Originating in China about 2,000 years ago, tea is currently the most consumed beverage in the world after water and well ahead of coffee, beer, wine, and carbonated soft drinks. It ranks fourth in the United States, with the average American consuming 155 bottles per year.
All tea is brewed from the dried leaves of the Camellia Sinensis bush and can be categorized into four types, depending on the level of oxidation: white (nonoxidized buds), green (nonoxidized leaves), oolong (partially oxidized leaves), and black (oxidized leaves).
Tea polyphenols, the main group of which are flavonoids known as catechins, are key components with many biological functions, including anti-inflammatory, antioxidative, and anticarcinogenic effects. All teas (excluding herbal teas) naturally contain between 100 and 300 mg of flavonoids per serving. During the production of black tea, other flavonoids called theaflavins and thearubigins are formed. Because green tea undergoes little processing, most of the catechin content is unchanged and is found in the form of epigallocatechin-3-gallate (EGCG), epigallocatechin, and epicatechin. Among tea catechins, EGCG is the most effective at eliminating free radicals.
According to National Health and Nutrition Examination Survey 1999-2002 data, 65.4% of flavonoids consumed by U.S. adults come from tea.
Tea leaves contain three main components that have metabolic effects: xanthic bases (caffeine and theophylline), essential oils, and polyphenolic compounds. Caffeine acts mainly on the central nervous system. Some effects of caffeine are enhanced by theophylline, which induces psychoactive activity, has a vasodilator effect, and has a much stronger diuretic effect than caffeine. Theophylline also has a relaxation effect on bronchial smooth muscle. Essential oils, known for aiding digestion, are volatile and evaporate from tea during the brewing time.
All of the compounds in tea are believed to offer health benefits. According to Chung Yang, PhD, director of the Center for Cancer Prevention Research at Rutgers University, "Tea and cancer prevention is an area that has been most extensively studied for the past 20 years, but there are also interesting results on the prevention of cardiovascular diseases, obesity, metabolic syndrome, diabetes, and neurodegenerative diseases."
Extracts of tea, especially green tea, and tea polyphenols have been shown to inhibit (prevent) the formation and development of tumors at different organ sites in animals and in cells studied in the laboratory, including skin, lung, esophagus, stomach, liver, small intestine, pancreas, colon, bladder, prostate, and mammary glands. Tea and tea components have also been shown to inhibit carcinogen-induced DNA damage in a number of cell studies. Many mechanisms have been proposed for the inhibiting action of tea against cancer cell growth, but polyphenols, especially EGCG, have received the most attention. Tea polyphenols have been shown to inhibit the expression of carcinogen-activating enzymes and increase the levels of enzymes that detoxify carcinogens. However, the inhibitory activity of caffeine, which is also present in tea, has also been demonstrated in lung and skin carcinogenesis models. Epidemiological reports on tea and cancer by the amount of tea required for cancer-preventive effects has varied widely in studies, from two to three cups of green tea per day to 10 cups per day.
Epidemiological evidence suggests that tea consumption is associated with cardiovascular benefits. Much of the research has focused on the catechins present in green tea. Catechins exert vascular protective effects by acting as antioxidants, antihypertensives, anti-inflammatories, antiproliferatives, antithrombotic agents, and lipid-lowering agents.
Some evidence suggests that tea may also affect glucose metabolism and insulin signaling. Several studies have examined the relationship between tea consumption and the risk of type 2 diabetes. Findings have ranged from adults who consumed at least four cups of black tea or at least six cups of green tea per day and had a 30% to 33% lower risk of developing type 2 diabetes than did those who consumed no tea to studies that found tea consumption was not associated with the risk of type 2 diabetes at all. A recent meta-analysis of coffee and tea consumption found that people who drank more than three or four cups of tea per day had an 18% lower risk of developing diabetes.
Several small clinical trials have investigated green tea's and green tea supplements' effect on weight loss. While some trials suggest benefits from green tea, including reductions in body weight, body mass index, body fat ratio, body fat mass, and waist and hip circumference.
A longer list of lesser-studied potential benefits to drinking tea include a reduction in oral bacteria that can cause halitosis and dental caries, a possible protective effect against Alzheimer's and Parkinson's diseases, protection against the damage of the UV rays from the sun, increased bone density, and improvements in the intestinal microflora.