People who regularly drink green tea may have a lesser risk of mental decline as they grow older, researchers have found.
Their study, of more than 1,000 Japanese adults in their 70s and beyond, found that the more green tea men and women drank, the lower their odds of having cognitive impairment.
The findings build on evidence from lab experiments showing that certain compounds in green tea may protect brain cells from the damaging processes that mark conditions like Alzheimer's and Parkinson's disease.
But while those studies were carried out in animals and test tubes, the new research appears to be the first to find a lower risk of mental decline among green-tea drinkers, according to the study authors.
They speculate that the possible protective effects of green tea may help explain Japan's lower rate of dementia, particularly Alzheimer's disease, compared with Europe and North America.
Dr. Shinichi Kuriyama and colleagues at Tohoku University Graduate School of Medicine report the findings in the current issue of the American Journal of Clinical Nutrition.
The study included 1,003 adults age 70 and older who completed detailed questionnaires on their diets over the previous month, as well as their overall physical health and lifestyle habits. They also completed a standard test of cognitive functions such as memory, attention and language use.
The researchers found that older adults who drank two or more cups of green tea per day were about half as likely to show cognitive impairment as those who drank three cups or less each week. Men and women who averaged one cup per day fell somewhere in between.
The connection between green tea and mental function persisted when the researchers accounted for overall diet and factors such as smoking and exercise habits.
However, the findings cannot demonstrate a cause-and-effect relationship.
The study was observational, not a controlled experiment, and there may be something about green-tea drinkers that explains the link between the beverage and sharper mental function, Kuriyama told Reuters Health.
For example, healthier, more active individuals may simply drink more green tea -- which, in Japan, is often consumed in social settings.
"We think that the potential protective effects of green tea should be confirmed in further studies," Kuriyama said.
Given the high prevalence and heavy burden of dementia, the researchers conclude, any benefit of drinking green tea could have a "considerable" public health impact.