Green tea

Green tea is a product made from the Camellia sinesis plant and it is also known as Camellia theifera, Constituant Polyphénolique de Thé Vert, CPTV, EGCG, Epigallo Catechin Gallate, Extrait de Camellia Sinensis, Extrait de Thé Vert, Extrait de Thea Sinensis, Green Sencha, Green Tea Extract, Green Tea Polyphenolic Fraction, GTP, GTPF, Japanese Tea, Kunecatechins, Poly E, Polyphenon E, PTV, Té Verde, Tea, Tea Extract, Tea Green, Thé de Camillia, Thé Japonais, Thé Vert, Thé Vert Sensha, Thea bohea, Thea sinensis, Thea viridis, Yame Green Tea, Yame Tea, and other names.

Green tea has been used in alternative medicine as a likely effective aid in treating genital warts, high cholesterol, and maintaining mental alertness.

Green tea has been used in alternative medicine as a possibly effective aid in treating clogged arteries, endometrial and ovarian cancer, low blood pressure, osteoporosis, changes in cervical cells due to human papiloma virus (HPV), white patches in the gums and the prevention of Parkinson's disease.

Other uses not proven with research have included various cancers (bladder, esophagus, pancreas, breast, colon, stomach, leukemia, mouth, prostate, lung); acne, heart disease, diabetes, infertility, high blood presure, obesity, respiratory infections, improvement of athletic perfomrance, wrinkles and others.

It is not certain whether green tea is effective in treating any medical condition. Medicinal use of this product has not been approved by the FDA. Green tea should not be used in place of medication prescribed for you by your doctor.

Green tea is often sold as an herbal supplement. There are no regulated manufacturing standards in place for many herbal compounds and some marketed supplements have been found to be contaminated with toxic metals or other drugs. Herbal/health supplements should be purchased from a reliable source to minimize the risk of contamination.

Green tea may also be used for purposes not listed in this product guide.

Seek emergency medical attention or call the Poison Help line at 1-800-222-1222. Drinking green tea in very high amounts can cause headache, nervousness, sleep problems, vomit, diarrhea, irregular heartbeats, dizziness, and convulsions and can be dangerous and even fatal.

Follow your healthcare provider's instructions about any restrictions on food, beverages, or activity.

Avoid using green tea together with other herbal/health supplements that can also affect blood-clotting. This includes angelica (dong quai), capsicum, clove, danshen, garlic, ginger, ginkgo, horse chestnut, panax ginseng, poplar, red clover, saw palmetto, turmeric, and willow.

Drinking alcohol with this product can cause side effects including jitteriness, headache and fast heartbeat.

Avoid using stimulant drugs such as cocaine or amphetamines when using this product.

Other drugs may interact with green tea, including prescription and over-the-counter medicines, vitamins, and herbal products. Tell each of your health care providers about all medicines you use now and any medicine you start or stop using.

Do not take green tea without medical advice if you are using any of the following medications:

• adenosine (Adenocard);

• stimulant drugs (amphetamines, ephedrine, nicotine, others);

• nadolol (Corgard);

• asthma medications;

• medications for depression or mental disorders;

• medications that slow blood-clotting: ardeparin (Normiflo), aspirin, clopidogrel (Plavix), diclofenac (Voltaren, Cataflam, others), dipyridamole (Persantine), ibuprofen (Advil, Motrin, others), naproxen (Anaprox, Naprosyn, others), dalteparin (Fragmin), enoxaparin (Lovenox), heparin, ticlopidine (Ticlid), warfarin (Coumadin), and others;

• any medications to treat cancer;

• estrogens and birth control pills;

• any antibiotics or medication for fungal infections;

• medication for seizures;

• any medications for heart disease.

This list is not complete. Other drugs may interact with green tea, including prescription and over-the-counter medicines, vitamins, and herbal products. Not all possible interactions are listed in this product guide.

