Soy and Breast Cancer Risk
What is soy?
The soybean or soya bean is an edible legume native to East Asia. Soy is considered a complete protein as it contains all 9 essential amino acids. Alongside the protein content, soy is rich in B vitamins, fibre, potassium and magnesium. Soy foods can be classified as fermented and unfermented. When soy is fermented it is cultured with bacteria, yeast or mould.
Unfermented: soy milk, tofu, soybeans, edamame
Fermented: miso, natto, tempeh, soy sauce
Why are there concerns about soy intake and cancers?
The reason there is controversy around soy and cancer comes from its high isoflavone content. Isoflavones are a type of phytoestrogen (plant oestrogen), which has a similar function to human oestrogen with much weaker effects. Soy isoflavones can bind to oestrogen receptors in the body. The concern is that oestrogen can promote the development, growth and spread of some cancers; therefore eating or drinking soy is going to be harmful.
Breast Cancer and Soy intake
Breast cancers are classified into hormone type: hormone positive (ER+/PR+), or hormone negative (ER-/PR-) breast cancer. The type of cancer will alter how those tumours respond to hormones. Oestrogen receptor positive (ER+) cancers, oestrogen can help the breast cancer progress, similarly with progesterone receptor positive (PR+).1
With these cancer types; hormone therapy is likely to be given to help minimise risk of recurrence, slow the speed of cancer growth or reduce the size of the tumour. It is these hormone positive cancer tumour types that have gathered more attention regarding the concerns around eating soy, however research has been conducted on all types of breast cancer.
Phytoestrogens do not always mimic oestrogen, in fact in some people they may block the action of oestrogen, and if this occurs within the breast then eating soy, in theory, could reduce the risk of breast cancer.
At present, the research does not provide a clear answer to whether eating soy reduces risk of breast cancer. It appears that the variations seen may be due to menopausal status, age at which soy in consumed and type of breast cancer. Conversely, when looking at animal and cell studies, high doses of isoflavones or soy protein extracts stimulate breast cancer growth.2,3
Asian countries have lower incidence rates of breast cancer than in America and Europe. It has been proposed that the reason for this is due to varying soy intake. One study suggests that there may be ethnic differences in levels of biomarkers for breast cancer risk, but there is little evidence that Asian women respond to soy foods differently to non-Asian women.4
There have been some large and well-designed studies looking into this. The largest and most detailed of these is The Shanghai Women’s Health Study. This study followed 73,233 Chinese women for 7 years (1996-2000). Women who ate soy had a 59% lower risk of premenopausal breast cancer. Risk was 43% lower when soy was eaten during adolescence. No significant association was found for postmenopausal breast cancer.6 A follow up study with the same cohort of women suggested that hormonal status, menopausal status and time window of soy exposure are important factors.6
Key findings of the study include:
– 22% lower risk of breast cancer when comparing the highest to lowest intakes of soy during adulthood.
– 28% lower risk of ER+/PR+ breast cancer in postmenopausal women. A 54% lower risk of ER-/PR- breast cancer in premenopausal women.
– A 47% lower risk of premenopausal breast cancer when comparing high to low intakes of soy during adolescence and adulthood.6
There are several prospective studies which all looked at slightly different outcomes and parameters but propose positive benefits of soy. The Breast Cancer Family Registry followed 6,235 women diagnosed with breast cancer living in USA and Canada for 9 years, they found that women who ate the highest amounts of soy isoflavones had a 21% lower risk of death compared with women with the lowest intakes.7 Another prospective study found that recurrence was lower with increasing isoflavone intake among ER+/PR+ tumours, but not ER-/PR-.8 A meta-analysis of prospective cohort studies found a 12% reduction in breast cancer deaths with each 5g per day increase in soy protein intake.9
A systematic review of randomised controlled trials (RCTs) found no changes in breast cancer risk factors with isoflavone intake, however the authors acknowledged that there were limitations to the studies included in this review.10
The advice to give to your patients
There are studies that show soy promotes ER+ breast cancer, however these are in cells and animals. It is worth noting that rodents metabolise isoflavones differently to humans. These findings have not been replicated in human studies.
