Like black tea, 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. Black tea and herbal teas such as yerba maté and chamomile, mint, sage, and Essiac™ tea are covered in other web pages. Green tea has been shown to have neuroprotective, antioxidant, anti-inflammatory, and antihypertensive properties and may reduce the risk of stroke. Green tea may 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. Increased consumption of 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 that are thought to be responsible for its anti-cancer properties. Important green tea polyphenols include epigallocatechin-3-gallate (EGCG), epicatechin, epigallocatechin, and epicatechin-3-gallate, as well as gallic acid, myricetin and quercetin. 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 has been shown to inhibit tumor growth and angiogenesis of estrogen receptor positive (ER+) breast cancer in mice without affecting normal tissue growth.
EGCG also has been found to inhibit growth and induce apoptosis of HER2/neu overexpressing (HER2+) breast cancer cells, including those resistant to Herceptin. In addition, EGCG has been shown to reduce the growth and induce apoptosis of the stem-like inflammatory breast cancer (IBC) cells.
EGCG has been shown to synergistically enhance the growth inhibition of ER+ cells by tamoxifen and Taxol (paclitaxel), thereby increasing the effectiveness of such therapy. In addition, green tea extract has been found to protect against tamoxifen-induced liver injury in rats. 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 been shown to have potentiating effect on cisplatin chemotherapy in a mouse cervical cancer model (cisplatin is sometimes used to treat triple negative (ER-/PR-/HER2-) breast cancer). It is been reported that the (modest) kaempferol content of green tea might help protect against Adriamycin-induced heart damage. However, green tea could also reduce the effectiveness of Adriamycin and other anthracycline chemotherapy due to its caffeine content. In addition, one 2019 study reported that green tea extract increased circulating estradiol among women who consumed 1315 mg total catechins per day.
Numerous population studies have found 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 (stages I and II) breast cancers. A 2015 Chinese study reported that the positive association between green tea drinking and reduced breast cancer risk in postmenopausal women was strongest among those with ER- disease.
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). Organic green tea is the best choice.
Green tea consumption has been found to further reduce the risk of breast cancer in combination with high dietary intake of mushrooms. In other words, there appears to be a synergistic effect between green tea and mushroom consumption in reducing breast cancer risk.
Drinking very 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. Green tea should be prepared with hot, but not boiling, water to maximize its polyphenol content.
Green and black tea consumption decreases the bioavailability of folic acid in the diet 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. Similarly, tea inhibits iron absorption and high levels of iron in benign breast tissue has been found to be associated with increased risk of subsequent breast cancer. Supplements containing folate or iron should not be taken with tea by those who require additional folate or iron.
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).
One 2019 Chinese study that reported greater progression-free survival after breast cancer diagnosis among heavy green tea drinkers also reported that consumption of oolong tea was associated with impaired survival.
Green tea supplements and herbal mixtures containing green tea in concentrated form have been shown to have the potential to cause liver damage and should be avoided. For example, a 2018 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.
Below are links to recent studies concerning this food. For a more complete list, including less recent studies, please click on green tea.