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(Last edited: Monday, 30 March 2015, 9:58 PM)
Also known as: Green Sencha Tea, Japanese Tea.
Scientific name: Camellia sinensis, it is from the exact same tea plant as Black tea commonly used throughout the world.
Botanical family: Theaceae.
Parts used: Bud leaf and stem.
Green tea has been consumed and enjoyed as a beverage for centuries. Green tea is produced by steaming fresh leaves at high temperature. Unlike black and oolong (partially fermented) teas, green tea is not fermented.
Improving cognitive performance and mental alertness is a traditional use for green tea. It is also used to treat stomach disorders, vomiting, diarrhea, and headaches. Green tea has a history of use for weight loss. It has a role in cancer prevention and treatment including breast cancer, cervical cancer, prostate cancer, colon cancer, gastric cancer, lung cancer, leukemia, and skin cancer related to ultraviolet radiation (e.g. sunburn) and other environmental causes. It has traditionally been used for its anti-viral effects on the wart virus including genital warts, perianal warts and cervical cell changes. Other uses include osteoporosis, Crohn's disease, Parkinson's disease, cardiovascular disease, diabetes, low blood pressure, chronic fatigue syndrome, tooth decay, kidney stones, and skin damage.
Topically, green tea is used as a wash to soothe sunburn, as a poultice for bags under the eyes, as a compress for headache or tired eyes, and to stop the bleeding of tooth sockets. Green tea in chewable candy is used for inflammation of the gums. Green tea is also used topically to prevent skin damage and cancer related to ultraviolet radiation (e.g. sunburn) and other environmental causes.
There are no concerns about safety when consumed as a beverage in moderate amounts of 1-4 cups of green tea per day. (1,2,3,4,5,6,7,8)
Green tea extract as a supplement containing 7% caffeine has been used safely for six months. (9)
Avoid using high doses long-term because Green tea contains a significant amount of caffeine. Typically a cup of green tea contains 35-60mgs of caffeine.
Children: While no safety concerns have been highlighted avoid caffeinated drinks in children and limit to 1 cup per day. (11,12) Avoid giving green tea to infants, as they have an increased risk of anemia. (88)
Pregnancy: There are no reported problems when used orally in moderate amounts of two cups per day.
Breastfeeding: There are no particular concerns when used orally in moderate amounts of 1-4 cups per day. If the baby has loose stools exclude green tea, black tea and coffee to see if caffeine is contributing to the problem.
Mental Alertness. Green tea and other caffeinated beverages prevent a decline in alertness and cognitive capacity when consumed throughout the day. (10,13,14)
Bladder cancer, esophageal cancer, and pancreatic cancer. cervical dysplasia. Drinking green tea is associated with a reduced the risk of bladder cancer, esophageal cancer, and pancreatic cancer. (15,16,17,18,19)
Green tea as an oral or topical preparation seems to reduce cervical dysplasia caused by human warts (papilloma virus) infection. (20).
High Cholesterol. Green tea taken orally lowers cholesterol and triglycerides.(21)(22)
Low Blood Pressure. Consuming caffeinated beverages including Green Tea increases blood pressure in elderly people with low blood pressure after meals which may prevent falls. (23,24)
Oral leukoplakia (lesions of the mouth that may progress to cancer). Drinking green tea orally decreases the size of lesions in patients with oral leukoplakia. (25)
Ovarian cancer; Women who regularly consume green tea or black tea, have a lower risk of developing ovarian cancer.(92,25,26,27)
Parkinson's disease. Consuming green tea orally helps prevent or delay the onset of Parkinson's disease. (28, 29)
Prevention of colorectal cancer.
Population studies suggest that consuming green tea does not have any effect on colon cancer risk. (30,31,32)
Breast cancer. Population studies suggest that green tea does not seem to reduce the risk of initially developing breast cancer in Asian populations; (33.34) however, in Asian-American populations some evidence suggests that drinking green tea might reduce the risk of developing breast cancer. (35)
Additional population research suggests that Asian women who have had early breast cancer (Stage 1-11) who drink 3-5 of more cups of green tea daily seem to have reduced risk of breast cancer recurrence. (36,33)
Cardiovascular disease. A large-scale population study in Japan suggests that consuming 3 or more cups of green tea daily significantly decreases the risk of cardiovascular and all-cause mortality compared to drinking less than one cup daily. (32)
Diabetes. Population studies suggests that Japanese adults who consume 6 or more cups/day of green tea have a 33% lower risk of developing type 2 diabetes.(37)
Gastric Cancer. There is conflicting evidence about the effects of green tea on gastric cancer risk.
Gingivitis (inflammation of the gums); Green tea extract in chewable candy appears to reduce inflammation. (38)
High Blood Pressure. There is evidence of variable effects of green tea depending on the physiological state of the tissues when taken by individuals. Population studies in Chinese people show that drinking 120-599 mL of green tea or oolong tea daily is associated with a 46% lowered risk of developing hypertension compared with non-habitual tea drinkers. Drinking more than 600 mL per day is associated with a 65% reduced risk. (39) However, clinical studies on people with normal and high blood pressure show that green tea or black tea has no effect on their blood pressure. (40) This apparent contradiction is common with herbal medicines who only have an action when it is required to normalise the physiological terrain. It is one of the benefits of multi constituent medicines. Active constituents can vary from person to person depending on their individual physiology.
