Green tea extract
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A green tea extract is a herbal derivative from green tea leaves (Camellia sinensis). Containing antioxidant ingredients – mainly green tea catechins (GTC) – green tea and its derivatives are sometimes used as dietary supplements and in alternative medicine.
History of tea extracts
Types of green tea extracts
During the strong infusion green tea leaves are processed by soaking in the aqueous solution of alcohol (the aspect content is about 2% w/w).
To obtain soft extracts, the solution made by strong infusion beforehand is further concentrated to 20–25% (the catechin content is about 20% w/w).
After the strong infusions have been concentrated to 40–50% solids (the catechin content is above 25% w/w)，they are sprayed and then become dehydrated extract and powder. The leftovers - water content, which is less than 5% w/w, and the extract - are usually processed as a powder containing inert processing aids to become suitable for a variety of uses (tablets, capsules, dry mixes, etc.).
Partly purified extracts
Further purification processes, for example, solvent extraction or column chromatography techniques, as well as new techniques such as membrane extraction and separation, are utilized in order to acquire a higher content of tea catechins.
Chemistry and biochemistry
The cardinal antioxidative ingredient in the green tea extract is green tea catechins (GTC), which comprise four major epicatechin derivatives; namely, epicatechin (EC), epigallocatechin (EGC), epicatechin gallate (ECG), and epigallocatechin gallate (EGCG). Of which, EGCG accounts for more than 40% of the total content.
Other components include three kinds of flavonoids, known as kaempferol, quercetin, and myricetin. A remarkably higher content of myricetin is detected in tea and its extracts than in many other plants, and this high concentration of myricetin may have some implications with the bioactivity of tea and its extracts.
Green tea extract is approximately two times more antioxidant-active than Vitamin C. The main attribution is supposed to be EGCG. One study by the Drug and Food department of Iran in 2008 compared antioxidant properties of various green tea compounds with those of vitamin C and vitamin E: the study concluded that green tea extracts -for a total of approximately 68 mg of combined phenols, flavonoids and anthocyanidin present from 1g of a sample of "Chinas green tea" leaves, the grade yielding the highest content of bioactive chemicals- had the equivalent antioxidant power found in 50 mg-275 mg of vitamin C and 156 mg-813 mg of vitamin E 
In alkaline solutions (pH > 8) GTC (green tea catechins) is rather unstable; in acidic solutions (pH < 4), however, GTC shows excellent stability. The stability in alkaline solutions varies between four components of GTC in green tea extracts. Recent study demonstrates that EGCG and EGC is more unstable than EC and ECG in a basic solution, giving an explanation to the fact that EGCG and EGC do not circulate in the basic sodium phosphate buffer fluid of human body.
In a high temperature environment, GTC is not stable: an epimerization change is likely to occur, because heating results in the conversion from EGCG to GCG. Thus it is considered inappropriate to infuse green tea or its extracts with overheated water.
Scavenging effect of lipid free-radicals (one antioxidant property) of polyphenols in green tea extracts can be observed in experiments. The ability of GTP in green tea extracts to eliminate lipid-derived free radicals is noticeably stronger (almost 50 times) than that of ginkgo biloba extracts. Further investigations indicate that the boosting level of superoxide dismutase (SOD) and glutathione dismutase (GSHPx) may account for the inhibitory effect of GTC against lipid oxidation (rancidification). Generally speaking, green tea extracts are more effective as antioxidants than black tea extracts are due to the better preservation of catechins contained in green tea extracts.
Moreover, the anticarcinogenic properties of green tea extracts have been a focus of recent scientific research. In many experiments, green tea extracts show inhibitory effects on cancer cells. Invitro assays have shown that catechin and caffeine, which are main components in green tea extracts, block the cell cycle of cancer cells (cytotoxicity) and induce programmed cell death; in vivo, green tea extracts also inhibit prostatic carcinoma transplanted in nude mice.
In addition, green tea extracts also contain a wide range of anti-inflammatory characteristics, so green tea extract may be helpful in treating chronic inflammatory states. The bactericidal activity against S.mutans is conspicuous in Japanese green tea extracts, and the maltose level in mouth is consistently lower after drinking tea. Therefore, green tea extracts may be effective in oral hygiene maintenance.
Dosage and side-effects
EGCG has a poor bioavailability when taken orally; the absolute bioavailability of EGCG in CF-1 mice and Sprague-Dawley rats was found to be only 26.5 and 1.6%, respectively. The bioavailability for humans is assumed to be in the same range.
Though green tea extracts show potential anticancer capacity - under some conditions by working together with other drugs and suppressing multidrug resistance in cancer cells - they are not suggested for use alone as a chemotherapy agent for cancer treatment.
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