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Green tea is made from the dried leaves of Camellia sinensis, a perennial evergreen shrub. Green tea has a long history of use, dating back to China approximately 5,000 years ago. Green tea, black tea, and oolong tea are all derived from the same plant.
Tea varieties reflect the growing region (for example, Ceylon or Assam), the district (for example, Darjeeling), the form (for example, pekoe is cut, gunpowder is rolled), and the processing method (for example, black, green, or oolong). India and Sri Lanka are the major producers of green tea.
Historically, tea has been served as a part of various ceremonies and has been used to stay alert during long meditations. A legend in India describes the story of Prince Siddhartha Gautama, the founder of Buddhism, who tore off his eyelids in frustration at his inability to stay awake during meditation while journeying through China. A tea plant is said to have sprouted from the spot where his eyelids fell, providing him with the ability to stay awake, meditate, and reach enlightenment. Turkish traders reportedly introduced tea to Western cultures in the 6th Century.
L-theanine is a predominant amino acid found in green tea. Preliminary research exists on the effects of this amino acid in comparison with the prescription drug alprazolam on experimentally induced anxiety. No benefit was found.
Research indicates that green tea may benefit arthritis by reducing inflammation and slowing cartilage breakdown. Further studies are required before a recommendation can be made.
Research has shown caffeine to cause improvements in airflow to the lungs (bronchodilation). However, it is not clear if caffeine or tea use has significant benefits in people with asthma. Better research is needed in this area before a strong conclusion can be drawn.
Overall, the relationship of green tea consumption and human cancer remains inconclusive. One clinical trial showed minimal benefit using green tea extract capsules for the treatment of hormone refractory prostate cancer. Further research is needed before a recommendation can be made.
There is early suggestive evidence that regular intake of green tea may reduce the risk of heart attack or atherosclerosis (clogged arteries). Further well-designed clinical trials are needed before a firm recommendation can be made in this area.
Common cold prevention:
In humans, preliminary date suggests that a specific formulation of green tea may help prevent cold and flu symptoms. Further well-designed clinical trials are needed to confirm these results.
Dental cavity prevention:
There is limited study of tea as a gargle (mouthwash) for the prevention of dental cavities (caries). It is not clear if this is a beneficial therapy.
More studies are required to determine if green tea and polyphenols have any therapeutic benefit for diabetes prevention or treatment.
Early research using a combination product called FertilityBlend has been associated with some success in helping women to conceive. Further well-designed research on green tea alone for this use is needed before a strong conclusion can be drawn.
Green tea has been shown to increase or have no effect on blood pressure in several studies in humans.
Laboratory, animal, and limited human research suggest possible effects of green tea on triglyceride levels. Better human evidence is necessary in this area.
A study conducted in healthy postmenopausal women showed that a morning/evening menopausal formula containing green tea was effective in relieving menopausal symptoms including hot flashes and sleep disturbance. Further studies are needed to confirm these results.
Several preliminary studies have examined the effects of caffeine, tea, or coffee use on short and long-term memory and cognition. It remains unclear if tea is beneficial for this use. Limited, low-quality research reports that the use of green tea may improve cognition and sense of alertness. Green tea contains caffeine, which is a stimulant.
There is limited animal and human study of green tea as a protective agent of skin from ultraviolet light skin injury. Some study results conflict. Comparisons have not been made with well-established forms of sun protection such as ultraviolet protective sunscreen. The effects of green tea on skin damage caused by the sun remain unclear.
Viral infection (human T-cell lymphocytic virus):
Preliminary research suggests green tea decreases viral load in carriers of the HTLV-1 virus. Additional well-designed controlled research is needed before a recommendation can be made for or against green tea in the treatment of HTLV-1 carriers.
Weight loss (maintenance):
There are several small human studies addressing the use of green tea extract (GTE) capsules for weight loss or weight maintenance in overweight or average weight individuals. Study results are mixed. Better research is needed before a strong recommendation can be made in this area.
Benefits of specific doses of green tea are not established. Most studies have examined green tea in the form of a brewed beverage, rather than in capsule form. One cup of tea contains approximately 50 milligrams of caffeine and 80 to 100 milligrams of polyphenol content, depending on the strength of the tea and the size of cup.
Studies have examined the effects of habitually drinking anywhere from 1-10 cups per day (or greater).
In capsule form, there is considerable variation in the amount of green tea extract (GTE); there may be anywhere from 100 to 750 milligrams per capsule. Currently, there is no established recommended dose for GTE capsules.
Green tea is not recommended for infants or children due to caffeine content.
People with known allergy/hypersensitivity to caffeine or tannin should avoid green tea. Skin rash and hives have been reported with caffeine ingestion.
Studies of the side effects of green tea specifically are limited. However, green tea is a source of caffeine, for which multiple reactions are reported.
Caffeine is a stimulant of the central nervous system, and may cause insomnia in adults, children, and infants (including nursing infants of mothers taking caffeine). Caffeine acts on the kidneys as a diuretic (increasing urine and urine sodium/potassium levels and potentially decreasing blood sodium/potassium levels) and may worsen incontinence. Caffeine-containing beverages may increase the production of stomach acid and may worsen ulcer symptoms. Tannin in tea can cause constipation. Certain doses of caffeine can increase heart rate and blood pressure, although people who consume caffeine regularly do not seem to experience these effects in the long-term.
An increase in blood sugar levels may occur. Caffeine-containing beverages such as green tea should be used cautiously in patients with diabetes. In contrast, lowering of blood sugar levels from drinking green tea has also been reported in preliminary research. Additional study is needed in this area.
People with severe liver disease should use caffeine cautiously, as levels of caffeine in the blood may build up and last longer. Skin rashes have been associated with caffeine ingestion. In laboratory and animal studies, caffeine has been found to affect blood clotting, although effects in humans are not known.
Caffeine toxicity is possible with high doses. Chronic use can result in tolerance, psychological dependence, and may be habit forming. Abrupt discontinuation may result in withdrawal symptoms.
Several population studies initially suggested a possible association between caffeine use and fibrocystic breast disease, although more recent research has not found this connection. Limited research reports a possible relationship between caffeine use and multiple sclerosis, although evidence is not definitive in this area. Animal study reports that tannin fractions from tea plants may increase the risk of cancer, although it is not clear that the tannin present in green tea has significant carcinogenic effects in humans.
Drinking tannin-containing beverages such as tea may contribute to iron deficiency, and in infants, tea has been associated with impaired iron metabolism and microcytic anemia.
In preliminary research, green tea has been associated with decreased levels of estrogens in the body. It is not clear if significant side effects such as hot flashes may occur.
Large amounts of green tea should be used cautiously in pregnant women, as caffeine crosses the placenta and has been associated with spontaneous abortion, intrauterine growth retardation, and low birth weight.
Caffeine is readily transferred into breast milk. Caffeine ingestion by infants can lead to sleep disturbances/insomnia.