Abstract 4665: Postdiagnosis soy food intake and breast cancer survival: Report from the After Breast Cancer Pooling Project

Abstract

Abstract Background: Soy constituents possess anti-estrogenic and other anti-cancer properties. Soy food consumption has been associated with lower risk of development of breast cancer. However, the ability of soy isoflavones to bind to estrogen receptors, stimulate cell proliferation, and potentially interact with tamoxifen therapy, has raised concern about soy food consumption among breast cancer survivors. Two recent reports from China and one report from the US have suggested that postdiagnosis soy intake may improve prognosis among breast cancer survivors. Methods: We evaluated postdiagnosis soy food intake and breast cancer outcomes using data from the After Breast Cancer Pooling Project, which includes 18,312 women diagnosed with invasive primary breast cancer between ages 20 and 83 years and who participated in four prospective cohorts, three from the US and one from Shanghai, China. In two cohorts, information on soy food intake was assessed using food frequency questionnaires designed to capture soy food intake; two cohorts only collected information on two soy foods (tofu and soy milk), and multiple imputation was applied to derive total isoflavone intake based on information gathered by the US study with comprehensive assessment of soy intake. Delayed entry Cox proportional hazards models were used to estimate hazard ratios (HRs) and 95% confidence intervals (CIs) for associations of isoflavones with total mortality and breast cancer recurrence, adjusted for age at diagnosis, estrogen receptor/progesterone receptor status, stage, chemotherapy, radiotherapy, hormonal therapy, smoking, body mass index, exercise, race/ethnicity, and education and stratified by study. Analyses included 16,048 women with information on soy food intake. Results: After an average follow-up of 9.2 years, a total of 2,592 deaths and 2,622 recurrences were documented. Mean soy isoflavone intake (mg), as assessed at an average of 13 months postdiagnosis (range: 1-48 months), was 3.2 (standard deviation (SD)=5.8) for US women and 45.9 (SD=38.3) for Chinese women. Pooled analyses based on study-specific deciles showed that, compared with the lowest deciles, higher isoflavone intakes were associated with HRs in the range of 0.85-0.89 for mortality and 0.85-0.83 for recurrence (not all HRs were significant). Corresponding HRs restricted to only US cohorts ranged from 0.84-1.04 for mortality and 0.87-0.88 for recurrence, although none were significant. Using common cut-points for the pooled data including all cohorts, HRs (95% CIs) for the highest intake category (>23 mg/day) vs. the lowest (≤0.48 mg/day) were 0.91 (0.70-1.19) for mortality and 0.85 (0.66-1.11) for recurrence. Additional analyses conducted among non-Asian US women showed a similar pattern. Conclusions: Soy food consumption was not associated with an increased risk of mortality or cancer recurrence among breast cancer survivors. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 4665. doi:10.1158/1538-7445.AM2011-4665