Does Green Tea Aid in Cancer Prevention?October 2001
By Ivy Greenwell, Photography by Matthew Pace
In general, both chronic gastritis (an inflammation of the stomach mucosa that often leads to gastric cancer) and gastric cancer are related to H. pylori infection, lower socioeconomic status, reliance on salt-preserved foods rather than refrigeration and dietary deficiencies of antioxidant vitamins and micronutrients such as selenium. H. pylori infection, high sodium intake and a diet poor in antioxidants appear to be the main risk factors for gastric cancer, as confirmed by numerous studies.
Most studies find smoking and heavy drinking to be additional risk factors. One study includes moldy cereals among high-risk foods; this is plausible since we know that mold toxins are carcinogenic. In addition, a degree of genetic susceptibility also appears to play a role. However, this risk is confounded by the finding of much higher prevalence of H. pylori infection among the offspring of parents with gastric cancer. It is also likely that the same carcinogenic dietary habits (heavily salted, smoked and pickled food, with low consumption of fresh fruits and vegetables) are being passed from one generation to the next. Nevertheless, even though diet and H. pylori infection play an extremely important part in the development of stomach cancer, genetic susceptibility should never be ignored. Anyone with a family history of this deadly disease should be considered at risk, and would be wise to observe the preventive protocol outlined in the sidebar.
The key to prevention
There is no question that green tea contains anti-carcinogenic compounds. In particular, catechins, the main group of polyphenols found in green tea, have been found in numerous in-vitro and in-vivo studies to show significant antiproliferative, anti-cancer activity. The most active catechin appears to be epigallocatechin gallate.
Chinese scientists recently discovered that epigallocatechin gallate inhibits angiogenesis (the production of new blood microvessels) in mice inoculated with human colon cancer. This blocking of new blood vessel growth may be an important part of the overall anti-cancer action of polyphenols, since it impedes tumor growth (Jung 2001).
Other anti-cancer mechanisms of green tea polyphenols involve irreversible cell-cycle arrest, decrease in the production of inflammatory prostaglandin E2, inhibition of TNF-alpha release, and of certain enzymes involved in tumor proliferation, such as ornithine decarboxylase, cyclooxygenase and lipoxygenase. Matrix metalloproteinases, a group of enzymes involved in tumor invasion and metastasis, can also be inhibited with the green tea polyphenols and theaflavins, polyphenols found in black tea (Isemura 2000). Finally, green tea polyphenols have also been found to interfere with the activation of the various carcinogens through enhancement of the detoxifying phase II enzymes.
Likewise, the ability of green tea polyphenols to reduce the damage of nitrites in the acidic environment of the stomach has been found to be much stronger than that of vitamin C. An early study showed that green tea extract significantly inhibits the development of gastrointestinal tumors in animals and humans by blocking the formation of carcinogenic compounds containing the nitroso- group, such as N-nitroso-proline (Yan 1993).
Powerful antioxidant properties of green tea polyphenols are an inextricable part of their overall anti-inflammatory and cancer-preventive benefits.
Thus, we already know a great deal about the mechanisms through which green tea polyphenols inhibit both the development and progression of cancer. However, a skeptic might point out that most of this knowledge has come out of in vitro and animal studies. Fortunately, we do have some human epidemiological studies that do indicate the anti-cancer effectiveness of green tea, including specifically protection against stomach cancer.
The most recent of these, published in the May 2001 issue of The International Journal of Cancer, is a case-control study conducted in Yangzhong, a region in China with a high incidence of chronic gastritis and gastric cancer. Over 600 subjects participated in the study: 133 stomach cancer patients, 166 chronic gastritis patients and 433 healthy persons. Those with stomach cancer or gastritis were less likely to be green tea drinkers than healthy individuals (Setiawan, Zhang 2001). After adjustment for factors such as age, gender, smoking and body-mass index, frequent long-term green tea drinkers had on the average only one half the risk for either gastric cancer or gastritis. The study strongly indicated that the more green tea people consumed, and the longer the period of time they consumed it, the lower their risk of gastric cancer.
Gastritis is a chronic inflammation of the stomach lining that may lead to cancer; thus, it is considered a precancerous condition. Preventing inflammation of the stomach lining may be a very important part of the mechanism through which green tea helps protect against stomach cancer.
It should be pointed out that in China green tea is a popular beverage and, unlike in the West, it is not necessarily associated with an overall healthy lifestyle. In fact, green tea is a folk cure for hangover, and green tea drinkers may also be smokers. Thus, the protection against gastritis and gastric cancer is most likely due to the anti-inflammatory and anticarcinogenic properties of green tea catechins.
This study confirms an earlier Japanese study that also found a significant drop in the risk of chronic atrophic gastritis with high consumption of green tea. Subjects who drank more than 10 cups of green tea a day had a 37% lower risk of gastritis (Shibata 2000).
Another large Japanese study found that the risk of stomach cancer decreased by 31% with the intake of seven or more cups of green tea per day (Inoue 1998). This finding closely parallels an early, large Shanghai study that found a 29% drop in risk for green tea drinkers as compared with non-drinkers, with the risk decreasing in proportion to the daily amount of newly brewed green tea being consumed. A smaller study in another Chinese province found that low consumers of green tea had a 72% greater risk of stomach cancer. Overall, the studies appear to be consistent: green tea and fresh fruits and vegetables are associated with lower risk of gastric cancer.
Note that these studies indicate that long-term consumption of high doses of green tea may be required for a significant degree of protection. Five cups a day may not be sufficient, especially in the presence of heavy salt consumption, smoking, H. pylori infection, multiple antioxidant deficiencies and other risk factors. In-vitro data also indicates that the induction of cell death in gastric tumor tissue by green tea polyphenols depends on concentration and length of exposure, suggesting that only habitual daily intake of large doses of green tea polyphenols may provide anti-cancer protection (Hibasami 1998). Ultimately, it will take an interventional study to provide more reliable answers.