Steroid sex hormones play a central role in breast
carcinogenesis. Evidence from in vitro and animal studies suggests that
phytoestrogens may inhibit the development of mammary
tumors through their role in regulating the synthesis, metabolism, and signal transduction of
steroid hormones. In a study of 117 case-control pairs of postmenopausal women in Shanghai, we investigated whether the association between urinary
phytoestrogen excretion and
breast cancer risk may differ by levels of endogenous
steroid sex hormones,
sex hormone binding globulin (SHBG), body mass index (BMI), and waist:hip ratio (WHR). Fasting morning blood and urine samples were collected for the analysis of urinary isoflavonoids and mammalian
lignans, as well as blood levels of SHBG and selected
steroid hormones. For
cancer patients, samples were collected before any
cancer therapy. Conditional logistic regression models were used to estimate odds ratios and 95% confidence intervals after adjusting for potential confounding factors. The inverse associations between urinary
phytoestrogens and
breast cancer risk were found to be more evident among women with a high BMI or WHR than those with a low level of these anthropometric measurements. Although a reduced risk of
breast cancer was observed among women with a high excretion rate of urinary isoflavonoids in all of the strata defined by blood SHBG and
steroid hormones, the inverse association was more pronounced among women with a high blood concentration of
estradiol, a low level of
estrone sulfate, or a low level of SHBG. The risks of
breast cancer were also reduced with increasing excretion rate of mammalian
lignans, although no test for a linear association was statistically significant in stratified analyses. Findings from this study suggest that the potential protective association of
phytoestrogens may be modified by BMI, WHR, and blood levels of SHBG, and
steroid hormones.