Comprehensive Review of Isoflavones and Lignans in the Prophylaxis and Treatment of Breast Cancer

Breast cancer is the second most commonly diagnosed cancer worldwide, with the highest mortality of all cancers. In most cases, breast cancer is estrogen-dependent, which means cancer cells express estrogen receptors. Phytoestrogens, due to their structural similarities with estrogen, are capable of binding to estrogen receptors and are therefore also referred to as selective estrogen receptor modulators. They can be either estrogen agonists or antagonists and influence the risk of breast cancer. Two prominent representatives of phytoestrogens are isoflavones, which are found in soy products, and lignans, which are found in flaxseed, vegetables, and certain nuts. Phytoestrogens may be used in breast cancer prevention. They can reduce the risk of breast cancer incidence and its recurrence, yet mostly in postmenopausal women, and the reasons for this have not yet been explained. Plausible mechanisms of cancer risk-lowering properties of phytoestrogens include antiestrogenic action, influencing estrogen metabolism, sex hormone concentrations, and menstrual cycle length. These actions are not observed in every consumer. Phytoestrogens are found in plants in glycoside form (bound with sugar molecules), and at least part of them must be metabolized to their biologically active metabolites (equol for isoflavones and enterolactone for lignans) by intestinal microbiota. Bacterial strains capable of phytoestrogen metabolism have been identified. This article thoroughly describes phytoestrogens’ metabolism and their associations with breast cancer risk and outcomes.