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Tea comes from a rounded, evergreen shrub that grows between 4 to 6 feet tall. Tea plants thrive in tropical and semi-tropical climates, such as those of India, China, and Argentina. Small, white flowers are present in autumn, but are often hidden by foliage. Tea leaves are elliptical, serrated, and about 4 inches long. Both green and black tea come from the same plant. After picking, leaves that are allowed to oxidize produce black tea. Many of the polyphenols present in green tea are absent in black tea because of this oxidative process. In order to produce green tea, the leaves are steamed. This keeps the polyphenols from turning into compounds with much less activity.
The main constituents of green tea are polyphenols, carotenoids, polysaccharides, chlorophyll, vitamins E and C, manganese, zinc, and potassium. The polyphenols are generally believed to be the most beneficial compounds in green tea, thanks primarily to their antioxidant activity. The most active polyphenols in green tea are catechins, in particular Epigallocatechin Gallate (EGCG). Green tea supplements are often standardized to contain specific catechin concentrations.
Over-consumption of green tea can cause mild insomnia and irritability, due to its natural caffeine content. According to the US Food and Drug Administration, an 8 ounce serving of green tea has about 15 milligrams of caffeine; in contrast, an 8 ounce serving of coffee has between 80-135 milligrams of caffeine. Tea infusions are regularly recommended for those attempting to wean themselves from coffee because tea has a moderate amount of caffeine. People seeking the antioxidant benefits of green tea without the caffeine often take supplemental extracts, which are usually standardized to provide high polyphenol concentrations. Many herbalists recommend consuming the polyphenol equivalent of six cups of green tea (480-640 mg catechins) per day, and there are no known side effects at this dosage.