Green Tea

Common names: Green tea also is known as tea, black tea, oolong tea.

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●●...Ethno or other evidence of efficacy.

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What is Green Tea?

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C. sinensis is a large shrub with evergreen leaves native to eastern Asia, where it is cultivated extensively. The plant has leathery, dark green leaves and fragrant white flowers. Cultivated tea plants are trimmed down to facilitate the harvest of the leaves.

What is Green Tea used for?

The dried, cured leaves of C. sinensis have been used to prepare beverages for more than 5,000 years. The method of curing determines the nature of the tea to be used for infusion, and green tea is 1 type of cured tea. Green tea is prepared from the steamed and dried leaves. By comparison, black tea leaves are withered, rolled, fermented, and then dried. Oolong tea is semifermented and considered to be intermediate in composition between green and black teas. Traditional Chinese medicine recommended drinking green tea for prevention of ill health, which is still regarded as a healthy practice in Asia. Green tea is less popular in the US than black tea.

Research stems from the belief that tea may have positive healing benefits because of its continued use as hydration for those suffering from illness and disease. Focused investigations on therapeutic effects began in Japan and China, and continued in Europe and the US. The polyphenol presence (beneficial antioxidant component) in tea may play a role in lowering cancer risk and the incidence of heart disease. Traditionally consumed as a beverage, green tea retains many chemicals of the fresh leaf.

The primary hypothesis for many of tea's health benefits involves the antioxidant capacity of the chemical constituents. This has been supported by in vitro experiments and epidemiological studies. Green tea is considered more active than black tea.

Population surveys and animal experiments provide evidence of green tea's potential as an anticancer agent. While this research is promising, there are no human clinical studies regarding the use of green tea for anticancer effects.

Epidemiological studies and in vitro experiments demonstrate that tea consumption may be beneficial against cardiovascular disease. While these results are promising, there are no human clinical trials regarding the use of green tea in cardiovascular disease.

Green tea exhibits antimicrobial actions against oral bacteria and provides a natural source of fluoride. Tea has antimicrobial activity against a variety of pathogenic bacteria and fungi, including HIV.

Animal studies suggest the antioxidant properties of tea may act as an anti-inflammatory agent. Population surveys suggest tea consumption helps protect against osteoporosis in older women, but animal studies suggest tannins may decrease the absorption of calcium to some extent. Experiments have suggested that the polyphenols in green tea may help in degenerative disorders such as Parkinson and Alzheimer diseases.

A trial of oolong tea in type 2 diabetes showed that tea reduces plasma glucose levels. Human studies suggest the thermogenic properties of green tea might contribute to weight control, but this has been debated.

Tea is historically known for its calming effects, probably because of L-theanine, an anino acid that produces tranquilizing effects. Theanine also is considered to counteract some of the stimulant effects of caffeine in tea and to lower cortisol levels during stress periods.

What is the dosage of Green Tea?

Studies suggest a daily dose of epigallocatechin gallate 800 mg (approximately equivalent to 16 Japanese-style cups of green tea) for 4 weeks to be safe and well tolerated. Research indicates that the addition of milk counteracts the favorable health effects of tea on vascular function.

Is Green Tea safe?

Contraindications have not yet been identified.

The Food and Drug Administration advises those who are or may become pregnant to limit caffeine intake.

Vitamin K present in green tea may antagonize the anticoagulant effect of warfarin.

There are no reports of clinical toxicity from daily tea consumption. Adverse reactions recorded in human studies using tea extracts include headache, dizziness, and GI symptoms. Tea may impair iron metabolism.

Research reveals little or no information regarding toxicity with the use of green tea.

References

Copyright © 2006 Wolters Kluwer Health

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