Flavonoids

Flavonoids constitute a large class of phytochemicals that are widely distributed in the plant kingdom, are present in high concentrations in the epidermis of leaves and skin of fruits, and have important and varied roles as secondary metabolites. More than 8000 varieties of flavonoids have been identified, many of which are responsible for the colors of fruits and flower. They are found in fruits, vegetables, tea, wine, grains, roots, stems, and flowers and are thus regularly consumed by humans. Although it has been widely known for centuries that derivatives of plant origin possess a broad spectrum of biological activities, it was first suggested that flavonoids may be important for human health in the 1930s when it was observed that a fraction from lemon juice could decrease the permeability of arteries and partially prevent symptoms in scorbutic pigs. At the time, it was suggested that these compounds should be defined as a new class of vitamins, vitamin P, and the substance responsible for the effects was identified as the flavonoid rutin. However, the data were not generally accepted and the term vitamin P was abandoned in the 1950s. There was renewed interest in flavonoids when a potentially protective role for flavonoids in relation to heart disease in humans was reported. Since that time, there has been a surge of interest in the potential role of flavonoids in human health, with research suggesting antioxidant effects, hormonal actions, antiinfectious actions, cancer-pre-ventative effects, the ability to induce chemical defense enzymes, and actions on blood clotting and the vascular system. However, concrete evidence that they positively influence human health is lacking, and adverse effects have also been reported for some polyphenols. The main subclasses of flavonoids are flavones, flavonols, flavan-3-ols, isoflavones, flava-nones, and anthocyanidins (Figure 1 and Table 1).

Other flavonoid groups that are thought to be less important from a dietary perspective are the dihydro-flavones, flavan-3,4-diols, coumarins, chalcones, dihy-drochalcones, and aurones. The basic flavonoid skeleton can have numerous constituents; hydroxyl groups are usually present at the 4-, 5-, and 7- positions. Sugars are very common, and the majority of flavonoids exist naturally as glycosides. The presence of both sugars and hydroxyl groups increases water solubility, but other constituents, such as methyl or isopentyl groups, render flavonoids lipophilic.

Although many thousands of different flavonoids exist, they can be classified into different subclasses. The main subclasses that are important from a human health perspective are the flavones,

OH O

Genistein

OH O

Genistein

Isoflavone

Isoflavone

OH O

OH O

Quercetin

OH O

Apigenin

OH O

Apigenin

Flavone

OH O

Naringenin

OH Gallic Acid

Epicatechin

OH O

Quercetin

Naringenin

OH Gallic Acid

Epicatechin

Flavan-3-ols

Figure 1 Structures of the major subclasses of flavonoids.

flavonols, flavan-3-ols, isoflavones, flavanones, and anthocyanidins (Figure 1).

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