Cancer-Preventive Effects of Drinking Green Tea among a Japanese Population
Cancer is the leading cause of death in many countries. Along with cardiovascular disease, it has been the most important target of modern preventive medicine. Green tea has recently attracted attention as a natural product possessing preventative effects against cancer, so a prospective cohort study to examine the association between green tea consumption and cancer in a human population was begun in 1986 by Kazue Imai, Litt.D., Kenji Suga, and Kei Nakachi, Ph.D. of the Department of Epidemiology, Saitama Cancer Center Research Institute in Saitama, Japan. This ongoing study is supported in part by Grants-in-Aid for Cancer Research from the Ministry of Education, Science and Culture, and from the Ministry of Health and Welfare of Japan, among others.
Although the indications were clear from numerous laboratory studies that -epigallocatechin gallate (EGCG), the main constituent of green tea, was significantly anti-carcinogenic, the preventive effects of either green tea or black tea on cancer development among humans had not been well investigated. Widely varying results had been reported in the few studies undertaken with a human population. Most of the studies that showed no association or a positive one between tea and cancer were carried out in Western countries, while a negative association was mostly reported by case-control studies in Asian countries, especially China and Japan, where inhabitants daily drink large amounts of green tea. The reason for this discrepancy is in part ascribable to the fact that heavy tea drinkers and tea lovers were far fewer in study populations of Western countries than in those of Asian countries. In order to confirm the protective effects of green tea intake against cancer incidence, a study to examine whether green tea prevents cancer development in a population that includes a considerable number of subjects who consume large amounts of green tea was needed.
Imai et al‘s study found a negative association between green tea consumption and cancer incidence, especially among females drinking more than 10 cups a day. The slowdown in increase of cancer incidence with age observed among females who consumed more than 10 cups a day is consistent with the finding that increased consumption of green tea is associated with later onset of cancer. Relative risk of cancer incidence was also lower among both females and males in groups with the highest consumption, although the preventive effects did not achieve statistical significance among males, even when stratified by smoking and adjusted for alcohol and dietary variables.
Analysis of mean age at cancer onset among a total of 384 cancer patients showed that increased consumption of green tea was associated with delay of cancer onset. The association was especially remarkable among female patients who consumed more than 10 cups daily; their mean age at cancer onset over all age ranges was about 9 years later than that among those who consumed less than 3 cups a day. Combining data for males and females, the delay of cancer onset associated with increased consumption of green tea was 4 years, although this delay was not statistically significant among the male population.
Additional information on drinking of green tea among Japanese may be worth noting. According to parallel surveys on the living habits of the general population in Saitama prefecture, medium-size (180 ml) tea cups were used by 70 percent of the study subjects, followed by small-size (120 ml) and large-size (230 ml) cups, used by 17 and 13 percent, respectively. Measurement of EGCG in ordinarily prepared green tea showed 20 to 29mg per 100 ml, depending on type of green tea. According to this measurement, it is estimated that 10 cups of green tea supplies 300 to 400 mg of EGCG, assuming 150 ml of green tea per cup.
This study continued to demonstrate the preventive effects of green tea consumption against cancer in all sites. Future analysis will show more clearly which organs are most likely to receive green tea’s cancer-preventive effects. Results of recent laboratory tests indicate that EGCG in green tea can rapidly reach various target organs and exert its preventive effects against cancer. The strong potency of green tea in preventing cancers of various organs observed in vivo and among humans points toward a new strategy of cancer chemoprevention without toxic effects.
November/December 1997, pp. 769-775