Green tea, or Chinese tea, is procured from Camellia sinensis, an evergreen shrub of the Theaceae family, mainly grown in Asia, Africa and the Middle East. Its fresh leaves are utilized for tea. It originated from the same plant as black tea, but it is obtained in a non-fermenting process; therefore, it is considered the most potent tea with the most negligible loss of herbal elements. As per fermentation degree and plant sources, tea is classified as green tea, white tea, yellow tea, black tea and dark tea. The number of bioactive elements decreases as the fermentation of the tea increases.
The extract is sold as a dietary supplement to regulate blood sugar, cholesterol, blood pressure, weight loss, and cancer prevention. Green tea contains polyphenols which include catechins, which are assumed to be accountable for the health benefits traditionally attributed to green tea. Catechins, such as epicatechin (EC), epicatechin-3-gallate (ECG), and epigallocatechin-3-gallate (EGCG) etc., are present, which are believed to be responsible for the antimutagenic and anticarcinogenic activity of the green tea. The most active catechin in green tea is epigallocatechin-3-gallate or EGCG, which has proved to prevent many processes linked with cell replication, thus letting tumour cells die. EGCG can also slow down or stop the development of blood vessels around tumours, a process known as angiogenesis.
(A) Cancer Prevention- Some studies have suggested that green tea helps prevent the following cancers:-
- breast cancer
- bladder cancer
- ovarian cancer
- colorectal cancer
- esophageal cancer 
- lung cancer
- prostate cancer
- skin cancer
- stomach cancer
Other than cancer, green tea is also used for treating:-
(C) Heart diseases like clogged arteries.
(D) High blood pressure.
(E) Inflammatory skin conditions
MECHANISM OF ACTION
Anticancer activities have been associated with polyphenol content, with chemopreventive qualities attributed to EGCG through apoptotic initiation and tumour antiangiogenesis. EGCG may hinder enzymes required in cell replication and DNA synthesis by interfering with cell-to-cell adhesion or intracellular signalling pathways needed for cell division.EGCG can also covalently bind to cysteine residues in proteins by autoxidation and modulate protein function. Studies have revealed that tea catechins repress hepatocyte growth factor receptor (MET kinase) activity in human colon cancer cells. It also hindered DNA replication in leukaemia cancer cell lines and modulated vascular endothelial growth factor (VEGF), causing apoptosis in leukemic cells.
Administration of green tea hinders UVB light-induced Carcinogenesis; when given ere and during carcinogen prophylaxis, it lessened incidence and number of stomach and esophageal tumours in mice .
Green tea is also believed to confer cardiovascular protection by raising HDL cholesterol, reducing LDL cholesterol and triglycerides, and preventing platelet aggregation. The flavonoid components of green tea may reduce lipoprotein oxidation hence avoid the blockage of arteries.
Although many studies claim green tea’s chemopreventive nature, whether it can completely inhibit the risk of developing cancer continues to be questionable, despite the numerous clinical and animal studies.
EFFECTIVENESS IN VARIOUS CANCER
Oral Cancer: The research team headed by Joshua Lambert deduces that EGCG can set off mitochondrial destruction in tumours and initiate apoptosis. The team examined by growing Oral Cancer cells and normal cells in Petri plates and exposed the contents to EGCG. EGCG selectively accepted the SIRT3 in Oral Cancer cells and influenced its mitochondrial function, whereas it didn’t harm the SIRT3 of healthy cells. The reverse happened. The researchers noted that EGCG enhanced the mitochondrial membrane potential in healthy cells, which is needed for effective metabolism.
Lung Cancer: A 2010 Taiwanese study examined tea drinking and smoking habits of approximately 500 people. The researchers determined that those who did not consume Green Tea are at higher risk of developing Lung Cancer. The study found that the risk was comparatively higher than those who do not drink green tea among smokers. Current research confirmed that EGCG could increase the levels of mi-R210 in Lung Cancer Cells. Cells with higher levels of mi-R210 divided with less rapidity than Lung Cancer cells with lower levels of mi-R210. Additionally, Lung Cancer cells with greater levels of mi-R210 lost the capacity to grow atop each other, thus controlling the severity of cancer
Leukaemia: A study published in the British Journal of Cancer examined whether there are any noticeable connections between the consumption of Green Tea and the risk factor of Leukemia by analyzing 107 adults diagnosed with chronic lymphocytic Leukemia. The case study outcomes show that Green Tea can minimize the risk of developing Leukemia, especially if consumed often in higher amounts. EGCG provokes caspase-dependent death in chronic lymphocytic Leukemia cells.
