An estrogen model: the relationship between body mass index, menopausal status, estrogen replacement therapy, and breast cancer risk.

We present a mathematical model that lends support to the hypothesis that estrogen levels mediate the complex relationship between body mass index (BMI), menopausal status, estrogen-only hormone replacement therapy (HRT), and breast cancer risk. The model predicts a decrease in the relative risk of breast cancer of 3% per unit increase in BMI (kg/m2) for premenopausal women and an increase in the relative risk of 4% per unit increase in BMI for postmenopausal women who are not HRT users. When comparing postmenopausal women who use estrogen-only HRT to postmenopausal women who do not use HRT, the model predicts an increased risk of breast cancer associated with use of estrogen that diminishes with increasing BMI, with a relative risk of 1.6 for women with BMI of 18, 1.2 for women with BMI of 25, and 1.0 for women with BMI ≥ 30. Model predictions agree with data from five major epidemiological studies.