Vitamin K present in green tea may antagonize the anticoagulant effect of warfarin. Green tea consumption reduces the bioavailability of folic acid and may interfere with the absorption of iron. Green tea may stop bortezomib (Velcade) from working properly. Patients taking bortezomib should not drink green tea or consume any green tea products.

The dried, cured leaves of C. sinensis have been used medicinally for more than 5,000 years. Traditional Chinese medicine has recommended drinking green tea for the prevention of ill health, and in Asia, this is still regarded as a healthy practice. 2 , 3 , 4 , 5

Many of the health benefits of tea drinking are attributed to the antioxidant capacity of the chemical constituents 4 , 8 and are largely borne out by in vitro experiments and epidemiological studies. 3 , 5 , 9 In vitro experiments show a direct effect of tea on reactive oxygen species and chelation of metal ions, such as iron and copper. 4 Green tea is considered to be more active than black tea 3 , 8 with epicatechin and catechins ranking most potent of 24 tested plant polyphenolic flavonoids. 3 , 8 The various methodologies of antioxidant experiments markedly affects the rankings. 10 , 11 , 12 , 13 , 14 , 15 Most clinical trials demonstrate that tea consumption improves plasma antioxidant capacity and biomarkers of oxidative stress. 3 , 4 , 8 , 15

In 2006, the FDA approved a green tea–based ointment containing sinecatechin (polyphenon E) for the treatment of anogenital warts. Randomized, double-blind clinical trials have demonstrated the efficacy of the ointment ( Veregen ), which is considered to act via antiviral, immunomodulatory, antioxidant, and antiangiogenesis mechanisms. 16

Epidemiological studies and animal experiments have provided sufficient evidence of green tea's potential as an anticancer agent to warrant consideration, as demonstrated by the continued investigational interest in tea. Comprehensive reviews are available, but are limited in their findings because the studies included are too heterogeneous for meaningful comparisons. 3 , 5 , 9 Long-term and case-control studies suggest an inverse association between tea consumption and the risk of cancer of the colon, urinary bladder, stomach, esophagus, lung, pancreas, prostate, and squamous skin cells. 3 , 9 Increased risk of recurrence of breast cancer 9 and delayed onset of cancers in general have been described. 9 , 17 Results of epidemiological studies differ due to factors such as social class and lifestyle.

A Cochrane meta-analysis of the effects of green tea in cancer has found insufficient and conflicting evidence to support a preventative role. 7 The observed anticancer effects are largely attributed to the catechins found in tea, while action on tumors by theanine may be due to enhancement of the immune response. 18 A Cochrane review found no evidence suggesting the efficacy of green tea in preventing progression to malignancy in leukoplakia. 19

Mechanisms for anticancer activity of green tea have been investigated in animal models and laboratory experiments, but not yet demonstrated in vivo in humans. Doses used experimentally may not reflect usual tea consumption, and there may be a combination of effects or a combination of active compounds acting to produce the relationships reported in epidemiological studies. 3 , 9 , 20 The polyphenols in green tea inhibit cell proliferation. 7 The polyphenol epigallocatechin gallate increases the activity of antioxidants in specific organs in mice and thereby increases the overall chemoprotective effect of antioxidants in those organs. 3 , 7 , 9 , 17 Epigallocatechin gallate may also facilitate direct binding to certain carcinogens. 3 , 7

Polyphenols, especially catechins may protect cells from tumor development by enhancing gap junction communication between cells. 7 , 9 Tea has been shown to block tumor growth by sealing the receptors of affected cells. 9 , 17 , 20 Polyphenols may assist inhibition of tumorigenesis in a variety of organs including skin, lungs, oral cavity, esophagus, stomach, small intestine, colon, liver, pancreas, ovary, and mammary glands. 9 , 17 , 20 Green tea polyphenols induced apoptosis in a variety of cells, including human lymphoid leukemia and human prostate cells, 9 , 17 , 20 , 21 , 22 altered estradiol and sex-hormone binding globulin levels associated with the risk of breast cancer. 3 , 23 A risk of esophageal cancer from tea drinking has been suggested in epidemiological studies, but this has also been attributed to the scalding temperatures at which the beverage may be consumed. 3

