Evidence for lack of absorption of soy isoflavone glycosides in humans, supporting the crucial role of intestinal metabolism for bioavailability

Background: The isoflavones daidzein and genistein occur naturally in most soyfoods, conjugated almost exclusively to sugars. Controversy exists regarding the extent of bioavailability of isoflavone glycosides, and the mechanism of intestinal absorption of isoflavones in humans is unclear. Evidence from intestinal perfusion and in vitro cell culture studies indicates that isoflavone glycosides are poorly absorbed, yet isoflavones are bioavailable and appear in high concentrations in plasma, irrespective of whether they are ingested as aglycones or glycoside conjugates. Objective: The objective was to determine whether isoflavone glycosides are absorbed from the intestine intact and reach the peripheral circulation unchanged. Design: Plasma was collected at timed intervals before and after healthy adults ingested 50 mg of one of the isoflavone beta-glycosides (daidzin or genistin) or 250 mL soymilk containing mainly isoflavone glycosides. Electrospray ionization mass spectrometry was used to detect daidzin and genistin after solid-phase extraction of these conjugates from plasma. Bioavailability of isoflavones was confirmed by gas chromatography-mass spectrometry analysis. Results: Specific and sensitive electrospray mass spectrometry failed to detect even traces of daidzin or genistin in plasma collected 1, 2, and 8 h after their ingestion as pure compounds or in a soyfood matrix. However, plasma was enriched in isoflavones that were hydrolyzable with a combined beta-glucuronidase and sulfatase enzyme preparation. Conclusion: Isoflavone glycosides are not absorbed intact across the enterocyte of healthy adults, and their bioavailability requires initial hydrolysis of the sugar moiety by intestinal beta-glucosidases for uptake to the peripheral circulation.