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• While it is known that tea contains flavonoids that are active in laboratory tests, studies have been undertaken to determine the degree of activity of these compounds in the human body.  These studies have evaluated the uptake of tea flavonoids from the gut and the changes in antioxidant capacity of plasma after tea consumption.  The goal of these studies is to determine whether tea consumption leads to reduced oxidative damage in the body.  Rietveld and Wiserman (2003) reviewed 11 studies which tested antioxidant parameters in relation to black or green tea consumption.  These studies used various assays to determine plasma antioxidant capacity.  The independent studies showed a consistent increase in antioxidant capacity of plasma after consumption of tea.  This indicates that the flavonoids do enter the human body and have an antioxidant function.

• Cigarette smoke contains multiple reactive oxidants that damage DNA and contribute to malignant transformation of cells and carcinogenesis.   The damaged cells are usually eliminated by repair enzymes.  Without this repair, mutations can result and accumulate, thus increasing an individual's risk of cancer.  Hakim et al. (2003) conducted a randomized, controlled trial to test the efficacy of regular consumption of commercially available green or black tea in reducing DNA damage in heavy smokers.  The researchers measured 8-hydroxydeoxyguanosine (8-OHdG) in urine, which represents a major form of oxidative DNA damage.  Drinking four cups of decaffeinated green tea daily for 4 months was associated with a statistically significant decrease in urinary excretion of 8-OHdG, while no change was observed with drinking black tea.  The researchers propose that regular use of green tea is a safe and feasible way to prevent chronic toxicities, but also recognize the need for additional studies with standardized teas and tea extracts.

Determination of Green Tea Dosage

• Pisters et al. (2001) conducted a Phase I trial in adults with incurable malignancy (refractory to standard therapy or for which no standard therapy existed) to determine the maximally tolerated dose of green tea extract in adult cancer patients.  In addition, the study examined safety, side effects, pharmacology, and anti-tumor activity of green tea extract in capsule form.  The dose-limiting side effects were related to gastrointestinal complaints and central nervous system stimulation .  The green tea extract used in the study contained 7% caffeine, which was thought to be responsible for the side effects.  The dose with tolerable side effects is equivalent to drinking seven to eight Japanese-style cups (120 mL) of green tea three times daily, or 1 gram of green tea extract per square meter body area.  While a decaffeinated product would likely be better tolerated, this study cautions that caffeine may be a important component of the chemoprevention activity of tea.  At the completion of the study, researchers observed no direct anti-tumor effect of green tea extract.

Studies of Green Tea in Specific Disease

• Green Tea and Cancer: The active ingredient in green tea that is thought to provide health benefits is polyphenol. There are several kinds of polyphenols: epigallocatechin gallate (EGCG), flavanols, and catechins. Polyphenols have shown to inhibit cell growth and contain strong antioxidant activity (proven with studies in mice). Although black tea does contain polyphenols, green tea contains significantly more of such substance. EGCG has been found to suppress the development of blood vessels, which means that blood supply to cancerous cells can be compromised; it also has been hypothesized to directly bind carcinogens and inhibit the growth of tumor cells. Catechins  have also been thought to inhibit tumorigenesis. Furthermore, a Japanese study in 1996 has shown that polyphenols can induce cell death in human lymphoid leukemia cells. It must be noted, however, that there have been few clinical trials to test such effects in humans. There needs to be further studies involving human trials before such benefits can be validated.

• Green Tea and Gastric Cancer: Among many researchers, it has been regarded that consumption of large quantities of green tea may have a protective benefit against the development of gastric cancer. This notion is supported by animal models that have demonstrated inhibition of tumor genesis after the administration of green tea extracts (polyphenols). As noted in a 2001 article in the New England Journal of Medicine, four of seven studies in China and Japan demonstrates an inverse relationship between green tea consumption and gastric cancer; two found an insignificant inverse relationship and one found an insignificant increase in risk. In a prospective-cohort study conducted by Tsubono et al results failed to find an inverse relationship between green tea intake and the risk of gastric cancer. It must be noted, however, studies that showed an inverse correlation involved the consumption of greater than 10 cups (1000 ml) of green tea per day; the Tsubono study only divided it's study categories into < 1 cup green tea per day, 1-2 cups green tea per day, 3-4 cups green tea per day, and > 5 cups green tea per day. In conclusion, it is still unclear whether consumption of green tea will reduce the chance of developing gastric cancer. While there are promising studies that show polyphenols found in green tea can have anticarcinogenic effects, consumers should not simply rely on such beverage to prevent the development of gastric cancer.