Green tea is a type of tea that is made from Camellia sinensis leaves and buds that have not undergone the same withering and oxidation process used to make oolong teas and black teas. Green tea originated in China, but its production and manufacture have spread to many other countries in Asia.
Several varieties of green tea exist, which differ substantially based on the variety of C. Sinensis used growing conditions, horticultural methods, production processing, and time of harvest. Although there has been considerable research on the possible health effects of consuming green tea regularly, there is little evidence that drinking green tea has any effects on health.
Polyphenols found in green tea include epigallocatechin gallate (EGCG), epicatechin gallate, epicatechins, and flavanols, which are under laboratory research for their potential effects in vivo. Other components include three kinds of flavonoids, known as kaempferol, quercetin, and myricetin. 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.
Consumption of green tea extract is linked to hepatotoxicity and liver failure.
Green tea leaves are initially processed by soaking in an alcohol solution, which may be further concentrated to various levels; byproducts of the process are also packaged and used. Extracts may be sold in liquid, powder, capsule, or tablet form. Decaffeinated versions are also available. Green tea extract supplements are accessible over the counter in various forms.
Regular green tea is 99.9% water, provides 1 Calorie per 100 mL serving, is devoid of significant nutrient content (table) and contains phytochemicals, such as polyphenols and caffeine.
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 green tea may enhance mental alertness due to its caffeine content, there is only weak, inconclusive evidence that regular consumption of green tea affects the risk of cancer or cardiovascular diseases, and there is no evidence that it benefits weight loss.
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.
The link between green tea consumption and the risk of certain cancers such as stomach cancer and non-melanoma skin cancers is unclear due to inconsistent or inadequate 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 found a minor correlation between daily consumption of green tea and a 5% 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 slightly 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 found that green tea consumption for 3–6 months may produce small reductions (about 2–3 mm Hg each) in systolic and diastolic blood pressures. A separate systematic review and meta-analysis of randomized controlled trials found that consumption of 5-6 cups of green tea per day was associated with a small reduction in systolic blood pressure (2 mmHg), but did not lead to a significant difference in diastolic blood pressure.
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 3–7 mg/dL), LDL cholesterol (about 2 mg/dL), and does not affect the concentration of HDL cholesterol or triglycerides. A 2013 Cochrane meta-analysis of longer-term randomized controlled trials (>3 months duration) 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).
In one study, daily consumption of green tea was correlated with a lower risk of death from any cause; an increase of one cup of green tea per day was 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 small 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.