Green tea is produced from the leaves of the Camellia sinensis, or tea plant. Oolong and black tea are also produced from the plant, but are processed and oxidized in different manners. Of the three, green tea contains the highest levels of polyphenols, the antioxidant substance that is believed to be beneficial in protecting against both cancer and atherosclerosis.
The tea plant is actually a variation of evergreen bush, with glossy green leaves and small white to pink flowers. The plants can reach a height of 30–40 ft (9–12 m) or taller in the wild, but are generally kept to a height of 6 ft (1.2 m) or less on the tea plantations and gardens where they are grown in China, Argentina, Japan, India, Indonesia, Kenya, Malawi, Sri Lanka, Turkey, Pakistan, Bangladesh, and Tanzania. Tea plants are cultivated in countries where warm, rainy growing conditions are abundant, and are also frequently grown in high altitude areas.
When tea plants reach maturity at three or four years of age, the young leaves and leaf buds—the parts of the plant highest in polyphenols—are harvested. Green tea is produced by steaming or roasting the leaves as soon as they are picked, and then rolling and drying the tea leaves to remove any moisture.
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Approximately 2.5 million tons of tea are grown and produced worldwide on an annual basis. Written records date the use of the plant as a beverage since at least the tenth century B.C. in China, and it is thought to be close to 5,000 years old. Tea is the most consumed beverage worldwide (after water). It is also one of the most popular herbal infusions in existence—drunk regularly by over half the world population.
The polyphenols in green tea that act as antioxidants may actually inhibit the growth of existing cancer cells. In some animal studies, injections of tea extracts reduced the size of cancerous tumors in animals. The active agent that is thought to have this effect is an antioxidant, epigallocatechin-3-gallate (EGCG).
Recent clinical studies have also indicated that regular use of green tea may reduce the risk of certain types of cancer, including oral, skin, prostate, colon, stomach, and rectal. In one clinical trial, patients with pre-cancerous mouth lesions who were treated with green and black tea extracts achieved a 38% decrease in the number of pre-cancerous cells. Late in 2001, researchers acknowledged one reason for green tea’s anticancer effect, but further human studies are needed to clearly define its role in cancer prevention.
The antioxidants in green tea may also be helpful in lowering cholesterol and preventing hardening of the arteries and ischemic heart disease. Low flavonoid intake has been linked to atherosclerosis in several studies. The data from one 1999 study, which followed more than 3,400 tea-drinking residents of Rotterdam, the Netherlands, concluded that regular, long-term tea consumption can have a protective effect against severe atherosclerosis.
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Another preliminary study published in 1999 in the American Journal of Clinical Nutrition found that green tea extract may increase energy levels and promote fat oxidation, and consequently, may be a useful tool in weight control. A recent study, reported on in early 2002, showed that topically applied green tea extracts can reduce harmful effects of radiation from the sun. Further study might show that green tea polyphenol applications can help prevent sunburns.
In addition to polyphenols, green tea contains several minerals, including fluoride and aluminum. The fluoride in green tea may be useful in fighting tooth decay. Green tea is also an antibacterial agent, and can help to prevent gingivitis and periodontal disease by killing E. coli and streptococcus bacteria. This antibacterial action can also be effective in treating halitosis, or bad breath, by killing odor-causing bacteria.
As an herbal remedy, green tea is often recommended to ease stomach discomfort, vomiting, and to stop diarrhea. The antibacterial action of tea is useful in treating infections and wounds.
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Preparations
Green tea leaves and tea bags can be purchased at most grocery, drug, and health food stores. It is graded by leaf size, with tea containing whole leaves and leaf tips considered the highest quality tea. Tea grades include Broken Orange, Pekoe, Broken Pekoe Souchong, Broken Orange Pekoe, Fannings, and Dust.
Although green tea is grown from a single plant, slight variations in tea processing (usually in the way the tea is rolled) have created a number of varieties of green tea. Popular green tea varieties include Gunpowder, Hyson, Dragonwell, Sencha, and Matcha.
Tea leaves should be kept in an air-tight container to retain flavor and prevent odors and moisture from being absorbed by the tea. It should also be stored in a cool place for no longer than six months before use.
The most common method of preparing green tea is as an infusion. The tea is mixed with boiling water, steeped for several minutes, and then strained or removed from the infusion before drinking. Approximately two teaspoons of loose tea, or a single tea bag, should be used for each cup of boiling water. A strainer, tea ball, or infuser can be used to immerse loose tea in the boiling water before steeping and separating it.
A second method of infusion is to mix loose tea with cold water first, bring the mixture to a boil in a pan or teapot, and then separate the tea from the infusion with a strainer before drinking.
Flavonoids—polyphenols with antioxidative properties— are released into the infusion as the tea steeps. The longer the steeping time, the more flavonoids are released by the tea leaves, although most will infuse into the water during the first five minutes of brewing. Longer steeping time also results in a higher caffeine content in the brewed tea.
Green tea leaves can be used in a poultice for treating insect bites and other skin irritations. Green tea leaves are chopped and boiled in water for two to three minutes. After the excess water is squeezed from the leaves, the green tea is applied to the area to be treated and wrapped in a bandage. Green tea also makes an effective astringent, and tea-soaked cloth or tea leaf poultice may help renew tired and puffy eyes.
The antibacterial activity of green tea also makes it appropriate for use in compresses for cuts and abrasions. A quick compress can be made by soaking a pad or bandage in hot tea, wringing out the excess fluid, and holding the pad firmly against the wound. Once the compress cools, the process can be repeated.
Precautions
The U.S. Food and Drug Administration (FDA) includes tea on their list of “Generally Recognized As Safe” substances. However, pregnant women and women who breast feed should consider limiting their intake of green tea because of its caffeine content. Tea can pass into breast milk and cause sleep disorders in nursing infants. Decaffeinated green tea is available that contains only trace amounts (5 mg or less) of caffeine. Women should check with their healthcare professional about drinking tea when pregnant or nursing.
Tea can stimulate the production of gastric acid, and individuals with ulcers may want to avoid drinking green tea for this reason. Those taking warfarin or any bloodthinning drugs should first consult with the physicians before consuming green tea, as it may counter the effects of the drug.
Side effects
Green tea contains caffeine, a central nervous system (CNS) stimulant that can cause restlessness, irritability, difficulty sleeping, tremor, heart palpitations, loss of appetite, and upset stomach. To avoid side effects, caffeine intake should be limited to 300 mg or less a day (the equivalent of 4–8 cups of brewed hot tea). Caffeinefree green tea preparations are available commercially.
The tannin in tea can cause nausea when drunk on an empty stomach and inhibit the absorption of nonheme iron. Individuals with iron-deficiency anemia who take iron supplements should avoid drinking green tea several hours before and after taking supplements. Iron absorption with tea can be increased by consuming foods rich in vitamin C with tea, such as a slice of lemon.
