'Flavonoids Abstract

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Flavonoids Abstract

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 injudging their many alleged health effects.

 

Flavonoids present in foods were considered non-absorbable because

they arebound to sugars as beta-glycosides. Only free flavonoids

withoutasugar molecule, the so-called aglycones were thought to be

able to pass

through the gut wall.

 

Hydrolysis only occurs in the colon bymicroorganisms, which at the

same timedegrade flavonoids.

 

We performed a study to quantify absorption of various dietary forms

ofquercetin.

To our surprise, the quercetin glucosides from onions

were absorbedfarbetter 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.

Themetabolism 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

colonicflora.

There is evidence for O-methylation, sulfation and

glucuronidation of hydroxyl groups in the liver.

Bacterial ringfission

of flavonoids occurs in the colon.

The subsequent degradationproducts,

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

Quantitative data on metabolism are scarce.

 

http://www.enzy.com/abstracts/display.asp?id=17

 

 

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