Among women in Asia, epidemiologic (observational) studies show higher soy consumption is associated with a decreased risk of breast cancer (1). However, clinical (intervention) studies involving adults show that the consumption of soy or soy isoflavones has no effect on markers (indicators) of breast cancer risk (2). These markers include breast cell proliferation and breast tissue density. One explanation for the protective effect observed in observational studies but the lack of benefit observed in clinical studies, is that protection occurs as a result of soy consumption during childhood and/or adolescence. Women who consume more soy as adults are also likely to have consumed more soy during childhood as early dietary habits track into adulthood.
All four of the observational studies that have evaluated the relationship between soy consumption early in life on later risk of developing breast cancer have found protective effects (3-6). This evidence is consistent with studies in rodents which show exposure to soy or soy isoflavones early in life reduces the development of chemically-induced mammary tumors (7,8). Several mechanisms explaining the protective effects of early soy exposure have been proposed. It is notable that soy intake may be working in a similar manner to becoming pregnant early in life, which is also very protective against breast cancer (9,10).
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2. Messina M, Caan BJA, D.I., Hardy MM, G. It’s time for clinicians to reconsider their proscription against the use of soyfoods by breast cancer patients. Oncology 2013;430:430-37.
3. Korde LA, Wu AH, Fears T, Nomura AM, West DW, Kolonel LN, Pike MC, Hoover RN, Ziegler RG. Childhood soy intake and breast cancer risk in Asian American women. Cancer Epidemiol Biomarkers Prev 2009;18:1050-9.
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8. Peng JH, Zhang F, Zhang HX, Fan HY. Prepubertal octylphenol exposure up-regulate BRCA1 expression, down-regulate ERalpha expression and reduce rat mammary tumorigenesis. Cancer Epidemiol 2009;33:51-5.
9. Russo J, Balogh GA, Russo IH. Full-term pregnancy induces a specific genomic signature in the human breast. Cancer Epidemiol Biomarkers Prev 2008;17:51-66.
10. Russo J, Mailo D, Hu YF, Balogh G, Sheriff F, Russo IH. Breast differentiation and its implication in cancer prevention. Clin Cancer Res 2005;11:931s-6s.