pmc.ncbi.nlm.nih.gov/articles/PMC4190255/
21. García-Mantrana, I., et al. (2015). Reduction of phytate in soy drink by fermentation with Lactobacillus casei expressing phytases from Bifidobacteria. International Journal of Food Microbiology. https://pubmed.ncbi.nlm.nih.gov/26003176/
22. Fuso, A., et al. (2025). Effect of soaking and roasting on antinutritional factors, oligosaccharides, and nutritional properties of legumes. Food Chemistry. https://www.sciencedirect.com/science/article/pii/S088915752500818X
23. Samtiya, M., et al. (2020). Plant food anti-nutritional factors and their reduction strategies: An overview. Food Production, Processing and Nutrition. https://doi.org/10.1186/s43014-020-0020-5
24. Lai, L. R., et al. (2013). Effect of lactic acid fermentation on the total phenolic, saponin, and phytic acid contents of soy milk. Food Chemistry. https://doi.org/10.1016/j.foodchem.2013.01.030
How Much Soy?
25. Messina, M. (2022). The health effects of soy: A reference guide for health professionals. Frontiers in Nutrition. https://www.frontiersin.org/journals/nutrition/articles/10.3389/fnut.2022.970364/full
Soy & Cholesterol
26. Jenkins, D. J. A., et al. (2010). Soy protein reduces serum cholesterol by both intrinsic and food displacement mechanisms. Journal of Nutrition, 140(12), 2302S-2311S. https://doi.org/10.3945/jn.110.124958
Karen Wisnovsky says
I’m not sure where you live but in the US soy is the most genetically modified plant grown for consumption. With this in mind do you still maintain that it should be eaten daily and is good for you? Please let us know your thoughts.
Nisha Melvani, RDN says
In the U.S., it’s true that most soy grown for animal feed and processed foods is genetically modified. However, the majority of whole soy foods meant for direct human consumption—like edamame, tofu, tempeh, and soy milk—are made from non-GMO soybeans, and many brands are labeled as such. You can easily find organic or certified non-GMO options if you prefer to avoid genetically modified crops.
radhika says
This is a wonderful article! Does this mean soy milk is also fine to use?
Nisha Melvani, RDN says
Thank you. Yes it is! I use unsweetened.
Kathy says
Thank you for the full break down. I used to avoid soy and learned a few years ago that it is safe. I have not had any issues since adding it back in (thyroid, breast cancer...) It has been an interesting journey to learn more ways to use tofu and tempe. I purchased a nut milk machine and am thrilled to make my own soy milk. It is so much cheaper and bonus I get to have the okara for other uses (favorite for keeping healthy ice-cream creamy).
Nisha Melvani, RDN says
Suxh a great idea to use it for ice cream! Thank you for reading my article.
Diana Young says
Thank you for making my life easier.Especially with your verified facts.your recipes are great.love prune smoothle.
Nisha Melvani, RDN says
The prune smoothie is one of my faves! Thank you for making my recipes.