"This is the first large-scale study to examine the combined effects of both green tea and coffee on stroke risks," said Yoshihiro Kokubo, M.D., Ph.D., F.A.H.A., F.A.C.C., F.E.S.C., lead author of the study at Japan's National Cerebral and Cardiovascular Center. "You may make a small but positive lifestyle change to help lower the risk of stroke by adding daily green tea to your diet."
Researchers asked 83,269 Japanese adults about their green tea and coffee drinking habits, following them for an average 13 years. They found that the more green tea or coffee people drink, the lower their stroke risks.
People who drank at least one cup of coffee daily had about a 20% lower risk of stroke compared with those who rarely drank it.
People who drank two to three cups of green tea daily had a 14% lower risk of stroke, and those who had at least four cups had a 20% lower risk than those who rarely drank it.
People who drank at least one cup of coffee or two cups of green tea daily had a 32% lower risk of intracerebral hemorrhage, versus those who rarely drank either beverage. (Intracerebral hemorrhage happens when a blood vessel bursts and bleeds inside the brain. About 13% of strokes are hemorrhagic.)
Participants in the study were 45-74 years old, almost evenly divided in gender, and were free from cancer and cardiovascular disease.
During the 13 years of follow-up, researchers reviewed participants' hospital medical records and death certificates, collecting data about heart disease, strokes and causes of death. They adjusted their findings to account for age, sex and lifestyle factors like smoking, alcohol, weight, diet and exercise.
Green tea drinkers in the study were more likely to exercise than non-drinkers.
Previous limited research has shown green tea's link to lower death risks from heart disease but has only touched on its association with lower stroke risks. Other studies have shown inconsistent connections between coffee and stroke risks.
Initial study results showed that drinking more than two cups of coffee daily was linked to increasing coronary heart disease rates in age- and sex-adjusted analysis. However, researchers did not find the association after factoring in the effects of cigarette smoking -- underscoring smoking's negative health impact on heart and stroke health.
A typical cup of coffee or tea in Japan was approximately six ounces. "However, our self-reported data may be reasonably accurate, because nationwide annual health screenings produced similar results, and our validation study showed relatively high validity." Kokubo said. "The regular action of drinking tea, coffee, largely benefits cardiovascular health because it partly keeps blood clots from forming."
Tea and coffee are the most popular drinks in the world after water, suggesting that these results may apply in America and other countries.
It is unclear how green tea affects stroke risks. A compound group known as catechins may provide some protection. Catechins have an antioxidant anti-inflammatory effect, increasing plasma antioxidant capacity and anti-thrombogenic effects.
Some chemicals in coffee include chlorogenic acid, thus cutting stroke risks by lowering the chances of developing type 2 diabetes. Further research could clarify how the interaction between coffee and green tea might help further lower stroke risks, Kokubo said.