Catechin belongs to the colorless flavanols, which are a subgroup of flavonoids. These are classified as secondary plant compounds (bioactive substances with potential health-promoting effects).Catechin has a basic flavonoid structure, which is also called a flavan. It is composed of two benzene rings, in the middle of which is an O-heterocyclic pyran ring. On the pyran ring, the second and third carbon are connected with a single bond. In addition, there is a hydroxy group on the 3rd carbon. The molecular formula is C15H14O6. Catechins form the monomeric building block for proanthocyanidins. These colorless bitter substances also belong to the flavanols.
Catechins make up about 17 to 40 percent of the dry weight in green tea. In black tea, the figure is a maximum of ten percent, since complex polyphenols such as theaflavins, theaflavic acids, theaflagallins and thearubigins are formed from a large proportion of the catechins during oxidation (fermentation). Furthermore, catechins are found in white as well as oolong tea, wine and various fruits.
Synthesis
As a secondary plant substance, catechin is synthesized (produced) only by plants and is found here in the marginal layers and in the outer leaves. Therefore, catechin is found primarily in plant foods, and its content varies depending on how the food is grown, the season, and the variety of the food. In 100 g of freshly brewed green tea, 4.47 mg of catechin can be found. Dark chocolate contains 24.20 mg per 100 g. Baking cocoa powder has the most significant amount of catechin in terms of quantity (64.33 mg per 100 g). In the plant organism, flavonoids such as catechin occur predominantly in bound form as a glycoside (binding to glucose) and to a lesser extent in free form as an aglycone (without a sugar compound).
Absorption
The nutritively (dietary) absorbed free and glycoside-bound flavonoids enter the small intestine. The flavonoid aglycones are absorbed into enterocytes (cells of the small intestinal epithelium) via passive diffusion. Some of the flavonoid glycosides are absorbed (taken up) via the sodium/glucose cotransporter-1 (SGLT-1). This transports sodium ions together with glucose into the cell by means of a symport (rectified transport). In this way, the flavonoid glycosides reach the mucosa epithelium (intestinal mucosa) intact. The flavonoid glycosides that are not absorbed in the small intestine are transformed into free phenolic acids and flavonoid aglycones by the microorganisms of the colon (large intestine). While some of these flavonoids passively enter the colonic epithelium, the other part continues to be degraded by the microflora and excreted in the feces (stool). Flavonoids are well bioavailable at > 15%. Brewing in water can result in a 50% loss of water-soluble flavonoids. To brew green tea optimally, a temperature of 85 °C is suitable. In a study it was found that the contents of catechin, gallocatechin as well as gallocatechin gallate continuously increase with the duration of the brewing time. The contents of epicatechin, epicatechin gallate, epigallocatechin and epigallocatechin gallate increase within the first 3 to 5 minutes. After that, their content in green tea decreases. Also on the basis of sensory points, green tea performs best after a brewing time of 3 to 5 minutes. The longer the green tea brews, the more bitter the taste as well as the aroma becomes.
Transport and distribution in the body
Absorbed flavonoids are transported to the liver via the portal vein. Here, conjugation or methylation with glucuronic acid or sulfate occurs via phase II reactions. Subsequently, elimination via bile occurs.
