The study, published in this month's issue of Cancer Epidemiology Biomarkers & Prevention , adds to an ever-growing body of studies linking the phytochemicals to improved breast health.
Indeed, population studies have shown that a diet rich in soy is associated with fewer cases of breast cancer, linked to the presence of soy isoflavones.
China has the world's lowest incidence and mortality from breast cancer - a disease that has over one million new cases every year worldwide.
The new study, led by Johanna Lampe from the Fred Hutchinson Cancer Research Center, Seattle, indicates that benefits may also extend to fibrocystic breast conditions, a very common and benign condition characterised by lumpiness and discomfort in one or both breasts.
The researchers recruited 196 women with breast cancer, 304 women with benign breast condition, and 1,002 healthy, breast cancer-free age-matched controls in Shanghai, China.
The benign conditions were further classified as proliferative (173 women) or nonproliferative (131 women).
Increased plasma levels of the isoflavones genistein and daidzein were associated with a reduced risk of both types of benign conditions, in addition to breast cancer.
Indeed, the highest plasma levels of genistein (more than 76.95 nanograms per millilitre) were 74 per cent less likely to have breast cancer, and 60 per cent less likely to have benign conditions, relative to women with the lowest average levels (less than 9.42 ng/mL).
"Isoflavone exposure was inversely associated with fibrocystic breast conditions and breast cancer, and the results suggest that effects on cancer risk occur early in carcinogenesis," wrote Lampe.
Further research is necessary to further explore these potential benefits of soy, with epidemiological studies in populations with lower soy consumption necessary.
Intervention trials could also add support to the potential link.
The research does fit with a growing body of research that supports the potential cancer-protecting properties of soy.
A recent animal study published in the journal Cancer Research (Vol. 66, Issue 2) reported that high dietary intake of soy protected against breast cancer in postmenopausal monkeys.
This supports another study from the University of Ulster that focussed on the inverse link between soy and breast cancer.
In this study, funded by the EU's "Quality of Life and Management of Living Resources" project, soy isoflavones were reported to inhibit breast cancer cell invasion in vitro.
Source: Cancer Epidemiology Biomarkers & Prevention December 2007, Volume 16, Pages 2579-2586, doi: 10.1158/1055-9965.EPI-07-0368 "Plasma Isoflavones and Fibrocystic Breast Conditions and Breast Cancer Among Women in Shanghai, China" Authors: J.W. Lampe, Y. Nishino, R.M. Ray, C. Wu, W. Li, M.-G. Lin, D.L. Gao, Y. Hu, J. Shannon, H. Stalsberg, P.L. Porter, C.L. Frankenfeld, K. Wahala, and D.B. Thomas