Green tea (Camellia sinensis), an evergreen shrub native to eastern Asia, is cultivated extensively in India, China, Japan, Indonesia, Sri Lanka, Turkey, and Pakistan, as well as other parts of the world, such as Africa and South America. The Chinese have used the leaves to prepare beverages for over 4,000 years, believing that the tea infusion can cure many diseases and prevent heart problems and cancer. Green tea also has been used to flavor food, and the essential oil has been applied in perfumes and cosmetics.
Green tea is prepared from the steamed and dried leaves of the shrub. Different methods of curing the leaves create different tea products. Black tea is produced by withering, rolling, fermenting, and then drying the leaves. Oolong tea is semifermented and considered an intermediate product between green and black tea.^1,2 Tea was introduced to Europe in the 17th century and became a favorite beverage, especially in the form of black tea. Today, almost two-thirds of the world’s population consume tea in different forms (78% black, 20% green and 2% oolong). Green tea is consumed mainly in Japan and China.³ Tea drinking in Asian countries has evolved into a cultural habit and a part of everyday life.

Chemical Constituents
Several chemical constituents of green tea have been identified. The variation in chemical composition is further complicated with fermentation and other treatments. The most abundant components of green tea are polyphenols, such as gallic acid and catechin, and their derivatives theogallin, gallocatechin, epiatechin, and epigallo catechin. The fresh leaves contain 3%–4% caffeine (depending upon development), theobromine (0.15%–0.2%), theophylline (0.02%–0.04%), and other methylxanthines.¹ With fermentation, catechins partially change into oligomeric quinones, including theaflavine, theaflavine acid and thearubigene, or to the non-water-soluble flavonoids such as quercetin, kaempferol and myrecetin.^1,4,5 Green tea contains B-vitamins and ascorbic acid, which are destroyed in the process of making black tea. The essential oil contains more than 300 components including aldehydes, phenylethyl alcohols, phenols, hexenal, hexenol, linalool, dihydroactinidiolide and p-vinylphenol.⁵

Physiologic Activity and Therapeutic Use

Freshly brewed green tea has an aroma with a slightly bitter taste. It stimulates the nervous system and has a diuretic effect. Possible pharmacologic actions of green tea include effects on lipid metabolism; antimicrobial, antioxidant, and antimutagenic effects; and a wide variety of anticancer actions. In vitro and animal studies evoked high hope that regular consumption of tea, especially green tea, would have a direct lowering effect on plasma LDL levels. However, research has not supported such claims.
Antimutagenic and Anticarcinogenic Effects: Green tea has demonstrated activity against various mutagens,⁶ and its anticarcinogenic activities have been extensively examined. The polyphenol components of green tea may possess chemopreventative properties. Their mechanisms of antimutagenesis and anticarcinogenesis include the modulation of extracellular and intracellular metabolic and proliferative processes.^6,7 There is evidence that induction of phase II enzymes glutathione-S-transferase and quinone reductase are enhanced by nearly all tea fractions.⁸

Tea consumption has been shown to protect against chemical-induced stomach, lung, esophagus, duodenum, pancreas, liver, breast and colon carcinogenesis in specific bioassay models.⁶ A combination of curcumin (from the spice turmeric) and the tea ingredient epigallocatechin-3 gallate proved to be even more effective in inhibition of malignant human oral epithelial cells.⁹ Green tea extract exposure to human stomach cancer cells inhibited growth and induced programmed cell death (apoptosis). These data suggest that green tea consumption may protect humans from stomach cancer.¹⁰

Green tea and tea components were studied in the modulation of antitumor activity of doxorubicin in mice with Erlich ascites tumors. The oral administration of green tea extract enhanced the inhibitory effects of doxorubicin on tumor growth 2.5-fold.¹¹ The results suggest that drinking green tea can positively influence cancer chemotherapy.

Studies in mice showed that green tea inhibited chemical-induced hepatotoxicity.¹² Naphthoquinone-induced hepatotoxicity was evaluated in rat hepatocytes by measuring lactate dehydrogenase (LDH) leakage. Cell viability improved after treatment with either tea extract or tea polyphenols. A mechanism of protection was suggested to be closely related to the protein thiol content modulated by tea polyphenols.¹³
Clinical Studies: Epidemiological and human clinical studies produced varying results. Most reported that green tea reduced risk of esophageal, pancreatic and stomach cancer. However, a positive association between green tea consumption and development of stomach and colon cancer also has been reported.¹⁴

In Japan, a large-scale study carried out from 1990 to 1995 included 1,706 histologically diagnosed cases of digestive tract cancers and a total of 21,128 non-cancer outpatients. High intake of green tea (7 or more cups/day) reduced the odds ratio of stomach cancer to 0.69 (95% confidence interval).¹⁵ Another study, including 8,000 women, revealed that drinking 10 cups or more of green tea daily delayed cancer onset. Stages I and II breast cancer patients who consumed 5 or more cups/day experienced lower recurrence with longer disease-free period compared to occasional tea drinkers.¹⁶ This indicates that consumption of green tea could play a role both in cancer prevention and as an adjunct to chemotherapy.

