Isoflavones belong to a large group of phytoestrogens that possess the most potent estrogenic activity. Raw soybeans contain 1.2 – 4.2 mg/g dry weight of isoflavones mainly genistein, daidzein and their conjugates (Kurzer and Xu, 1997). In ruminants, isoflavones are highly metabolized by rumen microorganisms that convert daidzein into equol and genistein into p-ethyl-phenol (Lundh, 1995). Isoflavones and their metabolites can be excreted in milk, thus, soybean products used in dairy cows feeding may represent, via milk, a source of isoflavones in the human diet. Isoflavones are particularly important due to their diverse pharmacological and antioxidant properties because they reduce the risk of several types of cancers including the breast, prostate and colon (Messina and Barnes, 1991), heart disease, menopausal symptoms (Chiechi, 1999) and bone health (Anderson and Garner, 1997).
The objective of this study was to determine the effect of dietary soybean isoflavones on their concentration and total output in bovine milk.