Abstract
Soy isoflavones are considered to have potentially beneficial effects for preventing Western diseases including hormone-dependent cancer and heart disease. In soybean and soy foods, glycoside forms of isoflavones are dominant over their aglycones, while aglycones are mostly dominant in fermented soy foods. As the glycoside forms of isoflavones are not absorbed intact, initial hydrolysis of the sugar moiety is required for their bioavailability. β-Glucosidase activity of the intestinal flora is thought to play a role in the hydrolysis of the glucosides of isoflavones. Absorbed soy isoflavones are conjugated in the liver and excreted into bile. They are deconjugated in the intestine and then go into enterohepatic circulation. The activity of β-glucuronidase of intestinal flora plays an important role in this deconjugation. Isoflavone aglycones are further metabolized in the intestine to either more estrogenic metabolites or inactive compounds. The metabolic activities of intestinal flora are essential for this metabolism of the isoflavones. Great inter-individual difference is reported in the production of these metabolites and the difference in the composition of intestinal flora is thought to be responsible for this variation. The variation in the metabolism of soy isoflavones might cause differences in the beneficial effects of soy foods. There are few reports on bacteria which are active in producing estrogenic metabolites.
