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.
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.
Caffeine may be excluded in green tea extracts in order to avoid side-effects; caffeine-free green tea extract supplements are now available.
Green tea extract supplements are accessible over the counter in various forms. Standardized green tea extract is 90 percent total polyphenols, and 1 capsule equals 5 cups of tea.
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.
Excessive intake of green tea extracts containing caffeine has side effects; an excessive concentration may act as a pro-oxidant to damage DNA and produce undesirable side effects.
Use of green tea extracts has been linked to occasional cases of acute liver failure.