An 11 year study from Japan involving over 40,000 subjects published this week in The Journal of the American Medical Association found that drinking green tea was associated with fewer deaths by stroke. Heart attack deaths were also decreased, but not as dramatically. The association was stronger for women than men, and stronger for greater amounts of tea drunk, though some benefit was noted for smaller amounts. The largest benefit was seen in those who drank more than 5 cups per day. No benefit was seen for those who drank black and oolong teas, and no benefit for any tea was seen in preventing gastric, lung, or colorectal cancers, the only cancers examined in the study.
These cardiovascular findings are no surprise, as there are previous studies supporting such a benefit. However, the large size of this prospective (following subjects for years) study adds a lot of credibility to the claim. The study does have limitations, which I think explain the lack of effect of black and oolong teas. Over 80% of tea consumption in Japan is green tea, and therefore the numbers of subjects drinking black or oolong teas may have been too small to detect a benefit. Alternatively, green but not oolong or black tea has large amounts of EGCG (epigallocatechin gallate), the antioxidant molecule thought to be responsible for cardiovascular benefit. Black and oolong teas have antioxidants as well, but these have not been quite so well-studied.
I find the cancer findings rather surprising, since they contradict many of the animal and test tube studies showing a cancer benefit. This study suffers from several serious limitations with regard to the cancer data. The incidence of colorectal cancer is relatively low in Japan compared to the US, and this low incidence makes it difficult to detect a benefit. For unknown reasons, the study did not track other common cancers, such as prostate and breast. For prostate cancer, the data from other studies are fairly strong in favor of a tea benefit.
This prominent study is certainly not the last word. We really need large interventional studies (giving half the subjects tea and the other half a placebo) to further solidify the cardiovascular data and to test the cancer data. These interventional studies need also to look at traditional risk factors such as high blood pressure, cholesterol levels, inflammation markers (CRP, SSA), etc. Such studies are the gold standard in clinical research, and there are very few such studies in tea research.
Another message from this study is that the most benefits are seen with large amounts of tea consumption–5 cups per day or greater. In Japan, it is not unusual to drink 10 or more cups per day. We must either drink much more tea, or think about taking high-quality capsules made from certain bioactive tea products. However, much more research is needed with capsules to ensure they have the same beneifts as drinking tea.
In summary, this large, generally well-done study adds to the body of evidence in support of tea as a dietary intervention that has cardiovascular benefits.