Endogenous Estrogens, Estrogen Metabolites, and Breast Cancer Risk in Postmenopausal Chinese Women.

BACKGROUND The role of estrogen metabolism in determining breast cancer risk and differences in breast cancer rates between high-incidence and low-incidence nations is poorly understood. METHODS We measured urinary concentrations of estradiol and estrone (parent estrogens) and 13 estrogen metabolites formed by irreversible hydroxylation at the C-2, C-4, or C-16 positions of the steroid ring in a nested case-control study of 399 postmenopausal invasive breast cancer case participants and 399 matched control participants from the population-based Shanghai Women's Health Study cohort. Odds ratios (ORs) and 95% confidence intervals (CIs) of breast cancer by quartiles of metabolic pathway groups, pathway ratios, and individual estrogens/estrogen metabolites were estimated by multivariable conditional logistic regression. Urinary estrogen/estrogen metabolite measures were compared with those of postmenopausal non-hormone-using Asian Americans, a population with three-fold higher breast cancer incidence rates. All statistical tests were two-sided. RESULTS Urinary concentrations of parent estrogens were strongly associated with breast cancer risk (ORQ4vsQ1 = 1.94, 95% CI = 1.21 to 3.12, Ptrend = .01). Of the pathway ratios, the 2-pathway:total estrogens/estrogen metabolites and 2-pathway:parent estrogens were inversely associated with risk (ORQ4vsQ1 = 0.57, 95% CI = 0.35 to 0.91, Ptrend = .03, and ORQ4vsQ1 = 0.61, 95% CI = 0.37 to 0.99, Ptrend = .04, respectively). After adjusting for parent estrogens, these associations remained clearly inverse but lost statistical significance (ORQ4vsQ1 = 0.65, 95% CI = 0.39 to 1.06, Ptrend = .12 and ORQ4vsQ1 = 0.76, 95% CI = 0.44 to 1.32, Ptrend = .28). The urinary concentration of all estrogens/estrogen metabolites combined in Asian American women was triple that in Shanghai women. CONCLUSIONS Lower urinary parent estrogen concentrations and more extensive 2-hydroxylation were each associated with reduced postmenopausal breast cancer risk in a low-risk nation. Markedly higher total estrogen/estrogen metabolite concentrations in postmenopausal United States women (Asian Americans) than in Shanghai women may partly explain higher breast cancer rates in the United States.