Lung Cancer. There is conflicting evidence about the effects of green tea on lung cancer risk. (32) (41)
Obesity. There is conflicting evidence about the effectiveness of green tea for obesity and weight loss. A meta-analysis of clinical studies suggests that, overall, taking green tea extract 576-714 mg/day along with caffeine seems to modestly reduce body mass index (BMI), body weight, and waist circumference compared to caffeine alone. But taking green tea extract without caffeine does not seem to significantly reduce weight or waist circumference. (42) (43)
Osteoporosis. Population research suggests that drinking green tea for ten years is associated with increased bone mineral density. (44)
Prostate cancer. Chinese men who consume green tea seem to have a lower risk of developing prostate cancer. (45) Preliminary clinical research suggests that men with advanced cancer who take green tea supplements (containing catechins 200 mg three times daily) for a year seem to have a reduced risk of progression to prostate cancer. (46)
Mechanism of action.
Polyphenols including flavanols, flavandiols, flavonoids, and phenolic acids are abundant in green tea. The flavanols are all referred to as catechins. These seem to be responsible for many of the proposed benefits of green tea. (19,20,48,49,50) and they may be responsible for its anti-inflammatory activity. (51)
They may inhibit the production of other inflammatory substances including COX-1and 2, leukotriene-B4 and the activity of 5-lipoxygenase and nitric oxide synthase.(52, 53) Green tea catechins may also protect cartilage by inhibiting proteoglycan and collagen breakdown.(54) Green tea polyphenols seem to lessen joint degeneration in laboratory models of rheumatoid arthritis. (55) Green tea also contains plant estrogens including beta-sitosterol. (41)
Green tea contains 2% to 4% caffeine or 10-80 mg caffeine per cup.(56) a typical amount is 35-60mgs per cup. The caffeine in green tea stimulates the central nervous system (CNS), heart, muscles, and possibly blood pressure.(57) Caffeine is thought to increase the release of neurotransmitters such as dopamine.(58) Caffeine also decreases airway resistance and stimulates respiration.(59) Caffeine may decrease GABA and Serotonin neurotransmitters in the Central Nervous System. (58) Caffeine stimulates stomach acid secretion, and increases fight or flight chemicals in the body.(60) Caffeine can have positive stimulating effects on the heart.(59) Caffeine can also immediately raise blood pressure, but might not have this effect in habitual users. (57)
Caffeine doesn't substantially affect the fluid status of people who drink caffeinated beverages on a regular basis. (61,62)
The caffeine content is also thought to be responsible for green tea's effects on mental performance. (13)
Some preliminary studies show that flavonoids found in green tea might reduce heart disease risk factors lipoprotein oxidation. However, green tea doesn't reduce inflammation, vascular reactivity, or lipid oxidation. (63,64,65,66,67)
Caffeine has been reported to cause increases and decreases in blood glucose.
Green tea may protect against some kinds of cancer.
Green tea polyphenols also appear to have activity against human wart virus and related cervical changes and genital warts (20,68) but the mechanism of action for this is not known.
The polyphenols in green tea appear to reduce the cellular adhesiveness of bacteria associated with dental disease. (38) Some evidence suggests that green tea might be useful in skin disorders such as excess hair, acne and also male pattern baldness. (70)
Green tea is thought to be beneficial for preventing skin damage and cancer from ultraviolet radiation. Areas of skin where green tea extracts were applied had fewer sunburned cells and less damage to skin cells. (71)
Green tea is also used for weight loss. Early evidence indicates that a green tea extract rich in catechins can increase calorie and fat metabolism. The caffeine, catechin, and theanine constituents of green tea might contribute to this effect. (72,73,74) Caffeine increases resting energy expenditure and cellular heat production. (75)
Tannins in green tea can reduce diarrhea and the polyphenols in green tea might have a beneficial effect on the gut flora.. (76)
For prevention of Parkinson's disease, caffeine in green tea may help to maintain levels of dopamine in the central nervous system by preventing the inhibition of dopaminergic transmission. These actions help reduce the expression of symptoms of Parkinson’s disease. (28)
Protecting people from developing Alzheimer's disease has got to be a major priority for aging Western populations. Early studies suggests that catechins in Green tea may prevent oxidation and cell death of neurons, which may help resist cell damage and maintain health. (77)
Population studies suggest that drinking green tea for at least ten years increases bone mineral density. The exact mechanism for the effects on bone is unknown, but several possibilities have been suggested. Tea leaves contain fluoride, which might slow osteoporosis. Tea also contains flavonoids and phytoestrogens, which might affect bone mineral density. Other proposed mechanisms include inhibition of bone resorption and effects on mineral metabolism by polyphenols and tannins. (44)
Green tea can cause digestive upset and dizziness, insomnia and agitation and confusion. These effects are more common with higher doses of green tea or green tea extract, equivalent to 5-6 liters of tea per day. (9,78)
There have been at least 14 cases of liver toxicity, usually linked to green tea extract products in pill form. (79,80) However, there has been at least one report associated with consumption of a green tea-containing beverage. (80) In most reported cases, green tea products were not assessed to determine if any contaminants were present. In most cases, liver function returned to normal after discontinuation of the green tea product. (79,80)
Interaction with herbs and supplements.