Epidemiological studies and in vitro experiments show that tea consumption is inversely associated with cardiovascular disease, although a direct cause-effect relationship has not been conclusively demonstrated. 2 , 3 , 24 , 25 Mechanisms of action under investigation include reduced low-density lipoprotein (LDL) oxidation, enhanced endothelial cell functioning, hypotensive effects, effects on atherosclerosis, platelet aggregation, and improved cholesterol profiles. 25 , 26

Cholesterol-lowering effects of tea have been investigated in numerous clinical trials, including healthy volunteers, as well as obese children and adults. 26 , 27 , 28 , 29 , 30 A meta-analysis of trials up to June 2007 evaluated tea flavonoids for reducing the risk of cardiovascular events. A reduction in LDL cholesterol was found for green tea, but no effect on high-density lipoprotein (HDL) cholesterol was established. 27 These results are based on a limited number of clinical trials, because inclusion criteria for the individual trials and test compounds used are often too heterogeneous for meta-analysis. 25 , 27 Further trials evaluating effects on cholesterol and other cardiovascular markers showed decreases in total and LDL cholesterol for subgroups, 29 , 31 decreases in serum triglycerides, 31 , 32 and a decreased total:HDL cholesterol ratio. 30

Hypotensive effects of green tea have been evaluated, with the meta-analysis reporting no effect for green tea and black tea increasing blood pressure, 27 and other trials reporting a decrease in systolic pressure, 28 , 29 decreases in diastolic pressure, 33 , 34 or no effect. 30 Studies evaluating other markers of cardiovascular stress (inflammation and oxidative stress) include reported decreases in serum amyloid-alpha and malondialdehyde, 29 while others noted no effect on endothelial vascular reactivity. 30

A neuroprotective property has been suggested for green tea extracts, especially for epigallocatechin gallate and theanine. The pharmacological impacts are uncertain; however, several researchers have proposed a number of mechanisms by which it may act on the CNS that include free radical scavenging, iron-chelation, anti-inflammatory effects as well as modulation of enzymes involved in processing amyloid precursor protein. Animal experiments have shown positive findings for Alzheimer and Parkinson disease models. 35 , 36 , 37 , 38 , 39 Anxiolytic properties have been demonstrated for the theanine component of green tea. 40

Few clinical trials evaluate green tea extracts or catechins among diabetic populations, 25 , 41 , 42 , 43 , 44 with the majority of findings reported among obese adults or healthy volunteers. 33 , 34 , 45 Many studies report modest positive findings influencing glycosylated hemoglobin (HbA 1c ); however, a number of studies also report no effect on insulin sensitivity, fasting blood glucose, and glucose tolerance. 25 , 44 Dosages of epigallocatechin gallate range from 84 to 386 mg/day to support glucose homeostasis. 25

Placebo-controlled clinical trials suggest that the thermogenic properties of green tea might contribute to weight control, 2 , 3 , 25 , 46 but there is debate regarding the role of caffeine in this effect. 46 , 47 Catechins may inhibit the enzyme responsible for the degradation of norepinephrine. 25 , 48 Most studies report positive findings for increased fat oxidation, changes in abdominal and subcutaneous fat, and decreased waist circumference. 28 , 31 , 32 , 34 , 41 , 45 , 48 , 49 Confounders include the quantity and possible adjuvant role of caffeine, as well as the intervention diets used. 25 Dosage ranges used in these trials include 270 to 800 mg/day of epigallocatechin gallate, 33 , 48 and 125 to 625 mg/day of catchins. 32 , 49

