Soya foods get the all clear for breast cancer

Breast cancer is the most commonly diagnosed cancer in women with the prevalence increasing by 20% worldwide between 2008 and 2012.(1) In general, survival rates for breast cancer have improved and this has been attributed to better screening, diagnosis at an earlier and more localised stage and improved treatment availability.

Additionally, there is growing evidence of links between breast cancer survival and several other lifestyle choices, including:

• Maintaining a healthy body weight

• Being physically active

• Eating foods containing fibre

• Eating a more plant-based diet

• A lower intake of total fat and, in particular, saturated fat

Diet, in particular, has been shown to influence breast cancer risk, prognosis and mortality.

Soya and breast cancer survival

Large epidemiological studies, systematic reviews and meta-analysis strongly indicate that breast cancer patients and survivors can safely consume moderate amounts of soya foods.

The Shanghai Breast Cancer Survival Study, was a cohort study which followed over 5,000 Chinese women diagnosed with breast cancer over a 4-year period.(2) The researchers analysed the relationship between soya food consumption and both overall mortality and breast cancer re-occurance and found that women who consumed the highest amounts of soya had a significantly decreased risk of overall mortality and reoccurence, compared to those who consumed the least soya foods.

A 2009 meta-analysis of 5 prospective studies (2 from US and 3 from China) made a similar conclusion.(3) The analysis included over 11,000 women diagnosed with breast cancer who were followed up for 4 to 7 years. The data showed that consuming soya after breast cancer diagnosis was associated with a significant reduction in mortality and risk of recurrence. The protective effect of soya consumption was observed in both Asian and non-Asian women.

A recent review (2019) of 12 studies including over 37,000 women from the USA and Asia also showed that pre-diagnosis soya and isoflavone intake was associated with improved overall survival and reduced breast cancer recurrence, particularly in post-menopausal women. Post-diagnosis soya consumption was not significantly associated with survival in this study.(4)

Soya consumption as part of a balanced lifestyle, may help reduce breast recurrence and improve overall survival rate.

Experts at the American Institute for Cancer Research (AICR) in 2021 stated that soya is safe for breast cancer survivors:(5)
“Consistent findings from population studies indicate no increased risk for breast cancer survivors who consume soyfoods. In fact, limited evidence shows potential for greater overall survival, and perhaps decreased recurrence, among women a year or more after diagnosis who include moderate amounts of soy.”

Similar statements have been made by the American Cancer Society,(6) the Canadian Cancer Society(7) and the World Cancer Research Fund (WCRF) International.(8)

Moderate soya consumption is defined by the AICR as 1 to 2 serving per day soya foods, such as soya drink or alternative to yogurt, tofu, soya mince and soya beans.

The expert panel of the WCRF regularly reviews all of the available evidence for the impact of lifestyle factors (including diet) on the survival and future risk of different cancer types. The last update specific to breast cancer was published in 2018.(8) The report concluded there may be an association between consuming soya-based foods and a better prognosis following breast cancer diagnosis.

Soya and breast cancer risk

Additionally, research has been carried out to determine whether soya food consumption increases the risk of developing breast cancer. Clinical trials consistently show that soya isoflavone intake does not adversely affect markers of breast cancer risk, including mammographic density and cell proliferation.(9)

A large cohort study, published in the European Journal of Epidemiology, observed that amongst 300,000 Chinese women, observed over a 10-year follow up period, there was no evidence of any association between soya food intake and breast cancer risk.(10)

The authors also carried out a dose-response meta-analysis, including 11 high quality studies, and observed that each 10 mg/day increment in soya isoflavone intake was associated with a small (3%) but significant decrease in breast cancer risk.(10)

Another cohort study, Japan Public Health Centre-based (JPHC) Prospective Study, examined the association between fermented and non-fermented soya food consumption and breast cancer risk. Over 15.5 years of follow-up period, no significant difference in overall breast cancer incidence was observed between the highest and lowest consumers of soya foods. However, fermented soya food consumption was associated with a decrease in non-localised breast cancer risk.(11)

Finally, a 2020 meta-analysis (including 5 cohort and 13 case-control studies) concluded that soya intake was inversely associated with risk of developing breast cancer. The highest consumers of soya foods had a 12% lower incidence of breast cancer, and this increased to 21% amongst pre-menopausal women. The finding was found independent of ethnicity and oestrogen receptor status.(12)

The research suggests that consuming a moderate amount of soya does not increase risk of breast cancer and may offer a mildly protective effect.

Soya consumption in early years may be important for reducing breast cancer risk in later life

A 2021 technical review of observational and clinical data investigated the impact of isoflavones on various health outcomes including breast cancer.(13) The authors indicate that soya consumption during childhood and/or teenage years may be the determining factor for the reduced breast cancer risk observed in soya consumers. Early exposure appears to influence developing breast cells to become less likely to be transformed into cancer cells in later life. Proposed mechanisms for this protective effect of early years exposure to isoflavones include increase in cell differentiation and BRCA1 gene expression.

Conclusion

Consuming one to two servings of soya foods daily as part of an overall healthy diet and lifestyle is not only safe for breast cancer patients and survivors, but it may even offer a mildly protective effect.