- Green tea is derived from the shrubs Camellia sinensis through a non-fermenting process.
- The evidence for an association between green tea consumption and the incidence of cancer is generally inconsistent.
- Drinking green tea appears safe at regular, habitual and moderate use.
Green tea is derived from the shrub Camellia sinensis, the same plant as used for black tea, but is not fermented.
A total of 27 systematic reviews and/or meta-analyses investigated the association between green tea consumption and the risk of cancer. In general, inverse associations for green tea consumption and risk of gastrointestinal tract, oesophageal (women), lung (non-smoking women), ovarian, prostate and breast cancer were found. Evidence for a protective role of tea intake on the development of cancers of liver, colorectum, pancreas, urinary tract, prostate, glioma, lymphoma, and leukaemia was, however, insufficient.
Drinking green tea appears safe at regular, habitual and moderate use (3-9 cups per day).
Summary last updated and revised in February 2014 by Jianping Liu and Xun Li.
Summary fully updated and revised in May 2012 by Jianping Liu and Xun Li.
Summary fully updated and revised in April 2012 by Jianping Liu and Xun Li.
Summary first published in September 2005, authored by Jianping Liu.
Jianping Liu, Xun Li , CAM-Cancer Consortium. Green tea (Camellia sinensis) [online document], http://cam-cancer.org/en/green-tea. February 13, 2014.
Green tea is derived from Camellia sinensis, an evergreen shrub of the Theaceae family, which grows mainly in parts of Asia, the Middle East and Africa. It comes from the same plant as black tea but it is produced in a non-fermenting process thus is believed to be the strongest tea with the least loss of herbal elements. According to the fermentation degree and plants sources, there are classifications as green tea, white tea, yellow tea, black tea and dark tea. With increasing fermentation, the number of bioactive elements decreases.
Green tea contains antioxidants. The main active ingredients identified in green tea include polyphenolic compounds such as epicatechin (EC), epicatechin-3-gallate (ECG), epigallocatechin (EGC), and epigallocatechin-3-gallate (EGCG), which are thought to be responsible for their anti-carcinogenic and anti-mutagenic activity. Other polyphenols in green tea include flavanols and their glycosides and depsides such as cholorogenic acid, quinic acids, carotenoids, trigalloylglucose, lignin, protein, chlorophyll, minerals (depending on the soil content, aluminum and manganese), caffeine, and very small amount of methylxanthines.
Application and dosage
Green tea can be consumed as a brewed beverage or as dietary supplements in form of liquids, capsules, or tablets containing green tea extract. Commercial products, or green tea for medicinal use, are provided by tea manufacturers or different pharmaceutical herbal medicine companies. Water for green tea should be around 80 to 85 °C; the higher the quality of the leaves, the lower the temperature; high-quality green teas can have new water added as many as five times. Currently, there is no established recommended dose for green tea extract. For cancer prevention, studies have examined the effects of habitually drinking between one to ten cups per day, although these findings require confirmation.
Green tea consumption began in China with legendary origins of more than 4,000 years ago. A legend suggests it was an Emperor called Shen Nung who accidentally discovered tea. Then green tea has an origin linked to the highest tier of Chinese society, which made it expensive and only accessible to the rich. It was not until the 14th century that green tea become widespread throughout China. It has been used for its taste and medicinal qualities in most of Asia. In the 16th century, green tea was introduced to Europe and the USA1.
The Kissa Yojoki (Book of Tea), written by Zen priest Eisai in 1191, describes how drinking green tea can have a positive effect on the five vital organs and discusses tea's medicinal qualities2.
Claims of efficacy/alleged indications
A variety of health benefits have been suggested including cancer prevention, reduced risk for cardiovascular disease, increased bone density, fat loss promotion and protection against neurodegenerative disease.
Mechanism of action
Human studies on the pharmacokinetics of green tea polyphenols have been conducted. Polyphenols from green tea have been shown to be powerful antioxidants with anticarcinogenic properties. EGCG, the most abundant and biologically active polyphenol in green tea, induces apoptosis and suppresses proliferation of cancer cells by modulating multiple signal transduction pathways. EGCG can also covalently bind to cysteine residues in proteins through autoxidation and subsequently modulate protein function3. Studies have shown that tea catechins inhibit hepatocyte growth factor receptor (MET kinase) activity in human colon cancer cells.
Prevalence of use
Tea is the most popular beverage next to water around the world, and green tea accounts for 20% of tea consumption. It is particularly popular in China, Japan, and other South-East Asian countries.
Green tea is sold as beverage or dietary supplement. It therefore does not need to be approved by the FDA or EMA (European Medicines Agency), but no medicinal claims associated with green tea can be made.
Cost and expenditure
The cost of green tea differs substantially due to variable growing conditions, horticulture, production processing and harvesting time. Therefore, on the Internet in China, prices for green tea generally range from 2.67 Euros/3.63 USD per 500g to 284.82 Euros/388.22 USD per 500g in bulk. The price of green tea in the USA and Europe is about 12.46 Euros/17 USD per 500 mg 100 caps.
A total of 27 systematic reviews and/or meta-analyses of epidemiological studies investigated associations between green tea consumption and the risk of developing cancer. Details of the included reviews are presented in Table 1. None of studies detected an increased risk for any cancer.
