The group from Karolinska Institutet explained that, "There is often conflicting information about the beneficial effects of coffee when we compared our results to that of a German study we discovered that their data showed the same trend, but the relationship was much weaker. We suggest that this may have something to do with the way the coffee was prepared, or the type of bean preferred. It is unlikely that the protective effect is due to phytoestrogens present in coffee since there was no reduction in the incidence of ER-positive cancer in this study."
So while it is evident that coffee may have beneficial effects in protecting women from ER negative breast cancer the exact mechanism and compounds involved are not yet clear and not all types of coffee are the same.
Source: Eurekalert