Perhaps the most common hypothesis in the epidemiologic literature is that alcohol increases breast cancer risk via alterations in circulating estrogen levels (Chen et al., 2011; Singletary and Gapstur, 2001). Consistent with this possibility, (Li et al., 2010) found a relationship between estrogen receptor (ER) positive tumors and alcohol drinking in postmenopausal women. Therefore, a possible explanation for the relationship between moderate alcohol consumption and breast cancer in postmenopausal women is that ongoing alcohol consumption could increase the growth rate, or affect other properties of ER positive cancers (Tyson et al., 2011), thereby increasing the probability that they would be diagnosed during the course of the study. A problem with this mechanism is that, as discussed below, moderate alcohol consumption does not increase serum estrogen levels in postmenopausal women.