Cancer in general
Three recent systematic reviews4,6,7 and a 2009 Cochrane review25 concluded that there is insufficient and conflicting evidence to establish an association between green tea consumption and incidence of cancer. In general, inverse associations for green tea consumption and risk of gastrointestinal tract, oesophageal (women), lung (non-smoking women), ovarian, prostate and breast cancer were found. Evidence for a protective role of tea intake on the development of cancers of liver, colorectum, pancreas, urinary tract, prostate, glioma, lymphoma, and leukaemia was, however, insufficient.
A 2009 meta-analysis of 12 studies indicated a borderline significant association between highest green tea consumption and reduced risk of lung cancer, and an increase in green tea consumption of two cups/day was associated with an 18% decreased risk of developing lung cancer8, A newly published systematic review only included 5 human-related studies, and found insufficient evidence to support green tea use for preventing lung cancer9. Another meta-analysis of 6 cohort/case control studies published in 2012 found significant nonlinear dose-response relationship between green tea consumption and risk of lung cancer, showing favourable effect of green tea consumption especially when the dose was more than 7 cups per day10.
None of the three systematic reviews/meta-analyses published in 201211 and 201312,13 found a significant association between high/medium/low dose versus non/least of green tea consumption and cancer risk in overall population, but there seems to be reduced cancer risk with green tea consumption among female11,12 and in the Chinese13 population.
According to the latest systematic review (2013) including 7 cohort and 10 case control studies14, the association between green tea consumption and stomach cancer remains conflicting since the publication of the two meta-analysis in 201015 and 200916 and warmer tea seems to be associated with lower cancer risk. One systematic review based on a Japanese population published in 2012 found a consistent risk reduction of stomach cancer with green tea consumption in women only based on prospective data, whilst the inverse association among the general population was consistent according to retrospective studies17.
A meta-analysis of 13 studies published in 2011 found an inverse association with a borderline significance between tea consumption and primary liver cancer in both men and women. Green tea consumption was associated with a moderate reduction in risk (RR=0.79; 95% CI=0.68-0.93) for primary liver cancer18,19.
A 2006 meta-analysis of 8 studies indicated a reduced risk of colorectal cancer with green tea consumption from combined results19. The inverse association was observed only in case-control studies. Two systematic reviews published in 2012 both found reduced cancer risk in higher dose green tea consumption20,21. Inverse association of cancer risk and green tea consumption seems to focus in Asian population including Shanghai and Singapore people.
Among the three newly conducted meta-analyses22,23,24, only one22 (including one cohort and nine case control studies of different types of tea, two cohort and two case control studies of green tea) found protective effects on bladder cancer when pooling the data for green tea consumption only.
A systematic review published in 2009 found conflicting results for an association between green tea consumption and prostate cancer6, while a later meta-analysis of seven studies published in 2011 indicated that consumption of green tea may have a protective effect on prostate cancer in Asian populations, especially in Chinese people25.
A 2010 meta-analysis of nine studies found that increased green tea consumption (more than 3 cups a day) was inversely associated with breast cancer recurrence in the analysis of case-control studies but not in the cohort studies26. An updated meta-analysis published in 2013 found no significant association among cohort nor case control studies, however, a linear but not significant dose-response association was detected27.
No severe adverse effects have been reported in association with medicinal use of green tea31. According to the current systematic reviews/meta-analysis, drinking green tea appears to be safe at moderate, regular and habitual use.
One systematic review identified a total of 34 cases of hepatitis following the consumption of preparations containing green tea between 1999 and 2008, however the review concluded that the toxicity related to concomitant medications could also be involved32.
According to the reports of clinical trials, adverse events such as diarrhoea33, nausea and headache34 are generally considered irrelevant with green tea consumption. However some other researchers suggest that there is presumably a relationship between side effects of nausea, emesis, insomnia, fatigue, diarrhoea, abdominal pain and confusion and green tea use although green tea is well tolerated in most parts35.
Clinical pharmacokinetic and animal toxicological information indicated that consumption of green tea concentrated extracts on an empty stomach was more likely to lead to adverse effects than consumption after food.
Green tea can act as an antioxidant and induce superoxide dismutase enzyme, which could scavenge the free oxygen radicals generated by radiotherapy36. Green tea consumption was shown to increase the plasma concentration of 5-FU in a pharmacokinetic study in rats and in vitro37. Conclusions from existing pre-clinical studies regarding potential antagonism between bortezomib and green tea components are inconsistent38,39.
Pregnant women, nursing mothers and patients with cardiac problems are usually advised to avoid or limit their intake to two cups daily40. People with known allergy or hypersensitivity to caffeine or tannin should avoid green tea31.
Consumption of high doses of green tea or green tea extract (equivalent to five litres per day) may cause nausea, vomiting, abdominal bloating and pain, dyspepsia, flatulence, and diarrhoea. Excess consumption of caffeine from green tea may also cause central nervous system stimulation such as dizziness, insomnia, tremors, restlessness and confusion, and diuresis, irregularities in heart rate, and psychomotor agitation31.
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- Zheng JS, Yang J, Fu YQ, Huang T, Huang YJ, Li D. Effects of green tea, black tea, and coffee consumption on the risk of esophageal cancer: a systematic review and meta-analysis of observational studies. Nutrition and cancer. 2013;65(1):1-16. Epub 2013/02/02.
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