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Mechanisms and Biomarkers (WG 4) page 18
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an alternative way to prevent food spoilage than the one using synthetic antioxidants
(Aeschbach et al., 1994).
Polyphenols and flavonoids - Polyphenols are complex groups of secondary metabolites
found in high levels in plants. The proanthocyanidins are known to be formed of flavan-3-ol
units and to form complexes with proteins and carbohydrates but their antioxidant activity
remains to be further established.
Flavonoids are composed of 6 classes (flavanones, flavones, flavonols, isoflavonoids,
anthocyanins and flavans) varying around the heterocyclic oxygen ring (Peterson and Dwyer,
1998). Honey, red wine, onions, apples, citrus, chocolate and sweets that contain some plant
constituents have flavonoids. Dietary intake estimates vary from 23 mg/day to 1000 mg/day
(Hertog et al., 1993). Absorption and metabolism of flavonoids are not still clearly
established. They are metabolised in the liver, small intestine or kidney and by the colonic
bacteria. The resulting conjugated and phenolic acids are excreted into bile, urine or feces
(Hollman et al., 1995) They are characterised by antioxidant but also antimutagenic activities
(Shimoi et al., 1994). Following ingestion of foods containing polyphenols, plasma
antioxidant capacity increased and lipoprotein oxidation decreased (Ursini et al., 1999).
Flavonoids are reported to scavenge efficiently the reactive oxygen- and nitrogen-species, and
also to chelate metal ions. Many studies were done on the structure-antioxidant activity
relationships with different structurally related compounds (Van Acker et al., 1996). In vitro
studies (Bors et al., 1995) using electron pulse radiolysis and cyclic voltametry approaches,
established a positive correlation between inhibition of enzymatic and non enzymatic lipid
peroxidation and the oxidation potentials (up to 200 mV) of the flavonoids. For some
flavonoids a higher antioxidant effect was observed when used against metal ion-induced
peroxidation suggesting that metal chelation plays a major role in the antioxidant capacity of
at least some of the flavonoids. The catechol moiety and/or some OH groups positioned at
particular positions (positions 3 and 5) give the capacity to chelate metals. Altogether these
data indicated that antioxidant activity of the different flavonoids depend on their chemical
structure: the pyrogallol group, the carbonyl group in conjugation with particular hydroxyl
groups may increase the electron delocalization and thereby stabilising the radical form of the
flavonoid, conjugation of the pyran ring found in anthocyanins also increases the stabilisation
of the radicals formed, a free hydroxyl group an position C3 may increase the lipophilicity of
the compound. Case control studies suggest that flavonoids may reduce cardiovascular risk
and stroke (reviewed by Hertog, 1998).