The role coffee and tea play in a person's risk of having a stroke got a little clearer recently as two large observational studies found that the beverages may actually provide a modest amount of protection.
For coffee, the more that was consumed -- up to a point -- the lower the risk.
In a study that followed 83,000 women for 24 years, researchers found that those who drank four or more cups of coffee a day were 20 percent less likely to have a stroke than those who drank less than one cup a month.
Those who drank two to three cups a day had a 19 percent reduced risk and those who drank five to seven cups a week had a 12 percent reduction in risk, according to the Nurses' Health Study published last month in the journal Stroke.
The reduced risk was more than double in those who did not smoke. Non-smokers who drank four or more cups of coffee a day were 43 percent less likely to have a stroke, but among smokers the risk reduction was only 3 percent.
The researchers warned that any beneficial effects of coffee cannot counterbalance the ill effects of smoking. They added that the effects can be applied only to healthy people. Coffee can worsen conditions such as insomnia, anxiety and high blood pressure, so patients should talk to their doctor about how coffee might affect those health problems, the researchers said.
And they said people should not take up drinking coffee as a way to reduce their risk of stroke.
"If people want to reduce their stroke risk, they should prioritize other lifestyle changes: quitting smoking, increasing physical activity and eating a diet centered around whole grains and fruits and vegetables," said senior author Rob van Dam, an assistant professor of medicine at the Harvard School of Public Health and Brigham and Women's Hospital.
He said more study is needed to prove that coffee protects against stroke, "but results are reassuring in that coffee consumption does not seem harmful for stroke risk."
Other caffeinated beverages such as tea, as well as decaffeinated coffee, were not associated with a reduction in risk.
However, a separate analysis published in January in the journal Circulation found that both green and black tea consumption seemed to protect against stroke.
The article was a review of nine studies involving 195,000 people. It found that those who drank three or more cups of tea a day had a 21 percent reduction in stroke risk compared with those who drank less than one cup a day.
Because it is such a popular drink, coffee and its effect on health, especially heart disease, have received a great deal of attention.
Studies have produced mixed results, but a 2006 article that studied 128,000 men and women for up to 20 years found no evidence that drinking coffee increased the risk of coronary heart disease. In fact, those who drank more than six cups a day had a reduced risk of heart disease, according to the study published in Circulation, although researchers cautioned that unfiltered coffee may raise cholesterol.
Some large observational studies also found that drinking coffee might reduce the risk of developing Type 2 diabetes.
Tea, on the other hand, has been associated with a reduction in heart disease risk.
A 2001 analysis of seven studies found that for every three cups of tea consumed a day, the risk of a heart attack declined by 11 percent.
Diane Book, an associate professor of neurology at the Medical College of Wisconsin, said that more research is pointing to a possible stroke benefit from drinking tea and coffee but that it has not reached the point of being proof.
All of the studies are observational. They are not the more rigorous, randomized trials that typically are done to prove the effectiveness of a drug or some therapy.
Book said such studies are not likely to be done. So for the time being at least, epidemiological research is likely to remain the best available science.
Still, a reduction in stroke risk of 20 percent or more would be significant, she said.
"That's a big effect," she said. "Our best drugs are only 20 percent to 30 percent" effective.
Book noted that all of the subjects in the coffee study were women and that it is not certain that the findings would apply to men.
Researchers also are not certain why coffee and tea may reduce the risk of stroke.
One theory is there are substances in both beverages that reduce inflammation and improve the function of the endothelium, the inner lining of cells in blood vessels.
In addition, tea contains substances known as catechins, which can protect brain cells when the blood supply is interrupted. Another substance in tea, theanine, also may help protect brain cells.
The tea article was funded by the Unilever Lipton Institute of Tea. The coffee study was funded by the National Institutes of Health.
(c) 2009, Milwaukee Journal Sentinel.
Visit JSOnline, the Journal Sentinel's World Wide Web site, at www.jsonline.com/
Distributed by McClatchy-Tribune Information Services.
Explore further: Coffee seen to protect against cirrhosis