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Research in human subjects suggests that soy does not increase the risk of cancer, and in fact, it may actually lower the risk of certain types of cancers. Karen Collins, MS, RDN, CDN, FAND, of the American Institute for Cancer Research explains “Population studies don’t link soy consumption with increased risk of any cancer, and limited evidence shows soy possibly protecting against lung cancer in people who have never smoked tobacco, and either no effect or decreased risk of prostate cancer. Observational studies also link moderate soy consumption (one to two servings a day) with lower breast cancer risk in Asia, where soy foods are commonly consumed throughout life” (1).

A 2019 meta-analysis of 23 prospective studies with a total of 330,826 participants found that a 10 mg/day increase in soy isoflavones (natural plant compounds found in soy foods) intake was associated with a 9% decreased risk of breast cancer mortality, and a 5-g/day increase in soy protein intake was associated with a 12% reduction in breast cancer death (2).

A 2022 meta-analysis reviewed the available research on soy intake concluded that “soy consumption, and therefore, isoflavone intake is beneficial for breast cancer prognosis and lowers breast cancer manifestation. Women with high soy and isoflavone consumption (>15 mg/day) have a lower risk of being diagnosed with breast cancer when compared to women that have no to low soy in their dietary plan (0-15 mg/day).”

Why do people think soy can raise breast cancer risk?

Firstly, the uncertainty is due from failure to understand the differences between oestrogen and isoflavones led to many false assumptions. Isoflavones have a similar chemical structure to human oestrogen, but they bind to the body’s oestrogen receptors differently and function differently (1). Dr Lise Alschuler, ND, FABNO, a medical doctor and breast cancer survivor, said in an interview “Soy binds to the oestrogen receptors, preferentially to the beta sub-type which is actually an anti-proliferative receptor” (4).

Secondly, the type of study being examined – animals or humans? Soy may be metabolised differently in animals, so the outcomes of animal studies may not be applicable to humans (5).

Thirdly, the type of soy. As with food generally, any GMO (genetically modified organism) soy products and those highly processed (extruded soy proteins i.e. soy protein isolate, soy oils and faux meats) are best avoided. Consume primarily the traditional and/or unprocessed (soya beans, edamame), minimally processed (soya milk, tofu, tempeh, soy nuts) and moderately processed (miso, natto, soy yoghurts) (5).

So in a nutshell:

Soy is a nutrient-dense source of protein that can safely be consumed several times a week, and probably more often, and is like to provide health benefits – especially when eaten as alternative to red and processed meat (5).

Generally, 15 mg or more of isoflavones is enough to offer health benefits (6).

Finally, longevity has been linked with soy intake. Tofu, which is made from soybeans and water, is a staple in the diet of the people of Okinawa, one of the world’s five Blue Zones – regions where people not only live the longest but also the healthiest. According to Dan Buettner’s book, The Blue Zone Solution, people in Okinawa eat eight times more tofu than Americans, and Buettner ranks it in his top longevity foods from the region (7).

(1) https://www.aicr.org/resources/blog/soy-and-cancer-myths-and-misconceptions/

(2) https://pubmed.ncbi.nlm.nih.gov/31278047/

(3) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8931889/

(4) https://www.fxmedicine.com.au/podcast/breast-cancer-medical-insights-dr-lise-alschuler

(5) https://www.hsph.harvard.edu/nutritionsource/soy/

(6) https://www.betterhealth.vic.gov.au/health/healthyliving/soybeans

(7) https://www.bluezones.com/2016/10/japans-longest-lived-women-hold-key-better-health/