Prostate Cancer: Susanne M. Henning recommends that men who consumed Green Tea frequently had reduced prostate tissue inflammation correlated with cancer growth than those who did not drink it. Her team analyzed 79 men with Prostate Cancer and observed that the polyphenols in Green Tea could target prostate tissue and mitigate the inflammation of the prostate
Ovarian Cancer: A case-controlled study conducted in China during 1999 and 2000 found meaningful dose-response relations between Green Tea and ovarian cancer, i.e. the extended duration and frequency of Green Tea consumption would decrease the risk of developing Ovarian Cancer.
Breast cancer: A 2017 research examined the breast density in women administered with an EGCG supplement for a whole year. Women with a higher breast density have an increased risk of getting diagnosed with Breast Cancer. Although the EGCG supplement did not alter the breast density in older women, it caused a significant reduction in breast density in younger women.
Colorectal cancer: The results of the study published in Carcinogenesis shows that Green Tea can obstruct Colorectal Cancer in its tracks by principally targeting colon carcinogenesis in its primary stages. However, the EGCG in Green Tea might not be able to affect or repress the growth of adenocarcinomas developed before the administration of Green Tea.
APPLICATION AND DOSAGE
Green tea can be taken as a brewed beverage or dietary supplements in liquids, capsules, or tablets consisting of green tea extract. Tea manufacturers or different pharmaceutical companies provide marketing products or green tea for medicinal use .
Currently, there is no confirmed prescribed dose for green tea extract. The EGCG content of green tea infusion differs considerably due to plant variety, environment, age of leaves, season, production conditions, and factors associated with infusion, such as timing. A recommended range of green tea doses has been used across research studies, most commonly 400-800mg of green tea catechins per day. Beneficial effects of green tea on cancer risk have been documented with higher doses, more than 7 cups per day. One study has demonstrated the maximum endured amount to be 3 g/ m², equivalent to 20 cups of tea.
Side effects are often mild, rare and associated with caffeine content. The most common side effects consist of gastrointestinal and central nervous system disorders, including excess gas, upset stomach, nausea, vomiting, heartburn, abdominal pain, insomnia, diarrhoea, muscle pain, anxiety, fatigue, agitation, restlessness, convulsions, ringing in the ears and confusion etc. [6,7]. A few studies have seen serious side effects, including raised liver enzymes, stomach pain, insomnia, confusion, diarrhoea and anorexia etc. . Higher doses can cause nausea and hypertension and be associated with heightened neurological toxicity, linked to higher caffeine levels in the blood.
Although the U.S. FDA( Food and Drug Administration) adds tea to their list of “Generally Recognized As Safe” substances, pregnant women, lactating women should restrict their consumption because of caffeine content because tea can pass into breast milk and can cause sleep disorders in nursing infants. Ingestion by infants has been associated with impaired iron metabolism and microcytic anaemia.
Pregnant women, lactating women and patients with cardiac problems are usually advised to avoid or limit their intake to two cups daily. People with associated allergy or hypersensitivity to caffeine or tannin should refuse green tea. Most healthy women can harmlessly consume up to 300mg caffeine daily, although over 200mg has been associated with developing risk of miscarriage in some women with slow caffeine metabolism.
Consuming higher doses of green tea or its extract may cause nausea, vomiting, abdominal bloating and pain, diarrhoea, etc.. Excess caffeine consumption from green tea may also cause central nervous system stimulation such as dizziness, insomnia, tremors, confusion, diuresis, irregularities in heart rate etc.. Caffeine doses greater than 600 mg per day, or approximately 12 cups of green tea, have been linked with cardiac arrhythmias.
- Adenosine: The caffeine content of green tea may repress the hemodynamic effects of adenosine .
- Atropine: The tannin content of green tea may decrease the absorption of atropine.
- Iron supplements: The tannin content may lessen the bioavailability of iron. Green tea should be consumed 2 hours before or 4 hours following iron administration.
- Codeine: The tannin content may decrease the absorption of codeine.
- Bortezomib: EGCG and other polyphenols can restrain the therapeutic effect of bortezomib and additional boronic acid-based proteasome inhibitors.
- Tamoxifen: EGCG was proved to increase the oral bioavailability of tamoxifen, increasing the potential for their interactions.
- P-glycoprotein: EGCG represses P-glycoprotein and may cause interaction with irinotecan or verapamil; it increases the half-life of irinotecan, possibly enhancing both activity and adverse potential.
- UGT (Uridine 5′-diphospho-glucuronosyltransferase) substrates: Green tea modulates UGT enzymes and can increase the side effects of medications metabolized by them.
- Atorvastatin: Green tea limited hepatic drug uptake and heightened plasma exposure to atorvastatin in a murine model. Clinical evidence is not known.
Ziprasidone: A 23-year-old man who was stable for several months after medication intake became psychotic within days following using a green tea extract for weight loss. His symptoms resolved after eliminating green teafrom his routine.
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