Cancer-fighting potential of green tea

Several case-control studies have suggested that the consumption of green tea provides protection against gastric cancer, the second leading cause of cancer deaths throughout the world. Two of the most popular varieties of tea, the most consumed beverage in the world, differ primarily by color-black and green. Although both varieties come from the same plant, black tea is exposed to air during harvesting, which oxidizes certain ingredients. Green tea leaves, on the other hand, are steamed and heated, which removes the enzyme that causes oxidation. Some of the compounds in green tea that are saved from oxidation include polyphenols, known to have strong antioxidant effects and thought to inhibit a variety of cancers, including stomach cancer.

Green tea is widely consumed in Asian countries, and four studies have shown a significant inverse association between green tea and gastric cancer. Two other studies noted a nonsignificant inverse association. Up to now, very little evidence as to the relationship has been available from prospective studies.

New analysis

A group of Japanese researchers recently published a paper in the New England Journal of Medicine reporting the results of a prospective cohort study that examined the relationship between green tea and the development of gastric cancer.

The study began in January, 1984 with self-administered questionnaires completed by more than 26,311 residents, 40 years of age or older, who resided in three different Japanese municipalities. The questionnaire included items about frequency of recent consumption of certain beverages (green and black tea and coffee) and 11 items about food, as well as questions about alcohol consumption, smoking, personal and family history of disease, and health insurance. Frequency consumption of green tea was based on a 100-ml portion and was divided into five categories: never, occasionally, 1-2 cups per day, 3-4 cups per day, and 5 or more cups per day.

Green tea consumption varied substantially among the study subjects (less than one cup/day-19%; 1-2 cups-17%; 3-4 cups-22%; 5 or more cups-42%). Men and women with higher intakes tended to be older and ate more rice, green or yellow vegetables, pickled vegetables, fruits, and bean paste more frequently. Heavy smoking (20 or more cigarettes per day) was associated with higher intakes of green tea among men, but not among women.

During 8 years of follow-up, 419 cases of gastric cancer were noted. After adjustment for sex, age, presence or absence of history of peptic ulcer, smoking status, alcohol consumption and other factors, the relative risks associated with drinking 1-2, 3-4, and 5 or more cups of green tea per day compared with less than 1 cup per day were not statistically related to the development of gastric cancer. This was true regardless of whether the data for men and women were combined or separated. Of note is that no association was found between stomach cancer and consumption of black tea or coffee, either.

Why are these results contradictory with other studies?

The authors report that their food frequency questionnaire included only a limited number of items and could not be used to calculate the consumption of total energy and other nutrients. Although they adjusted for the consumption of dietary items other than the green tea as much as possible, some residual confounding by other dietary characteristics may have occurred. A second possibility is that in previous case-control studies, patients with stomach cancer may have decreased their green tea consumption after experiencing initial symptoms of their cancer, but before they were diagnosed. This change in practice my have biased their recall of past intake in such a way that they underestimated their true consumption, resulting in inaccurate data. In fact, studies have shown that patients with stomach cancer often decrease their tea consumption as much as 2 years prior to diagnosis, and that recall of their earlier diet is strongly affected by their recent diet.

A related editorial states that this study constitutes the strongest epidemiologic findings to date as to the relationship between green tea and gastric cancer. While the authors of the editorial note the possible confounding dietary factors referred to above, they conclude that people who enjoy green tea should continue to do so, as long as there is no expectation that doing so will lower the risk of stomach cancer.