Results from epidemiological and experimental studies indicate that
phytoestrogens may protect against
breast cancer. Because one of the
biological effects of
phytoestrogens is probably estrogenic, it's possible that the preventive effect on
breast cancer differs by
estrogen receptor (ER) or
progesterone receptor (PR) status of the
tumor. We evaluated the associations between dietary
phytoestrogen (isoflavonoids,
lignans, and
coumestrol) intake and risk of
breast cancer and whether the ER/PR statuses of the
tumor influence this relationship. In 1991-2 a prospective population-based cohort study among Swedish pre- and postmenopausal women was performed, making questionnaire data available for 45,448 women. A total of 1014 invasive breast
cancers were diagnosed until December 2004. Cox proportional hazards models were performed to estimate multivariate risk ratios, 95% CI for associations with risk of
breast cancer. Intakes of
lignan, isoflavonoid, or
coumestrol were not associated with
breast cancer risk overall or before or after 50 y of age. The effects of
lignans or isoflavonoids were independent of receptor status. However, intake of
coumestrol was associated with decreased risk of receptor negative
tumors (ER-PR-) but not positive
tumors. The risk of ER-PR-
tumors was significantly lower (50%) in women with intermediate
coumestrol intake compared with those who did not consume any. In conclusion, we found no association between intake of isoflavonoids or
lignans and
breast cancer risk. Our results of a decreased risk of ER-PR-
tumors in women with intermediate intake of
coumestrol could be due to chance because of the low intake. The results should be confirmed in other studies.