Green tea contains volatile oils, vitamins, minerals, and caffeine, but the primary constituents of interest are the polyphenols, particularly the catechin called epigallocatechin gallate (EGCG). The polyphenols are believed to be responsible for most of green tea's roles in promoting good health.56
Green tea has been shown to mildly lower total cholesterol levels and improve the cholesterol profile (decreasing LDL "bad" cholesterol and increasing HDL "good" cholesterol) in most,57, 58, 59, 60 but not all,61 studies. Green tea may also promote cardiovascular health by making platelets in the blood less sticky.
Green tea has been shown to protect against the oxidation of cholesterol to a more toxic molecule (oxidized cholesterol).62 Consumption of green tea increases antioxidant activity in the blood.63 Oxidative damage to LDL can promote atherosclerosis. While population studies have suggested that consumption of green tea is associated with protection against atherosclerosis,64 the evidence is still preliminary.
Several animal and test tube studies have demonstrated an anticancer effect of polyphenols from green tea.65, 66, 67 In one of these studies, a polyphenol called catechin from green tea effectively inhibited metastasis (uncontrolled spread) of melanoma (skin cancer) cells.68 The polyphenols in green tea have also been associated with reduced risk of several types of cancer in humans.69, 70, 71 However, some human studies have found no association between green tea consumption and decreased cancer risk.72, 73
In a double-blind trial, people with leukoplakia (a pre-cancerous oral condition) took 3 grams orally per day of a mixture of whole green tea, green tea polyphenols, and green tea pigments orally, and also painted a mixture of the tea on their lesions three times daily for six months.74 As compared to the placebo group, those in the green tea group had significant decreases in the pre-cancerous condition.
Compounds in green tea, as well as black tea, may reduce the risk of dental caries.75 Human volunteers rinsing with an alcohol extract of oolong tea leaves before bed each night for four days had significantly less plaque formation, but similar amounts of plaque-causing bacteria, compared to those with no treatment.76
Green tea polyphenols have been shown to stimulate the production of several immune system cells, and have topical antibacterial properties--even against the bacteria that cause dental plaque.77, 78, 79
One study found that intake of 10 cups or more of green tea per day improved blood test results, indicating protection against liver damage.80 Further studies are needed to determine if taking green tea helps those with liver diseases.
Tea flavonoids given by capsule reduced fecal odor and favorably altered the gut bacteria in elderly Japanese with feeding tubes living in nursing homes.81 The study was repeated in bedridden elderly not on feeding tubes, and green tea was again shown to improve their gut bacteria.82 These studies raise the possibility of using green tea in other settings where gut bacteria are disturbed, such as after taking antibiotics. Further studies are needed to clarify the role of green tea in this respect, however.
High-tannin tea has been shown to reduce the need for blood removal from people with iron overload, or hemochromatosis, in an open study.83 The tea had to be taken with meals and without lemon or milk to be effective. Tea is believed to help in hemochromatosis by preventing iron absorption.
In a double-blind trial, men with precancerous changes in the prostate received a green tea extract providing 600 mg of catechins per day or a placebo for one year. After one year, prostate cancer had developed in 3.3% of the men receiving the green tea extract and in 30% of those given the placebo, a statistically significant difference.84 These results suggest that drinking green tea or taking green tea catechins may help prevent prostate cancer in men at high risk of developing the disease.
There are four case reports in which certain types of leukemia or lymphoma (low grade B-cell malignancies) improved after the patients began taking green tea extracts.85