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Adult Soy Intake
Several well conducted case-control studies show protective effects of soy consumption against breast cancer. In particular, investigations in Shanghai (1), in Japan (2), and among Asian-Americans (3) observed significant protective effects in subjects with high soy intake. A meta-analysis of 18 studies that investigated soy foods and breast cancer risk yielded a statistically significant risk estimate of 0.86 that translates into a 14% lower risk for women with high intake as compared to subjects reporting little or no soy food consumption (4). The association was similar in Asian and Western studies, but slightly stronger among pre- than postmenopausal women. In a study conducted in California, the reduction in risk was primarily observed in women born in Asia (4). When urinary isoflavonoid excretion was used as a biomarker for soy intake, several studies reported lower levels in breast cancer cases than in controls (5-6). In a cross-sectional study design, a number of investigations examined circulating hormone levels and mammographic densities as biomarkers of breast cancer risk. Serum estrogen levels were found to be lower among women in the higher soy intake categories in reports from Japan but not among British women (7-8). An investigation from Singapore reported lower breast density among women with higher soy intake while a study from Hawaii did not observe such a relation (9-10).


Early Life Soy Intake
Migrant studies indicate that it takes 2-3 generations for women moving from a low risk to a high risk country to adopt the risk of the host country. After 100 years in Hawaii, Japanese-Americans now have a breast cancer risk close to that of Caucasian women. Therefore, some etiologic factors may act during childhood or adolescence (11). So far, only two case-control studies explored the effects of soy intake early in life by asking the subjects about food consumption during adolescence. The Shanghai Breast Cancer Study reported an inverse relation between adolescent soy intake and breast cancer risk with a 50% lower risk for the highest compared to the lowest quintile of soy intake (12). Similarly, a study among Asian-Americans in California showed that women who reported high soy intake during both adolescence and adulthood had a significantly lower risk of developing breast cancer, while women who consumed tofu only during adult life had an intermediate reduction in breast cancer risk (13). Possibly, soy isoflavones, due to their weak estrogenic effects, influence breast growth by accelerating differentiation of breast tissue structures similar to an early pregnancy (14). A role of soy during early life appears more likely in light of findings that growth patterns, body mass index, and nutrition during adolescence have been related to adult breast cancer risk (15).


If you already have breast cancer
Concern that soyafoods have the potential to stimulate the growth of existing breast cancer is based on the similar chemical structure between estrogen and isoflavones and on experimental results in rodents (16-17). However isoflavones are very different from estrogen  and they also possess non hormonal properties associated with the inhibition of cell growth. There is however no clinical data that allows definitive conclusions to be drawn about the effect of isoflavones in breast cancer patients.

References

  1. Dai Q, Shu XO, Jin F, Potter JD, Kushi LH, Teas J, Gao YT, Zheng W. Population-based case-control study of soyfood intake and breast cancer risk in Shanghai. Br J Cancer 2001;85:372-378.

  2. Yamamoto S, Sobue T, Kobayashi M, Sasaki S, Tsugane S. Soy, isoflavones, and breast cancer risk in Japan. J Natl Cancer Inst 2003;95:906-913.

  3. Wu AH, Wan P, Hankin J, Tseng CC, Yu MC, Pike MC. Adolescent and adult soy intake and risk of breast cancer in Asian- Americans. Carcinogenesis 2002;23:1491-1496.

  4. Trock BJ, Hilakivi-Clarke L, Clarke R. Meta-analysis of soy intake and breast cancer risk. J Natl Cancer Inst 2006;98:459-471.

  5. den Tonkelaar I, Keinan-Boker L, Veer PV, Arts CJ, Adlercreutz H, Thijssen JH, Peeters PH. Urinary phytoestrogens and postmenopausal breast cancer risk. Cancer Epidemiol Biomarkers Prev 2001;10:223-228.

  6. Dai Q, Franke AA, Jin F, Shu XO, Hebert JR, Custer LJ, Cheng J, Gao YT, Zheng W. Urinary excretion of phytoestrogens and risk of breast cancer among Chinese women in Shanghai. Cancer Epidemiol Biomarkers Prev 2002;11:815-821.

  7. Nagata C, Kabuto M, Kurisu Y, Shimizu H. Decreased serum estradiol concentration associated with high dietary intake of soy products in premenopausal Japanese women. Nutr Cancer 1997;29:228-233.

  8. Verkasalo PK, Appleby PN, Davey GK, Key TJ. Soy milk intake and plasma sex hormones: a cross-sectional study in pre- and postmenopausal women (EPIC-Oxford). Nutr Cancer 2001;40:79-86.

  9. Jakes RW, Duffy SW, Ng FC, Gao F, Ng EH, Seow A, Lee HP, Yu MC. Mammographic parenchymal patterns and self-reported soy intake in Singapore Chinese women. Cancer Epidemiol Biomarkers Prev 2002;11:608-613.

  10. Maskarinec G, Meng L. An investigation of soy intake and mammographic characteristics in Hawaii. Breast Cancer Res 2001;3:134-141.

  11. Kolonel LN, Wilkens L.R. Migrant Studies. In: Schottenfeld D, Fraumeni JF, editors. Cancer Epidemiology and Prevention. New York: Oxford University Press, 2006: 189-201.

  12. Shu XO, Jin F, Dai Q, Wen W, Potter JD, Kushi LH, Ruan Z, Gao YT, Zheng W. Soyfood intake during adolescence and subsequent risk of breast cancer among Chinese women. Cancer Epidemiol Biomarkers Prev 2001;10:483-488.

  13. Wu AH, Wan P, Hankin J, Tseng CC, Yu MC, Pike MC. Adolescent and adult soy intake and risk of breast cancer in Asian- Americans. Carcinogenesis 2002;23:1491-1496.

  14. Hilakivi-Clarke L, Onojafe I, Raygada M, Cho E, Skaar T, Russo I, Clarke R. Prepubertal exposure to zearalenone or genistein reduces mammary tumorigenesis. Br J Cancer 1999;80:1682-1688.

  15. Michels KB, Willett WC. Breast cancer--early life matters. N Engl J Med 2004;351:1679-1681.

  16. Messina MS, Loprinzi CL. Soy for breast cancer survivors: a critical review of the literature. J Nutr 2001; 131(11): 3095S-108S

  17. Bouker KB, Hilakivi-Clarke L. Genistein: Does it prevent or promote breast cancer?Environ Health Perspect 2000; 108(8):701-708