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 ancer 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.
-- As reported in Preventative Medicine Magazine
November/December 1997, pp. 769-775 |
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