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Tea is the most widely consumed beverage in the world, except for water, and provides over 40 percent of the world's dietary CAFFEINE. In the United States, caffeine from tea accounts for about 17 percent of caffeine consumed; per capita caffeine consumption from tea is about 35 milligrams per day, which is a little over one-third of the daily caffeine provided by coffee beverages. Tea consumption in the United Kingdom is substantially higher, averaging 320 milligrams per capita per day and accounting for 72 percent of the United Kingdom's caffeine consumption.

Although tea contains a large number of chemical compounds, the relatively high content of polyphenols and caffeine is responsible for tea's pharmacological effects. The primary psychoactive component of tea is caffeine. Tea also contains two compounds that are structurally related to caffeine, theophylline and THEOBROMINE, however, these compounds are found in relatively insignificant amounts. On average, a 6-ounce (177-milliliter) cup of leaf or bag tea contains about 48 milligrams of caffeine, a little less than half the caffeine in the same amount of ground roasted coffee, and only slightly more than the amount found in 12 ounces of a typical COLA soft drink. Six ounces of instant tea contain 36 milligrams caffeine, on average. Individual servings of tea contain amounts of caffeine that can affect mood and performance of adult humans.

Although the term tea has been used to refer to extracts from a large number of plants, only teas derived from leaves of Camellia sinensis plants are of special interest here, because they contain caffeine. The term tea has come to be used especially for extracts of Camellia sinensis and that restricted usage is maintained in this entry.

Consumption of Camellia sinensis was first documented in China (where tea is called cha or chai) in 350 A.D., although there is some suggestion that the Chinese consumed tea as early as 2700 B.C. Tea was introduced to Japan around 600 A.D. but did not become widely used there until the 1400s. Through the China trade, tea became available in England in the 1600s, where it became the national drink. Tea was introduced into the American colonies around 1650 but in 1773 became a symbol of British rule. Americans protested the British tax on tea by raiding ships anchored in Boston Harbor and dumping boxes of tea into the water. This event, referred to as the Boston Tea Party, along with other similar protests that followed, became important in shifting the predominant caffeinated beverage in North America from tea to coffee.

India, China, and Sri Lanka are the major producers and exporters of tea—producing about 60 percent of the world's tea and providing about 55 percent of world tea exports. The United Kingdom, the United States, and Pakistan are the leading importers of tea.

Two types of tea, black and green tea, account for almost all of the tea consumed in the world. Black tea makes up over 75 percent of the world's tea; green tea accounts for about 22 percent. The method by which tea is manufactured determines whether black or green tea is produced. Black tea is dark brown in color and is produced by promoting oxidation of a key tea constituent. Green tea is yellow-green in color and is produced by preventing such oxidation, a less processed tea. Oolong tea, a less common type, is partially oxidized and is intermediate in appearance to that of black and green tea. Flavored teas were originally prepared by adding a range of fruits, flowers, and other plant substances to the tea prior to final packaging, although artificial flavors are often added today.