Soy is one of the more controversial foods out there. Do you avoid it completely? Or is it healthy? Let’s dig into the history and research on soy.
Soy contains high amounts of isoflavones. Isoflavones are found in many plants and are considered to be mildly phytoestrogenic. While this may seem like a bad thing, it really isn’t. Phytoestrogens can only bind very weakly to estrogen receptors, or they can mildly block estrogen receptors. These compounds are found in many other plants, like chickpeas, beans, and green peas. Soy is the most widely known of these phytoestrogenic foods. Japan traditionally eats high amounts of soy, yet the risk of breast cancer is lower there than the United States.
A recent review of the literature has found that dietary soy intake has found that it does not raise the risk of breast cancer, but in fact it lowers the risk of breast cancer. Studies have found that it can lower the risk of recurrence, even in estrogen receptor positive breast cancer. This seems to apply to dietary soy intake, not supplementation of isoflavones. With supplementation, a higher dose of isoflavones can be consumed and the effect is not known, especially with a history of breast cancer.
The best quality soy products to take in are tofu, tempeh, miso, edamame, or soy milk. Research supports that one serving per day is very safe. The upper limit of isoflavone intake that is considered safest (more could be safe but not yet tested) is about 75mg per day. One serving of tempeh, which has the most isoflavone content out of all the soy sources, is about 30mg of isoflavone.
Where did this idea that soy is bad come from? Studies done in the 1990s with soy were done exclusively on rodents. The dosages were higher in isoflavones, which is why we are cautious with supplementation of these compounds. Additionally, rodents are not humans. They process isoflavones differently than humans.
Soy is also considered a goitrogen. Goitrogens are substances that can alter thyroid hormones. Some other foods considered goitrogens are cruciferous vegetables like broccoli, kale, and cabbages. Many people fear soy because it has the potential to slow down thyroid production. However, the most recent studies have not found this to be the case. The conclusion of a meta-analysis where there were 40mg-200mg of isoflavones is that it does raise TSH slightly but that there is still adequate t3 and t4 production, and there were no significant clinical findings of the elevated TSH. This means there really was not any change to thyroid production. The older studies that showed issues with soy were with soy formula in infants. Once the infants were switched to a dairy formula or formula fortified with iodine the thyroid normalized. The question becomes was it the soy or the lack of iodine that caused the hypothyroid state?
Soy can be part of a healthy diet. Incorporating quality soy products into your diet may be protective of cancer. It can be a great protein source to include to help meet your nutrition goals.
1. Braakhuis AJ, Campion P, Bishop KS. Reducing Breast Cancer Recurrence: The Role of Dietary Polyphenolics. Nutrients. 2016;8(9):E547. doi:10.3390/nu8090547
2. Doerge DR, Sheehan DM. Goitrogenic and estrogenic activity of soy isoflavones. Environ Health Perspect. 2002;110 Suppl 3:349-353. doi:10.1289/ehp.02110s3349
3. Fritz H, Seely D, Flower G, et al. Soy, red clover, and isoflavones and breast cancer: a systematic review. PLoS One. 2013;8(11):e81968. doi:10.1371/journal.pone.0081968
4. Gómez-Zorita S, González-Arceo M, Fernández-Quintela A, Eseberri I, Trepiana J, Portillo MP. Scientific Evidence Supporting the Beneficial Effects of Isoflavones on Human Health. Nutrients. 2020;12(12):E3853. doi:10.3390/nu12123853
5. Hüser S, Guth S, Joost HG, et al. Effects of isoflavones on breast tissue and the thyroid hormone system in humans: a comprehensive safety evaluation. Arch Toxicol. 2018;92(9):2703-2748. doi:10.1007/s00204-018-2279-8
6. Li J, Teng X, Wang W, et al. Effects of dietary soy intake on maternal thyroid functions and serum anti-thyroperoxidase antibody level during early pregnancy. J Med Food. 2011;14(5):543-550. doi:10.1089/jmf.2010.1078
7. Messina M. Impact of Soy Foods on the Development of Breast Cancer and the Prognosis of Breast Cancer Patients. Forsch Komplementmed. 2016;23(2):75-80. doi:10.1159/000444735
8. Messina M. Soy and Health Update: Evaluation of the Clinical and Epidemiologic Literature. Nutrients. 2016;8(12):E754. doi:10.3390/nu8120754
9. Mittal N, Hota D, Dutta P, et al. Evaluation of effect of isoflavone on thyroid economy & autoimmunity in oophorectomised women: a randomised, double-blind, placebo-controlled trial. Indian J Med Res. 2011;133:633-640.
10. Otun J, Sahebkar A, Östlundh L, Atkin SL, Sathyapalan T. Systematic Review and Meta-analysis on the Effect of Soy on Thyroid Function. Sci Rep. 2019;9(1):3964. doi:10.1038/s41598-019-40647-x
11. Zhang FF, Haslam DE, Terry MB, et al. Dietary Isoflavone Intake and All-Cause Mortality in Breast Cancer Survivors: the Breast Cancer Family Registry. Cancer. 2017;123(11):2070-2079. doi:10.1002/cncr.30615