High estrogen levels, unhealthy estrogen metabolites, and cumulative number of menstrual periods are all believed to be contributors to unhealthy breast and/or endometrial tissues. Scientists discovered that Asian women eating a soy-rich diet have excellent breast and endometrial health. This observation certainly doesn't prove that soy consumption is the cause for the differences between Asian and American women, but it has led to much speculation and medical research in this health area.

Soy is not proven to prevent or cure cancer. Soy may support better breast and endometrial health by favorably altering estrogen levels, estrogen metabolism and increasing length of time between periods.(1-9) Recent studies suggest that adequate daily soy consumption lowers circulating estrogen levels in premenopausal women,(1-2) while increasing time between periods by 1-3 days (thus fewer periods during a lifetime).(2) In both postmenopausal and premenopausal women, a soy-rich diet favorably shifts the body's estrogen metabolism to produce smaller amounts of unhealthy estrogen metabolites.(3-5) Interestingly, the National Institutes of Health (NIH) has recently found that prescription estrogen therapy alone does not increase breast cancer risk (only when used in combination with progestin).

The FDA did not find evidence to warn any group of patients regarding soy use when it approved a heart health claim for soy protein in 1999 (i.e. soy protein lowers risk of heart disease). Critics told the FDA soy may cause harm because it contains a chemical similar to the female hormone, estrogen, that might disrupt normal hormone levels. The FDA said the concerns were not supported by conclusive scientific research. While chemicals in soy do exert hormonal effects, the impact is "very limited" and much lower than that of natural or synthetic estrogens.

As always, women are advised to consult their physician about this research if you are interested in breast and endometrial health, particularly if she have a personal or family history of cancer.

References:
1. Lu LJ, et al. Decreased ovarian hormones during a soya diet: implications for breast cancer prevention. Cancer Res 2000 Aug 1;60(15):4112-21.
2. Cancer. 2002 Feb 15;94(4):1166-74. The specific role of isoflavones on estrogen metabolism in premenopausal women. Kumar NB,et al.
3. Cancer Res 2000 Mar 1;60(5):1299-305. Increased urinary excretion of 2-hydroxyestrone but not 16alpha-hydroxyestrone in premenopausal women during a soya diet containing isoflavones. Lu LJ, et al.
4. Xu X, Duncan AM, Merz BE, Kurzer MS, Effects of soy isoflavones on estrogen and phytoestrogen metabolism in premenopausal women. Cancer Epidemiology Biomarkers Prev 1998 Dec;7(12):1101-8.
5. Cancer Epidemiol Biomarkers Prev 2000 Aug;9(8):781-6. Soy consumption alters endogenous estrogen metabolism in postmenopausal women. Xu X, Duncan AM, Wangen KE, Kurzer MS.
6. Eur J Cancer Prev 2003 Apr;12(2):165-9. Mammographic densities in a one-year isoflavone intervention. Maskarinec G, Williams AE, Carlin L.
7. Wu AH, et al.. 2002. Soy intake and other lifestyle determinants of serum estrogen levels among postmenopausal Chinese women in Singapore.. Cancer Epidemiol Biomarkers Prev Sep;11(9):844-51.
8. J Natl Cancer Inst. 2003 Aug 6;95(15):1158-64. Phytoestrogen intake and endometrial cancer risk. Horn-Ross PL, et al.
9. American Journal of Epidemiology 1997 Aug 15;146(4):294-306. Association of soy and fiber consumption with the risk of endometrial cancer. Goodman MT, et al.