Green tea is made by adding hot water to the cured leaves of the Camellia sinensis plant. Green tea leaves are unoxidized, whereas black tea leaves have been allowed to oxidize. White tea is made from uncured and unoxidized tea leaves of the same plant. Teas such as yerba maté and chamomile tea are covered in other webpages.
Green tea has been shown to have neuroprotective, antioxidant, anti-inflammatory, and antihypertensive properties and may reduce the risk of stroke. Green tea appears to improve glucose metabolism and has been shown to support weight loss by suppressing hunger, increasing satiety, and increasing energy expenditure and fat oxidation.
Green tea consumption has been found to be associated with increased bone mineral density and may help prevent osteoporotic fractures in older women. Green tea may reduce the risk of liver disease, although it appears not to reduce the risk of liver cancer. Consumption of tea has been found to be associated with reduced risks of melanoma and adult leukemia, as well as gastric, ovarian, endometrial, colorectal, and prostate cancer.

Breast cancer-related effects of drinking green tea

Green tea contains catechins and other polyphenols thought to be responsible for its anti-cancer properties. Important green tea catechins include epigallocatechin gallate (EGCG), epigallocatechin and epicatechin. Note that epigallocatechin gallate is also known as epigallocatechin-3-gallate. Green tea is also a good source of quercetin and contains modest amounts of kaempferol and myricetin.

Human studies

Numerous population studies have reported that heavy green tea consumption is associated with lower risk of breast cancer. There is also some evidence that green tea consumption might help prevent recurrence in early-stage breast cancer. For example, one Chinese study found that breast cancer patients who were regular tea drinkers were approximately half as likely to progress as non-tea drinkers.

Green tea and green tea extracts

Green tea and green tea extracts have been demonstrated to have chemopreventive effects in laboratory studies. For example, green tea polyphenols have been found to inhibit the transformation of normal breast cells into breast cancer cells in a laboratory model of carcinogenesis induced by long-term exposure to low levels of environmental carcinogens.

EGCG

EGCG inhibits breast cancer growth
EGCG has been shown to inhibit tumor growth and angiogenesis of estrogen receptor positive (ER+) breast cancer in mice without affecting normal tissue growth (in fact, cancer cells are more sensitive to EGCG than normal cells). EGCG also has been found to inhibit the growth and induce apoptosis (programmed cell death) of HER2 overexpressing (HER2+) breast cancer cells, including those resistant to Herceptin. In addition, EGCG has been shown to reduce the growth and induce apoptosis of triple negative (ER-/PR-/HER2-) and inflammatory breast cancer (IBC) cells.
EGCG enhances some breast cancer treatments
EGCG has been shown to synergistically enhance the growth inhibition of ER+ cells by tamoxifen and Taxol (paclitaxel), thereby increasing the effectiveness of both therapies. Green tea intake has been found to enhance the inhibitory effects of Adriamycin (doxorubicin) in Ehrlich ascites carcinoma tumor-bearing mice and in ovarian cancer cells. Green tea polyphenols have also been reported to have potentiating effect on cisplatin chemotherapy in a mouse cervical cancer model (cisplatin is also used to treat triple negative breast cancer). In addition, EGCG has been shown to help prevent cyclophosphamide-induced ovarian damage.
It has also been demonstrated that EGCG does not interfere with the treatment effectiveness of radiotherapy and may enhance it.
However, there is evidence that green tea could also reduce the effectiveness of Adriamycin and other anthracycline chemotherapy due to its caffeine content. In the case of such treatment, decaffeinated green tea is a good option — choose organic green tea that has been decaffeinated using the CO2 process, which retains far more of the beneficial EGCG than methods using methylene chloride or ethyl acetate.

Quercetin

Green tea flavonol quercetin has been shown to increase the effectiveness of both Adriamycin (doxorubicin) and Taxol (paclitaxel) chemotherapy in multidrug resistant ER+/PR+ breast cancer cells, in part by eliminating cancer stem cells. In addition, quercetin has been reported to increase the sensitivity of ER+/PR+ cells to 5-fluorouracil (5-FU), thereby increasing its treatment effects. Quercetin has also been found to inhibit the migration and adhesion of triple negative (ER-/PR-/HER2-) breast cancer cells and to significantly inhibit tumor progression in a mouse model of triple negative breast cancer. Finally, quercetin also acts as an iron chelator, which can help reduce iron's breast cancer-promoting effects in some women.

