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 other countries in East 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.
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.
Although using green tea as a health supplement green tea is associated with a slight improvement in overall quality of life, it is also associated with some potential adverse effects.
A 2020 review by the Cochrane Collaboration listed these as gastrointestinal disorders, higher levels of liver enzymes, and, more rarely, insomnia, raised blood pressure, and skin reactions.