The antibacterial effects of green tea against oral bacteria and several pathogenic strains common in the GI tract also have been well documented. Green tea polyphenols completely inhibited the growth and adherence of Porphyromonas gingivalis in a concentration of 250–500 mcg/mL.¹⁷ After 5 minutes of contact, green tea polyphenol solution (1 mg/mL) inhibited the proliferation of Streptococcus mutans.¹⁸ In general, green tea extracts proved to be bactericidal. They inhibited the growth of diarrhea-causing Staphylococcus aureus, Staphylococcus epidermidis, Vibrio parahaemolyticus, Campylobacter jejuni and Vibrio cholerae O1.¹⁹ The extracts also proved effective against pathogenic methicillin-resistant S. aureus (MRSA) and, to some extent, against penicillin-resistant S. aureus.²⁰ In vitro tests also showed that green tea was effective against some pathogenic fungi.²¹

The antioxidant activity of catechins has been studied in various experimental and human conditions. It is now generally accepted that catechins possess antioxidant activity in vitro and in vivo. Iron-induced oxidative stress was investigated in rat brains. Infusion of green tea extract prevented oxidative injury induced by iron. Both iron-induced lipid peroxidation products and iron-induced decrease in dopamine were suppressed.²²

A Japanese longevity study suggested that regular green tea consumption could contribute to an extended life span. More than 3,000 practitioners of chanoyu (Japanese tea ceremony) were followed between 1980 and 1988. Their standardized mortality ratio was lower (0.55) compared to that of the general Japanese female population (0.57).²³

Other components of green tea, such as caffeine and tannin, can be effective in treating different pathological conditions. Caffeine is an effective stimulant of the central nervous system and can be useful in treating headaches and enhancing both stomach acid production and renal excretion of water. Caffeine is frequently included in weight loss pills and used as a cardiotonic agent.

Green tea also has been employed in cases of hepatitis, for protection of the liver against chemical toxins, and (topically) in various skin disorders. Psoriasis and a number of other common skin diseases are treated with psoralens combined with exposure to ultraviolet A (PUVA). Although this treatment combination is highly effective, careful follow-up cohort studies have shown that it greatly increases risk for the development of cutaneous squamous cell carcinoma and melanoma.²⁴ It has been shown that green tea and constituent polyphenols protect against ultraviolet B-induced carcinogenesis and reduce the growth rate of established tumors in the skin. Zhao and coworkers showed that pre- and post-treatment with standardized green tea extract abrogate PUVA-induced photochemical damage to skin.²³

Precautions

Moderate consumption of green tea does not pose any hazard. However, women who are or intend to become pregnant should avoid caffeine-containing products. Also, there is evidence that highly condensed tannins and catechin could induce esophageal cancer. Precaution is necessary in patients who are taking anticoagulants. It has been reported that green tea may antagonize the effects of warfarin, an oral anticoagulant that requires close monitoring to prevent complications.²⁵ Tea combined with milk may prevent the undesired effects of tannins. Patients with high blood pressure, insomnia, asthma, heart conditions or elevated cholesterol should consult their physician before becoming a regular tea drinker.

Dosage

The beneficial effects of tea infusion are typically seen at the dosages normally consumed by humans. Overdose (quantities more than 300 mg caffeine, more than 5 cups/day) can lead to negative effect on the CNS and irritation of the stomach.

Conclusion

Green tea is a popular beverage in Asia and is becoming recognized in the U.S. both as a beverage and as an herbal remedy. Detailed multidisciplinary research on the effect of green tea is available, and the following conclusions can be derived: 1) green tea drinkers may enjoy a lower risk of developing certain cancers; 2) tea modifies oral and intestinal microflora, reducing undesirable bacteria; 3) tea and tea polyphenols selectively induce phase I and phase II enzymes, which may aid in detoxification of harmful metabolites; and 4) the antioxidant effect of green tea may control inflammatory processes, protect against DNA damage, and prolong life span.

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