Can I eat soya products if I have/had breast cancer?

Dietitian, Annica Rust, explains the controversy surrounding soya consumption and breast cancer.

---

Soya is a plant-based protein which contains a phytoestrogen (a very weak plant-based oestrogen) and isoflavones.⁶ The controversy surrounding soya consumption and breast cancer stems from the term phytoestrogen and hormone sensitive cancers. 

Isoflavones are a class of phytoestrogens that has a similar chemical structure to human oestrogen. However, they bind differently to oestrogen receptors and function differently accordingly.^1,2As such, the consumption of soya will not increase the oestrogen levels in your body.⁴

Oestrogen receptors

Earlier studies performed on mice suggested that soya isoflavones promoted the growth of oestrogen receptor positive (ER+) breast cancer. However, recent literature found that mice metabolised isoflavones differently than humans and found much higher blood levels of isoflavones in mice than in humans who were consuming soya foods.¹

We have different types of oestrogen receptors with different functions present in different parts of our bodies. If some of the receptors are activated, it can promote cell growth. However, it was found that isoflavones would rather bind itself to receptors that will supress tumour growth.¹ 

Studies that have been linked to the regular consumption of legumes are associated with a reduced risk for colorectal-, prostate- and breast cancer. The American Institute for Cancer Research (AICR) third Expert Report categorises evidence for the mentioned cancers as “Limited – No Conclusion”, meaning more studies are warranted to draw such a consulsion.³

Population-based studies also found no increased risk associated with breast cancer survivors who consume soya products. In fact, the limited evidence rather suggests a greater survival rate and decrease in cancer recurrence. The beforementioned includes women who had ER+ cancer, among the women who consume moderate amounts of soya.³

Drug interactions

Studies reap inconclusive results whether soya products have a positive effect on cancer treatments, such as tamoxifen or an aromatase inhibitor. More research is thus needed.^3,5

Moderate amount of soya

A moderate consumption of whole soya is considered to be 1 to 2 standard servings per day.³

Whole soya sources	Standard servings
Tofu	1/3 cup
Soya milk	1 cup
Edamame	½ cup
Soya nuts	¼ cup
Tempeh	½ cup
Miso	¼ cup
Soya mince	30g

What about soya in nutritional supplements?

Randomised controlled trials found that soya protein powder or soya isoflavone supplements had no effect on breast cancer markers used when diagnosing and monitoring breast cancer. 

Using medical nutritional supplements, such as meal replacement shakes and/or sip feeds, to achieve energy, protein and fat requirements are found to remain safe. However, soya in the form of soya pills and isoflavone-enriched powders should be used with caution given that isoflavones concentrations are much higher in soya pills and enriched powders than they are in whole soya products. ^1,2,6

Additionally, soya products contain soluble (absorbs water and is available for the fermentation by gut microflora) and insoluble (increases faecal bulk) fibre. Fermented soya products are high in vitamin K2 and vitamin B12, rich in magnesium, a source of omega 3 fatty acids and contains less saturated fat when compared to animal protein sources.⁶

Conclusion

There is no reason to cut soya from your diet, even when diagnosed with breast cancer or in the case of survivorship. However, there is also no need to include or to increase your soya intake. Accordingly, it remains the best to consume soya-based products in moderation.

---

References:

1. American Institute for Cancer Research. 2019. Soy and Cancer: Myths and Misconceptions. https://www.aicr.org/resources/blog/soy-and-cancer-myths-and-misconceptions/ [27 February 2021].
2. Eat Right. 2013. Soy and Breast Cancer. https://www.oncologynutrition.org/on/erfc/healthy-nutrition-now/foods/soy-and-breast-cancer [27 February 2021].
3. American Institute for Cancer Research. 2019. Soy: Intake Does Not Increase Risk for Breast Cancer Survivors. https://www.aicr.org/cancer-prevention/food-facts/soy/ [27 February 2021].
4. Memorial Sloan Kettering Cancer Centre. 2020. Nutrition and Breast Cancer: Making Healthy Diet Decisions. https://www.mskcc.org/cancer-care/patient-education/nutrition-and-breast-making-healthy-diet-decisions[27 February 2021].
5. https://www.mskcc.org/cancer-care/integrative-medicine/herbs/soy#references-86
6. Mahan, L.K. & Raymond, J.L. (eds).2017. Krause’s food and the nutrition care process. 14th ed. St Louis. MO: Elsevier Saunders.
7. Patel, S., Homaei, A., Rajy, AB & Meher, BR. 2018. Estrogen: The necessary evil for human health, and ways to tame it. Biomedicine & Pharmacotherapy, 102(1).

Header image by Adocbe Stock