Author: Dr Roxanne Becker

There are many myths and misconceptions surrounding soy. This fear-mongering began once it was discovered that soy has isoflavones, which are classified as phytoestrogens (or plant estrogens), due to their ability to bind to estrogen receptors in the human body, and animal research suggested they might increase the risk of breast cancer.

The effects of soy on the body

Phytoestrogens (isoflavones are a type of phytoestrogen found in soy) have a different molecular structure than human estrogen and therefore have different effects on the body. They act as selective estrogen receptor modulators (SERMs) whereby they are pro-estrogenic, mimicking the effects of estrogen, in some tissues, and anti-estrogenic, blocking the effects of human estrogen, in other tissues. For example, we see that soy reduces the frequency and severity of hot flashes during menopause (a pro-estrogenic effect) 1 and reduces the risk of breast cancer (an antiestrogenic effect).

Soy reduces the risk of breast cancer

Soy is protective against breast cancer, both before and after a breast cancer diagnosis. The protective effect of soy appears to be particularly potent when consumed in adolescence, as phytoestrogens may affect the developing breast tissue during puberty, resulting in a reduced risk of breast cancer later in life. 2 One meta-analysis found that the highest intakes of soy isoflavones were associated with 25% lower odds of breast cancer compared to the lowest intakes. 3 The protective effect was stronger in women from Asian countries compared to Western countries (OR 0.59 vs 0.92). This protective effect was seen in both pre- and postmenopausal women. Similarly, another meta-analysis published in 2006 found that soy was associated with a lower risk of breast cancer, but that this effect was stronger in pre-menopausal women. 4

Soy may also reduce the risk of breast cancer in women with a genetic predisposition towards developing it. One study found that the protective effect of soy was stronger in women who had the BRCA mutation than those without. The BRCA mutation is a genetic mutation that increases a woman’s risk of breast cancer. In this study, soy intake was associated with a 27% reduced risk of breast cancer in women who did not have the BRCA mutation, and an even greater 73% risk reduction in carriers of BRCA gene mutation. 5

Soy and breast cancer survival

Women who have already been diagnosed with breast cancer can also benefit from eating more soy. An analysis of over 9,500 breast cancer cases found that consuming more than 10mg of isoflavones per day was associated with a 13% lower risk of all-cause mortality, a 17% lower risk of breast cancer-specific mortality, and a 25% reduced risk of breast cancer recurrence. 6 To put this into perspective, 100g of tofu or 1 cup of soy milk contains about 25mg of isoflavones, which is more than double the amount found to be protective in the study. 7

The Shanghai Breast Cancer Survival Study similarly found that soy consumption was protective against breast cancer recurrence and death and that this protective effect was present in those who did and did not use tamoxifen. 8 Tamoxifen is a SERM medication that is used to treat breast cancer. Given that phytoestrogens are also SERMs, it raises the question of whether phytoestrogens interfere with tamoxifen treatment. Though limited research is available on the subject, human data has found that serum levels of tamoxifen and its metabolites are unrelated to self-reported intake of soy or serum concentrations of isoflavones. 9

The other beneficial effects of soy

The great news is that the benefits of soy extend beyond breast cancer. Soy is beneficial for cardiovascular health, as its consumption has been found to lower LDL cholesterol and triglycerides, lower blood pressure, improve vascular wall health, and reduce CRP, which has been found to translate into lower rates of heart disease. 7

Soy has also been found to reduce the risk of bone fractures in postmenopausal women by about one-third. 10,11 More than one glass of soy milk per day has been associated with a 70% lower risk of prostate cancer in men, 12 while having no feminising effects or reduced testosterone levels. 13

In conclusion, soy is a health-promoting food that should be included in everyone’s diet, from adolescents to adults, women to men. Aim for 1-2 servings soy products, such as tofu and soy milk, to see these benefits.

References:

1. Barnard ND, Kahleova H, Holtz DN, et al. The Women’s Study for the Alleviation of Vasomotor Symptoms (WAVS): a randomized, controlled trial of a plant-based diet and whole soybeans for postmenopausal women. Menopause. 2021;28(10):1150-1156. doi:10.1097/GME.0000000000001812
2. 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
3. Chen M, Rao Y, Zheng Y, Wei S, Li Y, Guo T, Yin P. Association between soy isoflavone intake and breast cancer risk for pre- and post-menopausal women: a meta-analysis of epidemiological studies. PLoS One. 2014 Feb 20;9(2):e89288. doi:10.1371/journal.pone.0089288
4. Trock BJ, Hilakivi-Clarke L, Clarke R. Meta-analysis of soy intake and breast cancer risk. J Natl Cancer Inst. 2006;98(7):459-471. doi:10.1093/jnci/djj102
5. Ko KP, Kim SW, Ma SH, et al. Dietary intake and breast cancer among carriers and noncarriers of BRCA mutations in the Korean Hereditary Breast Cancer Study. Am J Clin Nutr. 2013;98(6):1493-1501. doi:10.3945/ajcn.112.057760
6. Nechuta SJ, Caan BJ, Chen WY, et al. Soy food intake after diagnosis of breast cancer and survival: an in-depth analysis of combined evidence from cohort studies of US and Chinese women. Am J Clin Nutr. 2012;96(1):123-132. doi:10.3945/ajcn.112.035972
7. Messina M. Soy and Health Update: Evaluation of the Clinical and Epidemiologic Literature. Nutrients. 2016;8(12):754. Published 2016 Nov 24. doi:10.3390/nu8120754
8. Shu XO, Zheng Y, Cai H, et al. Soy food intake and breast cancer survival. JAMA. 2009;302(22):2437-2443. doi:10.1001/jama.2009.1783
9. Wu AH, Pike MC, Williams LD, et al. Tamoxifen, soy, and lifestyle factors in Asian American women with breast cancer. J Clin Oncol. 2007;25(21):3024-3030. doi:10.1200/JCO.2006.10.5023
10. Zhang X, Shu XO, Li H, et al. Prospective cohort study of soy food consumption and risk of bone fracture among postmenopausal women. Arch Intern Med. 2005;165(16):1890-1895. doi:10.1001/archinte.165.16.1890
11. Koh WP, Wu AH, Wang R, et al. Gender-specific associations between soy and risk of hip fracture in the Singapore Chinese Health Study. Am J Epidemiol. 2009;170(7):901-909. doi:10.1093/aje/kwp220
12. Jacobsen BK, Knutsen SF, Fraser GE. Does high soy milk intake reduce prostate cancer incidence? The Adventist Health Study (United States). Cancer Causes Control. 1998;9(6):553-557. doi:10.1023/a:1008819500080
13. Hamilton-Reeves JM, Vazquez G, Duval SJ, Phipps WR, Kurzer MS, Messina MJ. Clinical studies show no effects of soy protein or isoflavones on reproductive hormones in men: results of a meta-analysis. Fertil Steril. 2010;94(3):997-1007. doi:10.1016/j.fertnstert.2009.04.038