Plant lignans associated with high fiber foods such as cereal brans and beans are the principal precursor to mammalian lignans which have an ability to bind to human estrogen sites. Soybeans are a significant source of mammalian lignan precursor secoisolariciresinol containing 13–273 µg/100 g dry weight. Another phytoestrogen in the human diet with estrogen activity is coumestans, which are found in beans, split-peas, with the best sources being alfalfa, clover, and soybean sprouts. Coumestrol, an isoflavone coumarin derivative, is the only coumestan in foods. Soybeans and processed soy foods are among the richest foods in total phytoestrogens (wet basis per 100 g), which are present primarily in the form of the isoflavones daidzein and genistein. Human breast milk-containing or cow milk formula-containing diets provide 0.005–0.01 mg of isoflavone per day, while soy-based infant formulas provide 6–47 mg of isoflavone daily. This intake is several orders of magnitude greater than those provided by other nutrients. Although considerable research has examined the potential for soy consumption to lower the risk of breast cancer in women, there is insufficient population evidence to make a conclusion about this relationship, as of 2016. A 2011 meta-analysis stated: "Our study suggests soy isoflavones intake is associated with a significant reduced risk of breast cancer incidence in Asian populations, but not in Western populations." A 2016 review concluded that "current evidence from observational studies and small clinical trials is not robust enough to understand whether soy protein or isoflavone supplements may help prevent or inhibit the progression of prostate cancer." A 2010 review showed that neither soy foods nor isoflavone supplements alter measures of bioavailable testosterone or estrogen concentrations in men. Soy consumption has been shown to have no effect on the levels and quality of sperm. Meta-analyses on the association between soy consumption and prostate cancer risk in men concluded that dietary soy may lower the risk of prostate cancer. Rigorous dietary clinical research in people with cancer is inconclusive, as of 2018. One review noted that soy based foods may inhibit absorption of thyroid hormone medications required for treatment of hypothyroidism. A 2015 scientific review by the European Food Safety Authority concluded that intake of isoflavones from supplements did not affect thyroid hormone levels in postmenopausal women.