Summary
Drinking green tea reduces cardiovascular mortality over a 7-year period, according to Japanese researchers. There was no such benefit on cancer mortality.
Introduction
After water, tea is the most consumed liquid in the world. Among many types of tea, green tea is the most popular in Asia and the Middle East, but is now becoming increasingly popular in the Western world. The best green tea strains come from China and Japan, with India, Vietnam, and Sri Lanka also major producers.
For centuries, green tea has been credited with miraculous health properties, but these have not yet been established conclusively using Western evidence-based medicine techniques. This has now been partially redressed with a study from Japan, reported in the Journal of the American Medical Association. Here's a summary of the report.
What was done
The study in question was started in 1994, enrolling 40,000 people aged 40 to 79 living in the Ohsaki Prefecture in northeastern Japan. A questionnaire was sent to the participants, asking about their health, lifestyle, education level, and dietary intake (using a 40-item food-frequency format). Those with a history of stroke, coronary heart disease, or cancer were eliminated from the study at this stage.
The average recent consumption of 4 beverages was sought: green tea, oolong tea (a Chinese tea halfway between green and black tea), black tea, and coffee. The frequency of green tea consumption was categorized as: never, occasional, 1-2 cups/day, 3-4 cups/day, or 5 or more cups/day. (A cup in this part of Japan averages 100 mL.) As "never" and "occasional" answers only comprised 7% and 19% of the whole, data from these participants were combined into a single category.
The subjects were followed for up to 11 years for all-cause mortality and for up to 7 years for cause-specific mortality, using the Ohsaki National Health Insurance files; cardiovascular disease and cancer were the specific causes analyzed.
What was found
At baseline, there were roughly 5800, 4325, 3895, and 5040 participants in the four categories of green tea consumption. Those in the higher consumption categories tended to be slightly older, more likely to be unemployed, less likely to exercise, and to have high blood pressure and/or diabetes.
Over the 11-year follow-up period, green tea consumption was associated with reduced all-cause mortality, after adjustment for a large number of factors that might bias the findings. The Hazards Ratios, with "less than 1 cup/day" set as the standard 1.0, show the likelihood of death compared to the first category participants:
Less than 1 cup/day 1-2 cups/day 3-4 cups/day 5 or cups/day
Person-years 97,127 80,416 82,121 114,510
Deaths 1109 872 920 1308
Hazards Ratio 1.0 (set as standard) 0.96 0.90 0.84
It can be seen that those people drinking 5 or more cups of green tea a day had a 16% lower risk of all-cause mortality compared with those drinking less than one cup a day.
In the 7-year follow-up period, the hazards ratios for cardiovascular-related deaths in these categories were:
1-2 cups a day 3-4 cups a day 5 or more cups a day
0.87 0.77 0.74
Thus people drinking 5 or more cups of green tea had a 26% lower risk of death due to a cardiovascular disease, compared with those drinking less than one cup a day. In women, the risk was even lower: 31% (hazards ratio 0.69). The risk for stroke-related death was lower in the highest green tea consumption group than the risk for coronary heart disease; and that for ischemic (thrombotic or embolic) stroke was greatly reduced (51% reduction), while that for hemorrhagic stroke was not reduced.
For cancer deaths, the hazards ratios were not associated with any degree of green tea drinking (1.11, 1.16, and 1.11). And there were only weak or neutral associations between black tea or oolong tea and mortality.
What this study shows
This study shows associations between green tea drinking and lowered risk of mortality - all-causes and cardiovascular-related deaths. In particular, ischemic stroke was reduced, and heart attack to a lesser degree. The risk of cancer was not influenced by green tea drinking. Moreover, mortality was not affected by drinking black tea or oolong tea.
The polyphenols in green tea are thought to be the substances that bring the benefits, based on lab tests and animal experiments. However, they usually show effectiveness against cancer in such models, whereas in this study there was no effect on the frequency of cancer. Of course the antioxidant properties of polyphenols are often cited.
One might think that 5 or more cups a day of tea is an excessive amount. But this must be seen in relation to the small size of the cups used in this part of the world -100 mL, or 3.4 ounces. A US-style cup contains 8 ounces, so that 5 Japanese cups are equivalent to about 2½ to 3 cups in the USA.
This sort of study result should not be taken as irrefutable evidence that green tea has a protective effect. This can only come from prospective clinical trials, rather than retrospective analyses. However, there's no harm in becoming a green tea drinker while you're waiting a decade or more for such results.
