Green Tea Extract

What is green tea extract?

Green tea extract is made by lightly steaming and drying the leaves of the plant Camellia sinensis (R).

The main active ingredients in green tea are the polyphenols, which comprise 30-35% of the dry weight of green tea leaves (R). These polyphenols are also known as catechins, and the 4 most important are epicatechin (EC), epigallocatechin (EGC), epicatechin-3-gallate (ECG), and epigallocatechin-3-gallate (EGCG) (R, R). Other green tea phenolic compounds include gallic acid (GA), gallocatechin (GC), catechin (C), p-coumaroylquinic acid (CA), and gallocatechin-3-gallate (GCG). Green tea also contains condensed and hydrolyzable tannins (R).

While the ratio of each catechin is influenced by many intrinsic and extrinsic factors and even the technological processes used during production and extraction (R, R, R, R) it is generally acknowledged that EGCG is the most abundant and most potent green tea catechin (R, R).

Is green tea extract stable?

Green tea extract is among the more difficult botanicals to formulate as active ingredients in topical products (R). Green tea catechins, like most antioxidants, are highly unstable and easily oxidised in an ambient environment (R). They also tend to degrade more at higher temperatures, though this can be reduced by loading them into chitosan microparticles (R), or by adding other antioxidants such as butylated hydroxytoluene (BHT) (R).

Does green tea extract get absorbed?

Green tea catechins are hydrophilic by nature, limiting their passive diffusion through the hydrophobic stratum corneum (R). This is especially true of EGCG and ECG, which have larger molecular sizes and pass through the skin less easily than EGC and EC, the smaller catechins (R, R, R). We note, however, that a formulation containing 1% green tea polyphenols has been demonstrated to penetrate to the deeper layers of the stratum corneum (R).

The structure of the various catechins also affects their permeation profiles (R). EGC and EC are distributed in all layers of the skin, while EGCG and ECG are usually localised to the skin surface and stratum corneum (R). This is because the gallate group, despite being lipophilic, retards the permeation of EGCG and ECG through steric hindrance (R, R).

The extent of penetration of green tea catechins also relies on the vehicle used for topical delivery. A hydrophilic ointment containing 10% EGCG resulted in substantial intradermal uptake of up to 20% of the applied dose (R). Terpenes, especially α-terpineol, have also been successfully used to enhance the skin uptake of green tea catechins by perturbing the stratum corneum (R). Drug-in-adhesive patches also appear to improve the skin permeation of the catechins (R). More recently, chitosan microparticles loaded with green tea extract have been shown to be promising carriers for aiding permeation, as they protected the catechins from enzymatic degradation (R, R). Oil/water emulsions of catechin derivatives are also useful formulations for increasing their skin permeation (R, R).

Gelling agents do not seem to significantly affect the permeability of EC however (R). Likewise, EGCG encapsulated in liposomes did not seem to reach the deeper skin layers. It has been suggested that the galloyl catechins, particularly EGCG, have high affinity for and hence bind to the lipid bilayer of the liposomes, hindering their release from the carrier (R).

Although green tea catechins are capable of penetrating through mouse and pig skin (R, R, R), this does not happen with human skin (R, R, R) which is ideal as it means that the catechins are retained at their sites of action within the skin.

What happens after green tea extract is absorbed?

The green tea catechins EGCG and ECG are hydrolysed by esterases in the stratum corneum to EGC and EC (R, R).