Soy foods made from whole soybeans or isolated and purified soy proteins all contain relatively high concentrations of isoflavones, primarily in the form of various β-glycoside conjugates (Coward et al. 1993, Murphy 1982). A plethora of soy isoflavone extracts and supplements are now commercially available; however, little is known concerning their metabolism and effects. Much is known about the general metabolism of isoflavones in animals and humans, but only limited information exists on pharmacokinetics (King and Bursill 1998). When ingested, these conjugated isoflavones undergo hydrolysis by β-glucosidases in the jejunum, releasing the principal bioactive aglycones, daidzein and genistein. Further metabolism takes place in the distal intestine with the formation of specific metabolites (Joanneau et al. 1995). Although the latter enzymes are deficient in early life, there is adequate β-glucosidase activity to make isoflavones bioavailable to infants consuming soy formulas (Setchell et al. 1997). The aglycones and any bacterial metabolites are absorbed from the intestinal tract and conjugated mainly to glucuronic acid; then they undergo enterohepatic recycling. Intestinal metabolism is essential for their subsequent absorption and bioavailability (Setchell et al. 1984) because there is no evidence to support absorption of the conjugated forms of isoflavones. Furthermore, it is the aglycones that show an affinity for estrogen receptors and have other nonhormonal effects on the cell machinery.