The bioavailability of non-nutrient plant factors: dietary flavonoids and phyto-oestrogens

The bioavailability in human subjects of non-nutrient plant factors, includ ing dietary flavonoids and phyto-oestrogens, is of great importance relativ e to their reported health protective effects. These effects include protec tion against heart disease, and also in the case of the phyto-oestrogens, h ormone-dependent cancers. Epidemiological studies have shown flavonoid inta ke (mostly quercetin) to be inversely associated with mortality from CHD. Q uercetin is a potent antioxidant in vitro, and protection against the oxida tive damage to LDL implicated in atherogenesis has been suggested as a poss ible mechanism. Human subjects can absorb significant amounts of quercetin (particularly in the glucoside form) and it would appear to be sufficiently bioavailable to act as an antioxidant in vivo; however, following our rece nt study (J O'Reilly, TAB Sanders and H Wiseman, unpublished results), it i s currently less clear whether quercetin really can act as an antioxidant i n vivo. The isoflavone phyto-oestrogens genistein and daidzein are much les s effective antioxidants than quercetin in vitro, however, they are well-ab sorbed by human subjects and appear to be sufficiently bioavailable to act as antioxidants in vivo. In our recent study (O'Reilly et al. 1998) lower p lasma isoprostane concentrations and increased resistance of LDL to oxidati on were observed following the high-isoflavone dietary phase compared with the low-isoflavone dietary phase. Considerable inter-individual variation i n isoflavone metabolite excretion has been observed, in particular the prod uction of equol (the gut bacterial metabolite of daidzein; a more potent an tioxidant and more oestrogenic than daidzein), and this appears to be influ enced by habitual diet. Further studies on the bioavailability of these non -nutrient plant factors and related influencing factors are clearly still r equired.