Green tea supplements not recommended

While studies concerning green tea extracts and EGCG provide evidence of potentially beneficial effects of green tea, breast cancer patients should drink green tea rather than using high-dose green tea extracts or supplements. Although green tea consumption has been reported to be associated with reduced levels of circulating estrogen (in the form of estradiol (E2)), one study reported that green tea extract increased estradiol among women who consumed 1315 mg total catechins per day. There appears to be a level at which green tea polyphenols are estrogenic.
In addition, green tea supplements and herbal mixtures containing green tea in concentrated form have the potential to cause liver damage. A study commissioned by the European Food Safety Authority reported that taking 800 mg or more of EGCG per day in supplement form resulted in increases in a marker of liver damage compared to no EGCG supplementation. EGCG has also been reported to have the potential to cause kidney damage.

Additional comments

Selecting green tea

Organically-grown green tea is best
Tea is a often grown using relatively high levels of insecticides and herbicides. For example, some green tea imported into the U.S. from China has been found to contain high levels of pesticides, including the breast carcinogen DDT (banned in the U.S. since 1972). High levels of fungicides are also used in many tea fields. Organic green tea is the best choice.
Matcha tea
Japanese matcha tea normally has significantly higher catechin and caffeine levels than traditional green tea. To make matcha, young green tea leaves are ground into a powder (to which hot water is then added). This means that essentially the entire tea leaf is consumed, whereas green tea leaves normally are discarded after steeping. Matcha power incorporates high levels of insoluble fiber. The few studies that have included matcha tea have reported anti-cancer effects against both ER+ and ER- breast cancer cells.
However, we would be cautious about consuming matcha frequently, especially during treatment for ER+ breast cancer, since it is not clear whether estrogenic levels of catechins can be obtained by frequent matcha consumption (see the comments on green tea supplements above). In addition, it is important to purchase only high-quality organic matcha tea made using traditional methods. Matcha tea can easily be adulterated, for example, with food dyes and other chemicals.

Green tea in combination with other foods

Mushrooms
Green tea consumption has been found to further reduce breast cancer risk in combination with high dietary intake of mushrooms. In other words, there appears to be a synergistic effect in inhibiting breast cancer development when green tea and mushrooms are consumed together.
Folate
Green and black tea consumption decreases the bioavailability of folic acid by reducing the level of intestinal absorption and interfering with the folate pathway. This folate cycle inhibition is thought by some researchers to be one mechanism by which green tea protects against breast cancer. However, this is not a helpful characteristic to those, like pregnant women, who need to maintain their folate levels.
Iron
Tea also inhibits iron absorption. High levels of iron in benign breast tissue has been found to be associated with increased risk of breast cancer. Again, those with anemia or at risk for iron deficiency should be aware of this characteristic of tea. Supplements containing folate or iron should not be taken with tea by those who require additional folate or iron.

Caffeine linked to benign breast disease

Prepared green or black tea contains less than half the caffeine of the same amount of drip coffee. However, high caffeine intake may increase the risk of benign breast disease, particularly atypical hyperplasia, which is associated with increased breast cancer risk. Those sensitive to caffeine or with benign fibrocystic breast disease should take steps to limit their caffeine intake when preparing green tea (e.g., by using loose green tea leaves instead of tea bags and discarding the first infusion, or by drinking decaffeinated green tea).

Avoid very hot tea

Drinking extremely hot green tea (or very hot tea of any type) should be avoided. Rates of esophageal cancer are high in areas of the world where tea normally is consumed at high temperatures. In one study, the risk of esophageal cancer was found to be eight times higher for those who regularly drank very hot tea compared to those who drank warm or lukewarm tea. Note that it is the hot temperature that contributes to esophageal cancer, not the compounds found in green tea. In fact, one large Chinese population study found that relatively high urinary levels of green tea polyphenols were associated with lower risk of esophageal squamous cell cancer.

Sources of information provided in this webpage

The information above, which is updated continually as new research becomes available, has been developed based solely on the results of academic studies. Clicking on any of the underlined terms will take you to its tag or webpage, which contain more extensive information.
Below are links to 20 recent studies concerning green tea and its components. For a more complete list, including less recent studies, please click on green tea.