Green tea is a type of tea that is made from Camellia sinensis leaves that have not undergone the same withering and oxidation applied when processing Camellia sinensis into oolong tea and black tea. Green tea originated in China, but its production has spread to many countries in Asia.
Several varieties of green tea exist, which differ substantially due
to the variety of the tea plant, growing conditions, horticulture,
production processing, and time of harvest.
Research and health effects
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. Polyphenols found in green tea include epigallocatechin gallate (EGCG), epicatechin gallate, epicatechins and flavanols.
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.
Cancer
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.
Cardiovascular disease
Daily consumption of green tea has been associated with a lower risk of death from cardiovascular disease. In a 2015 meta-analysis of observational studies, an increase in one cup of green tea per day was associated with a 5% lower risk of death from cardiovascular causes. Green tea consumption may be associated with a reduced risk of stroke.A 2013 Cochrane review of randomized controlled trials concluded that green tea consumption for 3–6 months appears to lower systolic and diastolic blood pressures a small amount (about 3 mm Hg each). Additional analyses examining the effects of long-term green tea consumption on blood pressure have reached similar conclusions.Glycemic controlGreen tea consumption lowers fasting blood sugar but in clinical studies the beverage's effect on hemoglobin A1c and fasting insulin levels was inconsistent.
