- Chinese tea
- Green tea extract
- Green tea polyphenols
- Epigallocatechin gallate (EGCG)
For Patients & Caregivers
Green tea may help lower cholesterol. Evidence of its cancer preventive effects in humans is not conclusive.
Green tea contains substances called polyphenols, which scientists think contribute to its anti-cancer activity. Laboratory studies of one polyphenol, catechin epigallocatechin-3-gallate (EGCG), show that it may interfere with several of the processes involved in cell replication, causing tumor cell death (apoptosis). It also might slow the formation of blood vessels around tumors. Epigallocatechin (ECG), another polyphenol, stops leukemic cells from multiplying in laboratory studies. As a proven antioxidant, green tea may repair cell damage, but whether it can prevent cancer is uncertain. Tannins like those found in green tea generally have antibacterial properties. It is unknown how green tea might help protect the heart, but it reduces LDL (“bad”) cholesterol and increases HDL (“good”) cholesterol.
Animal studies indicate that oral consumption of green tea extract during fasting can increase the risk of toxicity. Human studies are needed.
- As an antioxidant
Studies have yielded mixed evidence.
- To prevent and treat cancer
Although laboratory studies show an anti-cancer effect, a few clinical trials and population surveys show mixed results. A number of studies in China have suggested that high intake of green tea may protect against cancers of the colon and stomach.
- To lower cholesterol and prevent heart disease
One clinical trial showed that theaflavin-enriched green tea extract can be used with other dietary approaches to lower certain type of cholesterol. Other studies have generally shown that green tea has no effect lipoprotein oxidation (a contributing factor in atherosclerosis).
- To improve mental functioning; for clear thinking
Caffeinated green tea may stimulate the nervous system.
- To lower high blood pressure
There is evidence that green tea can reduce the risk of developing high blood pressure.
- To prevent tooth decay
Laboratory studies show that green tea may prevent bacteria from attaching to teeth. This use has not been tested in clinical trials.
- To lose weight
One human study supports that green tea increases energy expenditure.
- To increase water loss (as a diuretic)
No studies support this use, although green tea contains caffeine, a diuretic.
- You are pregnant or breastfeeding: Caffeine passes into breast milk and may cause insomnia in the infant.
- You have a peptic ulcer: Green tea stimulates the production of gastric acid.
- You are taking adenosine: Caffeine may lessen its effects.
- You are taking atropine: Green tea may reduce its absorption from the gut and therefore lessen its effects.
- You are taking codeine: Green tea may reduce its absorption from the gut and therefore lessen its effects.
- You take warfarin or other blood thinners: In theory, very large amounts of green tea on the order of one-half to one gallon per day might lessen the effect of these drugs.
- You are using bortezomib (Velcade®): Polyphenols in green tea can inhibit the effects of this drug.
- You are taking drugs that are substrates of cytochrome P450 3A4: Green tea may increase the risk of side effects of these drugs.
- You are taking drugs that are substrates of UGT (Uridine 5’-diphospho-glucuronosyltransferase) enzymes: Green tea may increase the risk of side effects of these drugs.
- You are taking acetaminophen: Green tea increased liver injury caused by acetaminophen in mice when it was given after acetaminophen.
- Nausea, stomach upset
- The caffeine in green tea can cause insomnia, nervousness, or irritability.
- Liver toxicity: Several cases have been associated with consumption of green tea.
- Swelling, itchiness, and darkening of lower lip: In a 40-year-old woman following use of green tea for several years. Symptoms resolved after discontinuing consumption of green tea.
- Blood clots in small blood vessels: In a 38-year-old woman following green tea supplementation for weight loss.
For Healthcare Professionals
Green tea is a common beverage consumed in Asia. In recent years, green tea and its extracts have been used to prevent and treat hyperlipidemia, hypertension, atherosclerosis, and cancer. The active constituent is epigallocatechin-3-gallate (EGCG), which accounts for 40% of the total polyphenol content of green tea extract. Regular consumption of green tea may reduce the risk of hypertension (9) and positively affect mood (10). It may also enhance glucose tolerance in healthy individuals (11) (12), but does not improve insulin sensitivity or glycemic control in overweight or obese males (10) or in individuals with type II diabetes (13). A decaffeinated green tea extract was not associated with overall reductions in adiposity or improvements in BMD in overweight/obese postmenopausal women, but may reduce tissue and gynoid fat in those with higher BMI (60). Green tea may reduce mortality due to cardiovascular disease in both men and women (14). A theaflavin-enriched green tea extract can be used to lower low-density lipoprotein cholesterol (LDL-C) (8).
Studies of chemopreventive activity of green tea indicated positive results (1) (2) (3) (4) (5) (41). EGCG has been shown to modulate vascular endothelial growth factor (VEGF) leading to apoptosis in leukemic cells (7) and a green tea extract may benefit patients with chronic lymphocytic leukemia (52). In a population at high-risk for hepatocellular carcinoma, green tea polyphenols were found to mitigate fumonisin biomarkers (61). Data are not conclusive on whether consumption of green tea reduces the risk of colorectal and stomach cancers in women (6) (42). Another finding suggests that green tea drinking may increase the risk of breast cancer in postmenopausal women (62).
However, green tea was shown to reduce the risk of myelodysplastic syndromes (56) ; and supplementation with a blend of green tea, pomegranate, broccoli and curcumin resulted in a reduction in the rate of prostate-specific antigen (PSA) increase among men with prostate cancer following a PSA relapse post-radical treatment (59).