Anticoagulant/antiplatelet herbs and supplements:
There have been no reported cases of interaction with warfarin (this a pharmaceutical blood thinning agent).
Caffeine containing herbs and supplements: Other natural products that contain caffeine include coffee, black tea, oolong tea, guarana, mate, cola, and others. Some supplements for energy may contain caffeine; check the label.
Avoid large amounts of green tea along with supplements aimed at the market for sportspeople that contain creatine, caffeine or ephedra.(83,84)
Liver toxic herbs and supplements: Theoretically, avoid taking green tea supplements with other hepatotoxic herbs or supplements as there might be additive effects. (79,85) Check out herbs you want to use with Green Tea and avoid the combination if there are concerns about liver function.
Iron: Like black tea, green tea appears to reduce absorption of iron from foods. (86,87) However, a study of iron-deficient elderly patients suggests that concomitant use doesn't alter iron absorption in this population. (89) Iron levels are not affected in people with adequate iron intake. Theoretically, green tea might reduce the absorption of iron supplements. For most patients, this effect will not be clinically significant. On this account patients with iron deficiency are advised to consume black tea and green tea between meals. (87)
Interactions with drugs.
There is preliminary evidence that green tea might enhance the effects of doxorubicin (Adriamycin) on cancer cells. (69)
Caffeine accounts for most of the side effects for green tea. If you are on medication or need to start medication read the drug insert leaflet carefully. If there is an interaction with caffeine, or black tea or coffee be aware that green tea may have the same interaction.
Stimulants. Read the drug leaflet any nervous system stimulation medicine including amphetamines.
Drugs that inhibit caffeine metabolism; These include cimetidine (reduces stomach acid), fluconazole (anti-fungal), mexilitene (prescribed for heartbeat irregularities), fluvoxamine (antidepressant). Avoid green tea supplements or consult a medical herbalist.
Drugs that decrease caffeine clearance: terbinafine (prescribed for fungal infection of toe nails), quinolone antibiotics (the commonest prescribed is ciproflaxacin, apart from nalidixic acid all others end with –acin).
Hepatotoxic drugs. Avoid of green tea supplements.
Estrogens. The effect of caffeine on estrogens is complicated and depends on race and the dose of caffeine. High caffeine levels can elevate estrogens and low doses can stimulate. An average amount in daily intake of green tea is unlikely to be clinically significant but may be relevant to supplement doses.93
Drugs that reduce caffeine clearance; theophylline.
Warfarin. No reports have been noted with normal consumption, observe the usual vigilance.
Interactions with foods. Avoid excess caffeine intake when used in conjunction with black tea and coffee. Because the effects of caffeine are dose related avoid excessive amount of caffeine intake. The amount of caffeine in coffee is extremely variable, check the packaging or server, black tea averages 30-80mgs per cup. Check the amount in soft drinks and avoid high caffeine energy drinks.
Green tea appears to reduce absorption of iron from non-meat food sources. If you are concerned about this drink green tea between meals.
Adding milk to your green tea may reduce the benefits of green tea as the milk may reduce the absorption of some of the helpful constituents.
Interaction with laboratory tests.
Inform your medical provider if you are taking green tea supplements. Drinking 4-6 cups of green tea per day is unlikely to affect laboratory tests any more than 1-3 cups of coffee.
Interactions with diseases and conditions.
Generalized anxiety or nervous excitability: avoid caffeinated beverages or supplements.
Diabetes: caffeine may affect the presentation of hypoglycemic attacks. Avoid sudden increases or decreases of caffeine intake and monitor blood sugars more carefully if changing the amount of caffeine in your diet.
Glaucoma: Intake of caffeinated beverage (>/=180 mg caffeine) may not be recommended for patients with normotensive glaucoma or ocular hypertension.
Bleeding time: Caffeine is reported to have antiplatelet activity however, this interaction has not been reported in humans.
Heart rhythm irregularities: Caffeine in green tea can induce cardiac arrhythmias in sensitive individuals use with caution.
It is difficult to be precise with dosage of green tea as much depends on the duration of brewing and the quality of the raw herb.
Infusion: 1-6 cups for men per day and 1-4 cups for women per day if using one tsp. of the herb per cup of boiling water.
Powder/capsule: 1500 mg per day.
Oral: Doses of green tea vary significantly, but usually range between 1-10 cups daily. The commonly used dose of green tea is based on the amount typically consumed in Asian countries, which is about 3 cups per day, providing 240-320 mg of polyphenols.
The typical dose of caffeine:
Jpn J Cancer Res 1988;79:1067-74.