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TraditionWARNING: DISCLAIMER: The below uses are based on tradition, scientific theories, or limited research. They often have not been thoroughly tested in humans, and safety and effectiveness have not always been proven. Some of these conditions are potentially serious, and should be evaluated by a qualified healthcare provider. There may be other proposed uses that are not listed below.
Adults (over 18 years old)
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.
Children (under 18 years old)
Green tea is not recommended for infants or children due to caffeine content.
SafetyDISCLAIMER: Many complementary techniques are practiced by healthcare professionals with formal training, in accordance with the standards of national organizations. However, this is not universally the case, and adverse effects are possible. Due to limited research, in some cases only limited safety information is available.
People with known allergy/hypersensitivity to caffeine or tannin should avoid green tea. Skin rash and hives have been reported with caffeine ingestion.
Side Effects and Warnings
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.
Pregnancy and Breastfeeding
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.