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

The bioavailability in human subjects of non-nutrient plant factors, including dietary flavonoids and phyto-oestrogens, is of great importance relative to their reported health protective effects. These effects include protection against heart disease, and also in the case of the phyto-oestrogens, hormone-dependent cancers. Epidemiological studies have shown flavonoid intake (mostly quercetin) to be inversely associated with mortality from CHD. Quercetin is a potent antioxidant in vitro, and protection against the oxidative damage to LDL implicated in atherogenesis has been suggested as a possible 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to act as an antioxidant in vivo; however, following our recent study (J O'Reilly, TAB Sanders and H Wiseman, unpublished results), it is currently less clear whether quercetin really can act as an antioxidant in vivo. The isoflavone phyto-oestrogens genistein and daidzein are much less effective antioxidants than quercetin in vitro, however, they are well-absorbed 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 plasma isoprostane concentrations and increased resistance of LDL to oxidation were observed following the high-isoflavone dietary phase compared with the low-isoflavone dietary phase. Considerable inter-individual variation in isoflavone metabolite excretion has been observed, in particular the production of equol (the gut bacterial metabolite of daidzein; a more potent antioxidant and more oestrogenic than daidzein), and this appears to be influenced by habitual diet. Further studies on the bioavailability of these non-nutrient plant factors and related influencing factors are clearly still required.