Animal experiments have demonstrated increased bone density with administration of green tea extract. Bone strength and fracture rates were not reported; therefore, extrapolation to a human application is difficult. 50 Mechanisms of action suggested an antioxidant and anti-inflammatory action, as well as direct action on osteoblasts (to increase survival) and osteoclasts (suppression), and an immunological action, including modulation of cytokines. Animal studies also suggest the tannin component of green tea may decrease the absorption of calcium to some extent 2 , 50 ; while the provision of fluoride is recognized as 3 cups (720 mL) of green tea per day, providing approximately 4 mg/day of fluoride. 50

Epidemiological and case control studies suggest tea consumption helps protect against osteoporosis in older women; however, not all find a positive association for increased bone density, and data for bone fractures are conflicting. 2 , 3 , 50 Differences in study populations makes a meta-analysis difficult. Long-term human studies are needed that include outcomes of bone fracture and examination of the bone micro-architecture before conclusive recommendations can be made.

Large epidemiological studies have shown an inverse association between green tea consumption and the incidence of stroke. Smaller case control studies, however, provide equivocal results. 51 , 52 A meta-analysis of 10 trials meeting the inclusion criteria for the analysis found support for 3 cups (720 mL) of green tea or more per day in reducing the incidence of stroke and reducing the mortality from stroke. 52 The meta-analysis included 3 trials from Japan and Finland that provided stronger support for this finding than the 3 trials included from US-based populations, which had wider confidence intervals. 52 Suggested mechanisms of action involve impacts on hypertension, atherosclerosis, and thrombogenesis. 51

Green tea extracts (2% to 3%) have been evaluated for use as a topical photo-protective agent. Clinical studies have demonstrated a dose-independent protective effect via mechanisms other than a screening effect (the sun protection factor for the test materials was minimal). Erythema and inflammation consequent to solar-simulated UV exposure was reduced and was suggested to be related to protection against cutaneous immunosuppression and an antioxidant effect. 53 , 54 A 2-year clinical study, however, found no superiority of green tea polyphenols taken orally over placebo in photoaging by clinical and histological assessment. 55

Tea has been demonstrated to have in vitro activity against a number of pathogenic bacteria 56 , 57 , 58 and fungi. 59 Activity by epigallocatechin gallate against HIV has been demonstrated in vitro, with direct binding to CD4. 2

Evidence for protection against dental caries comes from kinetic and animal models, with few human trials published. Green tea exhibits antimicrobial actions against oral bacteria 2 , 3 , 60 , 61 and provides a natural source of fluoride. 3 , 7 , 50 Oolong and green tea inhibited bacterial adherence to tooth surfaces and inhibited the rate of acid production in animal studies. Black and green tea have been shown to inhibit amylase activity. 3 , 60

Pharmacokinetic studies of tea have been conducted in humans, with measurable levels of various chemical compounds found in the plasma, feces, and urine. Chemical constituents are able to cross the blood-brain barrier. 61 , 62 , 63 , 64 , 65 , 66 , 67 Bioavailability of active compounds appears unaffected by the addition of milk to tea, and can be enhanced in a fasted state. 3 , 5 , 14 , 67 , 68 Plasma levels of epigallocatechin gallate are increased 5-fold when green tea extracts are taken in the fasted state and may lead to potentially toxic doses. Extracts must, therefore, be taken with meals. 69

A daily intake of 3 to 5 cups/day (720 to 1,200 mL) of green tea will provide at least 250 mg/day of catechins. 7

Topical application of sinecatechins 3 times a day for a maximum of 6 weeks. 16

400 to 716 mg/day of catechins have been used in trials in divided dosages. 29 , 30

Dosages of epigallocatechin gallate range from 84 to 386 mg/day to support glucose homeostasis. 25

Dosage ranges used in trials include 270 to 800 mg/day of epigallocatechin gallate 33 , 48 and 125 to 625 mg/day of catchins. 32 , 49