Flavonoids

Flavonoids are a widespread (about 800 substances are known) polyphenolic group of substances in the plant world, which are present in numerous fruits and vegetables.
The name flavone (flavus lat. for yellow) is derived from plants such as the dyer oak, which were used to dye materials yellow. After the widespread use of flavones of other colours or colourless flavones became known, the group of substances was called flavonoids .

Flavonoids are chemically derived from the basic structure of flavan (2-phenylchroman). They consist of two aromatic rings connected by a tetrahydropyran ring. Flavonoids are divided into different subgroups according to the structural differences in their C-ring:

• Flavanols (examples: catechin, epicatechin)
• Flavanonlone (Example: Taxifolin)
• Chalcones (example: isoliquiritigenin)
• Anthocyanidins (flavenols) (example: cyanidine, malvidine, petunidine)
• Flavonols (example: quercetin, rutin, myrcetin)
• Aurones (example: aureusidine)
• Flavones (Example: Luteolin)
• Flavanones (example: hesperetin, eriodictyol)
• Isoflavones (example: genistein, licoricidin)

The general flavonoid structure can be modified by phenolic hydroxyl groups, conjugations with sugars, methoxy groups, sulfation and glucuronidation. These modifications contribute to structural diversity and have a decisive influence on their bioavailability and biological effects.

Flavonols:

• Onions: 185±332mg
• Kale 110 mg/kg
• green salad 94 mg/kg
• broccoli 30 mg/kg).

Anthocyanins provide the red or blue colour pigments in fruits. Their content in dark fruits is about 10 times higher than that of flavenoids. However, the compounds are less stable than flavonoids. Strawberries contain 30 mg anthocyanins/100 g, raspberries 40 mg/100 g, blueberries 165 mg/100 g, blue grapes 165 mg/100 g and sweet cherries 180 mg/100 g (Böhm et al).

Flavanols: Green tea is very rich in monomeric catechin-like flavonoids such as epicatechin, epigallocatechin, epicatechin gallate and epigallocatechin gallate (EGCG). In black tea the monomers are partly fermented and oxidized to dark-colored polymers like thearubigin and theaflavin.

Isoflavones: Soya products are a good source of isoflavones such as genistein, which belongs to the phytoestrogens (vegetable oestrogens).

With our daily food we take up to 1 g of polyphenols (including flavonoids and phenolic acids). About 40% comes from beverages (cola, coffee, tea, beer, wine), 30% from fruits and fruit juices, 15% from vegetables and 15% from nuts, potatoes and cereals (Pierpoint WS). Especially widespread are the flavonols(quercetin, onion, tea) and the anthocyanidins (in red fruits). Oligomers proanthocyanidins and other phenolic compounds called polyphenols also occur.

Flavonoids possess a variety of biological properties that contribute to tumor-protective effects in animal models. These results have not been proven in human biology.
Flavonoids are good antioxidants. The antioxidative effects and radical scavenging properties of flavonoids are structure dependent and have been investigated and found in various pilot studies in humans. Flavonoids have antioxidative effects and influence inflammatory processes, the hormone balance (especially phytoestrogenic isoflavones from soya) and cell growth. Protective properties consist in the ability to eliminate damaged cells by inducing programmed cell death (apoptosis) or to activate cell differentiation processes. Furthermore, flavonoids have antiphlogistic effects by inhibiting enzymes of the arachidonic acid cascade (cyclooxygenases 1 and 2, phospholipases and lipoxygenases).

Flavonols: Epidemiological studies show an inverse relationship between myocardial infarction (I21.9) and flavonol intake. An anticarcinogenic effect could not be clearly demonstrated.
Isothiocyanates: Other important plant active ingredients are sulphur-containing compounds such as isothiocyanates, which are precursors of glucosinolate, especially in cabbage-like vegetables such as broccoli. Isothiocyanates also modulate the metabolism of foreign substances in humans after ingestion of a diet rich in cabbage.

Flavenoids induce apoptosis (Galati G).
Fujiki et al. showed on the basis of a cohort study with 8000 participants that the daily intake of at least ten cups of green tea (Japanese cup size) has a tumour-protective effect. Epidemiological studies show an inverse relationship between breast, prostate and colon cancer and a traditional Asian, low-fat, soy-rich diet [Adlercreutz H].

Quercetin glycosides and kaempferol-3-glucoside: Several studies have shown that of the flavonoids contained in red vine leaves, mainly quercetin glycosides and kaempferol-3-glucoside(also called astragaline) have an anti-edematous effect (systemic and localized glaucoma in chronic venous insufficiency). The red coloration is due to anthocyanins.

In general, flavonol and flavonaglycones are absorbed in the small intestine, conjugated to glucuronic or sulphuric acid in the liver or methylated at the hydroxyl groups and excreted through the bile. In contrast, glycosides are neither hydrolysed in the stomach nor split by enzymes in the gastrointestinal tract. They are therefore not absorbed in the small intestine and reach the large intestine, where they are deglycosylated by microorganisms and further broken down by splitting the C-ring. The resulting metabolites (phenolic acids) are then excreted in the urine.

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