In fact, soy may be protective for cancer as it can bind to the oestrogen receptors in cells and block them, potentially acting as a tumour suppressor.
If you have already been diagnosed or survived treatment, there is no increased risk of consuming soy foods. There may even be a decreased recurrence in women who have a moderate amount of soy with ER+ cancer.
At present we would recommend taking soy foods, rather than supplements. With the current research it is difficult to recommend specific doses. Therefore, moderate soy intake via food and drink is the current recommendation.
Including soy in your diet may have added benefits to heart health by helping to lower cholesterol and potential to decrease blood pressure in people with hypertension.11, 12
Further research is definitely required to establish if there is an optimal amount of soy we should be including in our diet, and the best way to take this, either via whole foods, soy protein, isoflavone supplements. Recent research has indicated that increasing intake of soy was significantly associated with a lower risk of overall cancer incidence.13 At present we should reassure our patients that it is safe to eat soy regardless of cancer type and recommend to eat soy foods and drinks rather than supplements until further research is done.
- Breast Cancer Now. Hormone Receptors Breast Cancer. Available at: https://breastcancernow.org/information-support/facing-breast-cancer/diagnosed-breast-cancer/hormone-receptors-breast-cancer. Accessed 05.04.23
- Allred CD et al. Soy diets containing varying amounts of genistein stimulate growth of estrogen-dependent (MCF-7) tumors in a dose-dependent manner. Cancer Res. 2001;61(13):5045-50.
- de Lemos ML. Effects of soy phytoestrogens genistein and daidzein on breast cancer growth. Ann Pharmacother. 2001;35(9):1118-21.
- Maskarinec G et al. Soy Food Intake and Biomarkers of Breast Cancer Risk: Possible Difference in Asian Women? Nutr Cancer. 2017;69(1):146-153.
- Lee SA et al. Adolescent and adult soy food intake and breast cancer risk: results from the Shanghai Women’s Health Study. Am J Clin Nutr. 2009;89(6):1920-6.
- Baglia ML et al. The association of soy food consumption with the risk of subtype of breast cancers defined by hormone receptor and HER2 status. Int J Cancer. 2016 Aug 15;139(4):742-8.
- Zhang FF et al. Dietary isoflavone intake and all-cause mortality in breast cancer survivors: The Breast Cancer Family Registry. Cancer. 2017;123(11):2070-2079.
- Guha N et al. Soy isoflavones and risk of cancer recurrence in a cohort of breast cancer survivors: the Life After Cancer Epidemiology study. Breast Cancer Res Treat. 2009;118(2):395-405.
- Nachvak SM et al. Soy Isoflavones, and Protein Intake in Relation to Mortality from All Causes, Cancers, and Cardiovascular Diseases: A Systematic Review and Dose-Response Meta-Analysis of Prospective Cohort Studies. J Acad Nutr Diet. 2019;119(9):1483-1500.e17.
- Finkeldey L et al. Effect of the Intake of Isoflavones on Risk Factors of Breast Cancer-A Systematic Review of Randomized Controlled Intervention Studies. Nutrients. 2021;13(7):2309.
- Jenkins et al. Cumulative Meta-Analysis of the Soy Effect over Time. J Am Heart Assoc. 2019; 8: e012458
- Ramdath et al. Beyond the Cholesterol-Lowering Effect of Soy Protein: A Review of the Effects of Dietary Soy and Its Constituents on Risk Factors for Cardiovascular Disease. Nutrients. 2017; 9(4):324
- Fan Y et al. Intake of Soy, Soy Isoflavones and Soy Protein and Risk of Cancer incidence and mortality. Front Nutr. 2022; 9: 847421.