Flavonoids are a class of secondary plant phenolics with significant antioxidant and
chelating properties. In the human diet, they are most concentrated in fruits, vegetables,
wines, teas and cocoa. Their cardioprotective effects stem from the ability to inhibit
lipid peroxidation, chelate redox-active metals, and attenuate other processes involving
reactive oxygen species. Flavonoids occur in foods primarily as glycosides and polymers
that are degraded to variable extents in the digestive tract. Although metabolism
of these compounds remains elusive, enteric absorption occurs sufficiently to reduce
plasma indices of oxidant status. The propensity of a flavonoid to inhibit free-radical
mediated events is governed by its chemical structure. Since these compounds are based
on the flavan nucleus, the number, positions, and types of substitutions influence
radical scavenging and chelating activity. The diversity and multiple mechanisms of
flavonoid action, together with the numerous methods of initiation, detection and
measurement of oxidative processes in vitro and in vivo offer plausible explanations
for existing discrepancies in structure-activity relationships. Despite some inconsistent
lines of evidence, several structure-activity relationships are well established in
vitro. Multiple hydroxyl groups confer upon the molecule substantial antioxidant,
chelating and prooxidant activity. Methoxy groups introduce unfavorable steric effects
and increase lipophilicity and membrane partitioning. A double bond and carbonyl function
in the heterocycle or polymerization of the nuclear structure increases activity by
affording a more stable flavonoid radical through conjugation and electron delocalization.
Further investigation of the metabolism of these phytochemicals is justified to extend
structure-activity relationships (SAR) to preventive and therapeutic nutritional strategies.