Regular green tea is 99.9% water, provides 1 calorie per 100 mL serving, is devoid of significant nutrient content and contains phytochemicals, such as polyphenols and caffeine. Polyphenols found in green tea include epigallocatechin gallate (EGCG), epicatechin gallate, epicatechins and flavanols, which have antioxidant, anticarcinogen, anti-inflammatory, and anti-radiation biochemical effects in vitro.
Other components include three kinds of flavonoids, known as kaempferol, quercetin, and myricetin. A remarkably higher content of myricetin is detected in tea and its extracts than in many other plants, and this high concentration of myricetin may have some implications for the experimentally-observed bioactivity of tea and its extracts in vitro.
Although numerous claims have been made for the health benefits of green tea, human clinical research has not provided conclusive evidence of any effects. In 2011, a panel of scientists published a report on the claims for health effects at the request of the European Commission: in general they found that the claims made for green tea were not supported by sufficient scientific evidence. Although the mean content of flavonoids and catechins in a cup of green tea is higher than that in the same volume of other food and drink items that are traditionally considered to promote health, flavonoids and catechins have no proven biological effect in humans.
There is no conclusive evidence that green tea helps to prevent or treat cancer in people. A review of existing studies concluded that while suggestive evidence existed, it did not amount to a clear indication of benefit.
Daily consumption of black tea (but not green tea) has been associated with a significant reduction in death from all cancers. There is limited evidence to suggest that green tea consumption may be associated with a slightly lower risk of esophageal cancer in the Chinese population, a lower risk of lung cancer in women, and a lower risk of oral cancer in Asian people. A 2015 meta-analysis of nine prospective cohort studies concluded that a high amount of green tea consumption may be associated with a lower risk of liver cancer in Asian women. This association was not seen in Asian men or when one cup of green tea was consumed daily. Similarly, another analysis of observational data conducted in 2012 suggested that green tea consumption may have a favorable effect on lung cancer risk. The observed effect was strongest in those who consumed more than seven cups of green tea daily. A 2011 meta-analysis of epidemiological studies found limited evidence that green tea consumption may be associated with a moderately reduced risk of liver cancer in Chinese and Japanese people. Limited evidence suggests that green tea consumption is not associated with the risk of developing pancreatic cancer or prostate cancer. The link between green tea consumption and stomach cancer risk is unclear due to inconsistent evidence.
Green tea interferes with the chemotherapy drug bortezomib (Velcade) and other boronic acid-based proteasome inhibitors, and should be avoided by people taking these medications.
Observational studies have shown a correlation between daily consumption of green tea and a lower risk of death from cardiovascular disease. In a 2015 meta-analysis of such observational studies, an increase in one cup of green tea per day was correlated with a 5% lower risk of death from cardiovascular causes. Green tea consumption may be correlated with a reduced risk of stroke. Meta-analyses of randomized controlled trials have found that green tea consumption for 3–6 months appears to lower systolic and diastolic blood pressures a small amount (about 3 mm Hg each).
Green tea consumption lowers fasting blood sugar but in clinical studies the beverage’s effect on hemoglobin A1c and fasting insulin levels was inconsistent.
Drinking green tea or taking green tea supplements decreases the blood concentration of total cholesterol (about 7 mg/dL), LDL cholesterol (about 2 mg/dL), and does not affect the concentration of HDL cholesterol. A 2013 Cochrane review performed a meta-analysis of longer-term randomized controlled trials (>3 months duration) and concluded that green tea consumption lowers total and LDL cholesterol concentrations in the blood.
A 2015 systematic review and meta-analysis of 11 randomized controlled trials found that green tea consumption was not significantly associated with lower plasma levels of C-reactive protein levels (a marker of inflammation).
Daily consumption of green tea is significantly correlated with a lower risk of death from any cause; an increase of one cup of green tea per day is linked with a 4% lower risk of death from any cause. A separate analysis found that an increase of three cups of tea or green tea per day was associated with a lower risk of total mortality in Asians and women.
There is no conclusive evidence that green tea aids in weight loss.
Moderate, regular, and habitual consumption of green tea is safe; however, there are reports of liver toxicity in humans after consuming high doses (10–29 mg/kg/day) of green tea extract dietary supplements, and high doses may act as a pro-oxidant to damage DNA.