Epigallocatechin gallate (EGCG) 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).
Chemically, the monomeric flavanol EGCG is a carboxylic acid ester of epigallocatechin and gallic acid. The basic flavonoid structure is also called flavan and consists of two benzene rings with an O-heterocyclic pyran ring in the middle.
Epigallocatechin gallate occurs as the main constituent and main active ingredient in green tea. Other constituents include the catechins epicatechin, epicatechin gallate and epigallocatechin, as well as other flavonoids such as kaempferol, quercetin and myricetin.
Next to water, tea is the most commonly enjoyed beverage worldwide. In Asia, tea – especially green tea – is believed to have not only invigorating but also highly health-promoting properties. As a result, scientific interest in green tea – and the epigallocatechin gallate it contains – has increased in recent years.
Synthesis
As a secondary plant compound, EGCG is synthesized (produced) only by plants and is found here in the marginal layers and outer leaves. Therefore, EGCG is found primarily in plant foods, with levels varying depending on how the food is grown, the season, and the variety of the food. Green tea contains the most significant amount of EGCG in terms of quantity (70.2 mg per 100 g of freshly brewed tea). In 100 g of pecans there are 2.3 mg of EGCG.
In the plant organism, flavonoids such as EGCG 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.
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. Already after 3 minutes the EGCG content is at its highest with 50.69 mg/100 ml. In a study it was found that the contents of epicatechin, epicatechin gallate, epigallocatechin and epigallocatechin gallate increase within the first 3 to 5 minutes. As the brewing time increases, their content in green tea decreases. In contrast, the contents of catechin, gallocatechin and gallocatechin gallate increase continuously with the duration of infusion. 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 steeped, the more bitter the taste as well as the aroma.
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.
The bioavailability of EGCG in plasma is lower than that of other catechins such as epigallocatechin and epicatechin. After healthy subjects ingested 697 mg of green tea, only 0.07% to 0.2% of EGCG content was detected in plasma.