Introduction classification chemical structures and occurrence of flavonoids in plant foods

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Clinical observations, basic science and several epidemiological studies have contributed to an emerging body of evidence for a potential role of flavonoids in the prevention of cardiovascular disease (CVD) (Hollman and Katan, 1999). Flavonoids have been shown to inhibit the oxidation of plasma low-density lipoprotein (LDL), decrease platelet function and to modulate cytokines and eicosanoids involved in inflammatory responses (De Whalley et al., 1990; Murphy et al., 2003). Several epidemiological studies suggest a protection of a high flavonoid intake on the mortality of coronary heart disease (CHD) (Hollman et al., 1996a; Rimm et al., 1996a; Knekt et al., 1996; Yochum et al., 1999). Some prospective studies on major flavonoid sources, such as tea, have, however, shown large discrepancies in the relative risk (RR) of death from CHD with RRs ranging from 0.42 (Hertog et al., 1993a), 0.62 (Yochum et al., 1999), 1.08 (Rimm et al., 1996a), to 1.6 (Hertog et al., 1997). Also a recent study on the risk of CVD in women failed to show a protective effect of flavonoid intake on the risk of CVD (Sesso et al., 2003). Although the picture from epidemiological studies on the relationship between risk of CVD and intake of flavonoids is inconsistent, the majority of the studies suggest an inverse association between intake of flavonoids and the risk of CVD, which is supported by basic and clinical studies on flavonoids, indicating that flavonoids may have a protective action that deserves further investigations before final conclusions can be drawn.

*The author previously published under the name S.E. Nielsen.

Biosynthetic Flavonoid Pathway Fruits
Fig. 9.1 Biosynthesis pathways of dietary flavonoids in plants.

The flavonoids constitute a large class of polyphenols that are found ubiquitously in the plant kingdom and are thus present in fruits and vegetables regularly consumed by humans. They account for a variety of colours in flowers, berries and fruits, from yellow to red and dark purple. The flavonoids are biosynthesised from phenylalanine (ring B) and three acetate units (ring A), giving the chalcones as the first identifiable intermediate (see Fig. 9.1) (Herbert, 1989). Ring closure of the chalcones gives rise to the flavanones, which can be further oxidised or derivatised to flavanonols, flavones or flavonols. Reduction of the carbonyl group in the 4-position and subsequent removal of the hydroxyl group result in the formation of the catechins, whereas oxidation of the C-ring affords the anthocyanins. Rearrangement of the flavonoid skeleton by an intramolecular 1,2-shift of the B-ring gives the isoflavonoids. Substituents such as hydroxyl, methoxyl and sugar moieties give rise to a multitude of different compounds, and more than 4000 different naturally occurring flavonoids have so far been described (Middleton and Kandaswami, 1994).

In plants, the majority of the flavonoids are found as glycosides with different sugar groups linked to one or more of the hydroxyl groups. They are mainly found in the outer parts of the plants, such as leaves, flowers and fruits, whereas the content in stalks and roots is usually very limited. The flavonoids located in the upper surface of the leaf or in the epidermal cells, have a role to play in the physiological survival of plants. They contribute to the disease resistance of the plant, either as constitutive antifungal agents or as phytoalexins (Harborne and Williams 2000). The flavonoids have also UV-B-protecting properties. They absorb light in the 280 ± 315 nm region, and with their almost universal presence in green leaves, they protect the underlying photosynthetic tissues from damage (Harborne and Williams, 2000). Also in the peel of fruits, the flavonoids can act as UV-B filters, e.g. in apple skin, quercetin glycosides have UV-B protective capacity and accumulates in the skin of certain apple sorts, when exposed to UV-B radiation (Solovchenko and Schmitz-Eiberger, 2003).

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