Does Soy Cause Breast Cancer?

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, due to their ability to bind to estrogen receptors in the human body. But does this increase the risk of breast cancer? Should people avoid soy?

Type of estrogen receptors in the human body

There are two main estrogen receptors (ER), namely ERα and ERβ. ERβ acts has tumour suppressive effects when stimulated, and can counteract the stimulatory and proliferative effects of ERα stimulation. Essentially, when estrogen binds to ERα, it results in the growth of cancerous cells, and when estrogen binds to ERβ, it results in the opposite effect. Phytoestrogens preferentially bind to ERβ in breast tissue, which could explain the protective effect of soy. (1) It has also been proposed that phytoestrogens play a role in regulating the genes that are involved in DNA repair, apoptosis (cell death), inhibition of angiogenesis (the formation of new blood vessels – an important part of cancer growth) and inhibition of metastasis (the spread of cancer). (1)

What does the science say?

Breast cancer rates are historically estimated to be 4-7 times higher in the USA, where women follow a Western diet, compared to China and Japan, where women follow a traditional Asian diet, high in soy foods. Epidemiological studies have shown that as Japanese women immigrate to the USA, so their risk of breast cancer increases. (2) A metanalysis published in 2014, looked at 30 studies of pre- and postmenopausal women in western and Asian countries, and found that the highest intakes of soy had around a 40% lower likelihood of developing breast cancer, compared to those consuming the least. (3) However, it does appear that the earlier in life a woman begins to consume soy, the greater the protective effect it has. (4)

Soy intake can also reduce mortality and the risk of recurrence in breast cancer survivors. The Shanghai Breast Cancer Survival Study followed over 5,000 breast cancer survivors for 60 months. It found that those consuming more than 62g of soy per day, had significantly lower rates of death and recurrence, compared to those consuming less than 20g of soy per day. 5-year mortality rates were 13.1% and 9.2% and 5-year recurrence rates were 13.0% and 8.9%, respectively, for women in the lowest and highest quartiles of soy protein intake. (5)

Further studies have looked at soy intake and breast cancer mortality in both Chinese and US women, and found that regardless of the geographical location, soy intake was associated with a lower risk of breast cancer mortality and recurrence. (6) And soy intake both before and after diagnosis were important. (7)

Conclusion

Soy is protective against breast cancer, breast cancer mortality, and breast cancer recurrence. And the earlier a person starts to consume soy on a regular basis, the more protective the effects.

References

1. Lecomte S, Demay F, Ferrière F, Pakdel F. Phytochemicals Targeting Estrogen Receptors: Beneficial Rather Than Adverse Effects?. Int J Mol Sci. 2017;18(7):1381.

2. Ziegler RG, Hoover RN, Pike MC, Hildesheim A, Nomura AM, West DW, Wu-Williams AH, Kolonel LN, Horn-Ross PL, Rosenthal JF, Hyer MB. Migration patterns and breast cancer risk in Asian-American women. J Natl Cancer Inst. 1993 Nov 17;85(22):1819-27.

3. Chen M, Rao Y, Zheng Y, Wei S, Li Y, Guo T, et al. (2014) Association between Soy Isoflavone Intake and Breast Cancer Risk for Pre- and Post-Menopausal Women: A Meta-Analysis of Epidemiological Studies. PLoS ONE 9(2): e89288.

4. 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.

5. Shu XO, Zheng Y, Cai H, Gu K, Chen Z, Zheng W, Lu W. Soy food intake and breast cancer survival. JAMA. 2009 Dec 9;302(22):2437-43.

6. Nechuta SJ, Caan BJ, Chen WY, Lu W, Chen Z, Kwan ML, Flatt SW, Zheng Y, Zheng W, Pierce JP, Shu XO. 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 Jul;96(1):123-32.

7. Ho SC, Yeo W, Goggins W, Kwok C, Cheng A, Chong M, Lee R, Cheung KL. Pre-diagnosis and early post-diagnosis dietary soy isoflavone intake and survival outcomes: A prospective cohort study of early stage breast cancer survivors. Cancer Treat Res Commun. 2021;27:100350.