Flavonoids
Authors: Miranda Cristobal L , Maier Claudia S , Stevens Jan F 
Published: April 2013

The flavonoids form a group of approximately nine thousand plant metabolites. Chemically, they can be classified as polyphenols or phenolics. Subgroups of flavonoids include flavones, flavonols, flavanols, flavanones, chalcones, aurones, isoflavones, anthocyanins, and proanthocyanidins. Subgroup representatives vary structurally by their oxygenation, methylation, prenylation, and glycosylation pattern. Their name is derived from ‘flavus’ (Greek, meaning ‘yellow’), indicating that many representatives are yellow plant pigments. Physiologically, flavonoids play roles in allelopathy, attraction of pollinators, protection against damage from sunlight, protection against herbivores and microbes, and in plant growth and development. Many health effects of fruits, vegetables, and dietary supplements (nutraceuticals) have been attributed to the presence of flavonoids. Flavonoids exhibit antioxidant, anti‐inflammatory, anticancer, antiobesity, cardioprotective, and neuroprotective activities. Modulation of cell signalling pathways rather than antioxidant activity may explain the health‐promoting effects of flavonoids

in vivo

. Most flavonoids are poorly absorbed, extensively metabolised and primarily excreted as glucuronides and sulfates.

Key Concepts:

Flavonoids are synthesised by virtually all green plants except algae. They are biosynthesised from phenylalanine and three molecules of malonic acid to form chalconaringenin, from which virtually all other flavonoid skeleta are derived, that is, flavanones, flavones, flavonols, aurones, isoflavones, anthocyanins, and proanthocyanidins.

In plants, flavonoids are important for growth and development, attraction of pollinator animals, nitrogen‐fixation in leguminous plants, and for protection against damage by herbivores, microbes, UV and reactive oxygen species.

Flavonoids ingested by animals and humans are mainly in the form of glycosides which are hydrolysed in the intestines. The resulting free forms (aglycones) can be degraded to phenolic acids or conjugated with glucuronic acid, sulfate, and methyl groups by the intestinal microflora and the liver.

Plasma and intracellular concentrations of flavonoids are low because of poor absorption, extensive metabolism and quick excretion from the body. Isoflavones are the most bioavailable flavonoids in humans.

Flavonoids, as aglycones or conjugate metabolites, may confer health benefits related to cancer, inflammatory disease, cardiovascular disease, neurological disorders, metabolic syndrome, obesity and osteoporosis. Epidemiological studies show that consumption of flavonoid‐rich foods is associated with reduced risk of developing cancer and cardiovascular disease.

Modulation of cell signalling pathways via NF‐κB, Nrf2 and protein kinases, rather than antioxidant activity, may mediate most of the biological activities of flavonoids

in vivo

. The direct antioxidant activity of flavonoids has not been established in humans.

Flavonoids may interfere in the absorption and metabolism of drugs and nutrients but they themselves are of low or negligible toxicity at the concentrations found in foods.

Clinical studies are needed to better understand the pharmacokinetics and health‐promoting effects and side effects, if any, of flavonoids when used as dietary supplements or as pure compounds in pharmacological doses.