Caffeinated green tea may cause insomnia and nausea. Use of decaffeinated products may be preferred due to lower incidence of adverse events, but data are inconsistent regarding the relative efficacy of caffeinated versus decaffeinated teas. Patients undergoing chemotherapy should avoid consuming green tea products. Animal studies indicate that oral consumption of green tea extract during fasting can increase the risk of toxicity (49). Human studies are warranted.
The tannins in green tea may have antibacterial properties (28) and can produce anti-diarrheal effects. Green tea is thought to confer cardiovascular protection by increasing HDL cholesterol, decreasing LDL cholesterol and triglycerides (8) (29), as well as by blocking platelet aggregation. Flavonoids present in green tea may reduce lipoprotein oxidation (30). Green tea also contains caffeine, which has stimulatory effects and is responsible for the majority of adverse effects and drug interactions. It is unknown whether removing caffeine alters green tea’s activities (25).
The mechanism by which green tea influences blood pressure is thought to be mediated through the catechin, epigallocatechin-3-gallate’s (EGCG) modulation of vascular constriction. EGCG induces nitric oxide (NO) production through the activation of endothelial NO synthase, resulting in vasodilation (27).
In a recent study, green tea extract was shown to modulate effective brain connectivity during working memory processing in healthy subjects (55). EGCG was found to inhibit aggregation of tau protein, thereby reducing toxicity in neuronal model cells (57).
The anticancer activity of green tea is thought to be related to its polyphenol content. Its chemopreventive attributes are associated with EGCG, which is thought to induce apoptosis and tumor antiangiogenesis (20). EGCG may inhibit enzymes involved in cell replication and DNA synthesis by interfering with cell-to-cell adhesion or via inhibition of intracellular communication pathways required for cell division (21). In vitro data indicate that concentrations of 30 mcg/mL EGCG and (-)-epigallocatechin (EGC) inhibit lipoxygenase-dependent arachidonic acid metabolism by 30-75% in normal human colon mucosa and colon cancers (22). Other studies in human colon cancer cell lines suggest that EGCG inhibits topoisomerase I, but not topoisomerase II (23). EGCG also inhibits DNA replication in vitro in leukemia cancer cell lines (24). EGCG has been shown to modulate vascular endothelial growth factor (VEGF) leading to apoptosis in leukemic cells (7).
Administration of green tea inhibits UVB light-induced carcinogenesis (25), and when given before and during carcinogen treatment, reduces the incidence and number of stomach and esophageal tumors in mice (26). Topical EGCG may be useful as chemoprevention for skin cancer, but additional research and formulation are necessary (3).
- Although the U.S. Food and Drug Administration (FDA) includes tea on their list of “Generally Recognized As Safe” substances, pregnant women and women who breastfeed should limit their intake of green tea because of caffeine content.
- Because tea can pass into breast milk, it may cause sleep disorders in nursing infants. Green tea ingestion in infants has been linked to impaired iron metabolism and microcytic anemia.
- Individuals with peptic ulcers may want to avoid drinking green tea because it can stimulate the production of gastric acid (29).
- Nausea and GI upset, possibly due to tannin content. Insomnia, irritability, and nervousness can occur due to caffeine content.
- Hepatitis: Several cases have been associated with consumption of green tea (44) (46) (47) (58).
- Pruritic swelling/darkening of lower lip: In a 40-year-old woman following use of green tea for several years. Symptoms resolved after discontinuing green tea consumption (45).
- Thrombotic thrombocytopenic purpura: In a 38-year-old woman following green tea supplementation for weight loss (48).
- Adenosine: The caffeine content in green tea may inhibit the hemodynamic effects of adenosine (18).
- Anticoagulants / Antiplatelets: Theoretically, consumption of large amounts of green tea (.5-1 gallon/day) may provide enough vitamin K to antagonize the effects of anticoagulants and antiplatelet agents, though this effect has not been reported in humans (29) (34).
- Atropine: The tannin content in green tea may reduce the absorption of atropine.
- Iron supplements: The tannin content in green tea may reduce the bioavailability of iron. Green tea should be taken either 2 hours before or 4 hours following iron administration.
- Codeine: The tannin content in green tea may reduce the absorption of codeine (18).
- Bortezomib: EGCG and other polyphenols in green tea can inhibit the therapeutic effect of bortezomib (Velcade®) and other boronic acid based proteasome inhibitors (37).
- Tamoxifen: EGCG was shown to increase the oral bioavailability of tamoxifen, increasing the potential for their interactions (38).
- Verapamil: The bioavailability of Verapamil increased significantly in the presence of EGCG, thought to be due to P-glycoprotein inhibition by EGCG (39).
- Irinotecan: A study found EGCG to inhibit transport of irinotecan and its metabolite SN-38 into biliary elimination, resulting in their prolonged half-life which can increase toxicity (40).
- Cytochrome P450 3A4 substrates: Green tea extract inhibits CYP 3A4 enzyme and can affect the intracellular concentration of drugs metabolized by this enzyme (43) (44).
- UGT (Uridine 5’-diphospho-glucuronosyltransferase) substrates: Green tea modulates UGT enzymes in vitro and can increase the side effects of drugs metabolized by them (51).
- Acetaminophen: Green tea was shown to increase acetaminophen-induced hepatotoxicity in mice when administered following acetaminophen (53).
- Nadolol: Green tea extract inhibits OATP1A2 transporter and can reduce the absorption and plasma concentration of substrate drugs, like nadolol (54).