Abstract
B55
The role of soy isoflavones in preventing recurrent breast cancer is not well understood. Soy isoflavones are similar in structure to endogenous estrogens and have been demonstrated in vitro to have weak estrogenic, anti-estrogenic, antioxidant and anti-proliferative properties. Breast cancer survivors sometimes increase their soy intake post-diagnosis, through diet and supplement use, to prevent cancer recurrence or to alleviate post-menopausal symptoms when used as an alternative to hormone replacement therapy. Therefore, we sought to characterize soy isoflavone intake and assess its role in breast cancer recurrence among a cohort of 1,971 female breast cancer survivors enrolled in the Life After Cancer Epidemiology (LACE) study who completed a soy food frequency questionnaire (FFQ). Genistein and daidzein intakes, the primary soy isoflavones, were assessed through a self-administered modified Block FFQ and a supplemental soy FFQ, on average 23 months post-diagnosis. The use of soy supplements was also recorded. Delayed entry Cox proportional hazards models were used to generate hazard ratios (HR) and 95% confidence intervals (CI) to describe the effect of soy intake on the time to recurrence, adjusted for (yes/no) soy supplement use, body mass index (BMI) 1 year before diagnosis, menopausal status, tobacco pack-years, tumor stage, hormone receptor status, age, and race. The mean (+ standard deviation) genistein and daidzein intakes among the LACE cohort were 2.4 (+ 7.1) and 1.7 (+ 4.9) milligrams per day, respectively, and were collinear (r = 0.997). 53 women reported soy supplement use. Women in the highest category of genistein intake (> 13 mg/day) had a suggested decreased risk of recurrence when compared to those who had no genistein intake (HR = 0.88; 95%CI: 0.45-1.74; p for trend = 0.20). This effect was modified by estrogen (ER) and progesterone (PR) receptor status (p for interaction = 0.04) but not significantly by menopausal status (p for interaction = 0.80). However, there was suggestion of a protective effect of high genistein intake among postmenopausal women (n = 1,211) (HR = 0.75, 95%CI: 0.29-1.90, p for trend = 0.07) and among women with ER or PR positive tumors (HR = 0.47, 95%CI: 0.17-1.32, p for trend = 0.15). The results were similar when modeling daidzein intake. One suggested mechanism for the selective effect of soy in hormone receptor positive tumors is that soy isoflavones may bind to hormone receptor positive tumors to exert their anti-estrogenic effect, thereby decreasing the risk of recurrence. We conclude that high genistein intake, comparable to levels consumed in Asian populations, may reduce the risk of recurrence in women who have hormone receptor positive tumors.
Sixth AACR International Conference on Frontiers in Cancer Prevention Research-- Dec 5-8, 2007; Philadelphia, PA