Flavonoids

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Flavonoids JoAnn Guest Apr 17, 2003 12:31 PDT Medical Abstract Title:

 

Bioavailability and Health Effects of Dietary Flavonols in Man

 

Author:

Hollman PC, Katan MB

Source:

Arch Toxicol Suppl. 1998; 20:237-248.

 

Abstract:

 

Flavonoids are polyphenolic compounds that occur ubiquitously in foods

of plant origin. Over 4,000 different flavonoids have been described,

and they are categorized into flavonols, flavones, catechins,

flavanones, anthocyanidins, and isoflavonoids.

 

Flavonoids have a variety of biological effects in numerous mammalian

cell systems, as well as in vivo.

 

Recently much attention has been paid to their antioxidant properties

and to their inhibitory role in various stages of tumor development in

animal studies.

 

Quercetin, the major representative of the flavonol subclass, is a

strong antioxidant, and prevents oxidation of low density lipoproteins

in vitro. Oxidized low density lipoproteins are atherogenic, and are

considered to be a crucial intermediate in the formation of

atherosclerotic plaques.

 

This agrees with observations in epidemiological studies that the

intake of flavonols and flavones was inversely associated with

subsequent coronary heart disease.

 

However, no effects of flavonols on cancer were found in these studies.

The extent of absorption of flavonoids is an important unsolved problem

in judging their many alleged health effects.

 

Flavonoids present in foods were considered non-absorbable because they

are bound to sugars as beta-glycosides. Only free flavonoids without a

sugar molecule, the so-called aglycones were thought to be able to pass

through the gut wall. Hydrolysis only occurs in the colon by

microorganisms, which at the same time

degrade flavonoids.

 

We performed a study to quantify absorption of various dietary forms of

quercetin.

To our surprise, the quercetin glucosides from onions were absorbed far

better than the pure aglycone.

 

Subsequent pharmacokinetic studies with dietary quercetin glycosides

showed marked differences in absorption rate and bioavailability.

Absorbed quercetin was eliminated only slowly from the blood. The

metabolism of flavonoids has been studied frequently in various animals,

but very few data in humans are available.

 

Two major sites of flavonoid metabolism are the liver and the colonic

flora. There is evidence for O-methylation, sulfation and

glucuronidation of hydroxyl groups in the liver. Bacterial ring fission

of flavonoids occurs in the colon. The subsequent degradation products,

phenolic acids, can be absorbed and are found in urine of animals.

Quantitative data on metabolism are scarce.

 

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