Flavonoids

Flavonoids are a widespread (around 800 substances are known) polyphenolic group of substances in the plant world and are found in numerous fruits and vegetables.
Flavones (flavus, Latin for yellow) were named after plants such as the dyer's oak, which were used to dye goods yellow. After the widespread use of other colored or colorless flavones became known, the group of substances was named 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 based on the structural differences in their C-ring:

• Flavanols (example: catechin, epicatechin)
• Flavanonols (example: taxifolin)
• Chalcones (example: isoliquiritigenin)
• Anthocyanidins (flavenols) (example: cyanidin, malvidin, petunidin)
• Flavonols (example: quercetin, rutin, myrcetin)
• Aurones (example: aureusidin)
• 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 their structural diversity and significantly influence their bioavailability and biological effects.

Flavonols:

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

Anthocyanins are the red or blue color pigments in fruits. Their content in dark fruits is about ten times higher than that of flavonoids. 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, some of the monomers were fermented and oxidized to dark-coloured polymers such as thearubigin and theaflavin.

Isoflavones: Soy products are a good food source of isoflavones such as genistein, which belongs to the phytoestrogens (plant estrogens).

We consume up to 1 g of polyphenols (including flavonoids and phenolic acids) in our daily diet. Around 40 % comes from drinks (cola, coffee, tea, beer, wine), 30 % from fruit and fruit juices, 15 % from vegetables and 15 % from nuts, potatoes and cereals (Pierpoint WS). Flavonols(quercetin, onion, tea) and anthocyanidins (in red fruits) are particularly widespread. There are also oligomeric proanthocyanidins and other phenolic compounds known as polyphenols.

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

Flavonols: Epidemiologic studies show an inverse relationship between myocardial infarction (I21.9) and flavonol intake. An anticarcinogenic effect has not been clearly demonstrated.
Isothiocyanates: Other important plant-based active ingredients are sulphur-containing compounds such as isothiocyanates, which occur as glucosinolate precursors, particularly 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 in a cohort study with 8000 participants that the daily intake of at least ten cups of green tea (Japanese cup size) has a tumor-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 the flavonoids contained in red vine leaves, mainly quercetin glycosides and kaempferol-3-glucoside(also known as astragalin), have an anti-oedematous effect (systemic and local use 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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