Green tea (Camellia sinensis)

Scientific names

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.

Ingredients

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.

History/providers

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 European and the USA 1.

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 qualities 2.

Claims of efficacy/alleged indications

Green tea contains antioxidants and research reports a variety of green tea’s health benefits are reported 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 function 3. 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.

Legal issues

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 RMB 22.00yuan/500g to RMB 2350.00 yuan/500g in bulk. Green tea if well packed will be expensive. The price of green tea in the USA and Europe is about 17 USD per 500 mg 100 caps.

A total of 12 systematic reviews and/or meta-analyses of epidemiologic studies investigated associations between green tea consumption and the risk of cancer. Details of the included reviews are presented in Table 1.

Cancer in general

Two recent systematic reviews 4-5 and a 2009 Cochrane review 6 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, ovarian, prostate and breast cancer were found. Evidence for a protective role of tea intake on the development of cancers of the lung, liver, colorectum, pancreas, urinary tract, glioma, lymphoma, and leukaemia was, however, insufficient.

Ovarian cancer

Two systematic reviews 7-8 of 18 studies found uniformly inverse association between green tea consumption and ovarian cancer risk.

Prostate cancer

A 2011 meta-analysis 9 of 7 studies investigating the association between green tea and black tea consumption with prostate cancer risk indicated that consumption of green tea may have a protective effect on prostate cancer in Asian populations, especially in Chinese people.

Liver cancer

A 2011 meta-analysis 10 of 13 studies found an inverse association with a borderline significance between tea consumption and primary liver cancer, and green tea consumption was associated with a moderate reduction in risk for primary liver cancer.

Stomach cancer

A 2010 meta-analysis 11 of 18 studies found a reduced risk of stomach cancer with intake of green tea. However, another meta-analysis 12 published in 2009 of 13 studies (9 studies were covered in the 2010 meta-analysis 11) indicated that the highest green tea consumption had no preventive effect on stomach cancer in the meta-analysis of cohort studies and even had an increased risk of stomach cancer using crude data.

Colorectal cancer

A 2006 meta-analysis 13 of 8 studies indicated a reduced risk of colorectal cancer with green tea consumption from combined results. The inverse association was observed only in case-control studies.

Breast cancer

A 2010 meta-analysis 14 of 9 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 studies.

Lung cancer

A 2009 meta-analysis 15 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 cancer.

Green tea, as a popularly consumed beverage, is generally non-toxic 16. No severe adverse effects have been reported in association with medicinal use of green tea 17. The above Cochrane review concluded that drinking green tea appears to be safe at moderate, regular and habitual use 6.

Adverse events

In response to the publication of adverse event case reports involving green tea products, one systematic review by the US Pharmacopeia checked the safety of green tea extracts 18. The review found a total of 216 case reports on green tea products including 34 reports concerning liver damage among which 27 pertaining to liver damage were categorized as possible causality and 7 as probable causality. 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 in the fed state. Based on this safety review, it was determined that when dietary supplement products containing green tea extracts were used and formulated appropriately, the unawareness of significant safety issues would prohibit monograph development, provided a caution statement was included in the labelling section. Following this decision, monographs may be produced for green tea extracts, and verification may be offered for programmes related to that dietary ingredient.

Interactions

Green tea can act as an antioxidant and induce superoxide dismutase enzyme, which could scavenge the free oxygen radicals generated by radiotherapy 19. Green tea consumption was shown to increase the plasma concentration of 5-FU in a pharmacokinetic study in rats and in vitro 20.Conclusions from existing pre-clinical studies regarding potential antagonism between bortezomib and green tea components are inconsistent 22,23.

Contraindications

Pregnant women, nursing mothers and patients with cardiac problems are usually advised to avoid or limit their intake to two cups daily 21. In general, the stimulatory effect from green tea is considerably less than that of coffee 16.

People with known allergy or hypersensitivity to caffeine or tannin should avoid green tea 17.

Warnings

Consumption of high doses of green tea or green tea extract (equivalent to five liters per day) may cause nausea, vomiting, abdominal bloating and pain, dyspepsia, flatulence, and diarrhoea 17. Excess consumption of caffeine from green tea may also cause central nervous system stimulation such as dizziness, insomnia, tremors, restlessness and confusion, and diuresis (increasing urine output), irregularities in heart rate, and psychomotor agitation 17.