Numerous studies in recent years have reported that drinking coffee may be good for the cardiovascular system and might even help prevent strokes. Just last month, Swedish researchers announced results of a large study showing that coffee seemed to reduce the risk of stroke in women by up to 25%.
Not long ago, researchers thought quite the opposite about coffee and the heart, says Dr. Thomas Hemmen, director of the UC San Diego Stroke Center: "Coffee is fun and it tastes good, so people assumed for many years that it would be bad for you."
Studies conducted in the 1970s and 1980s offered little in the way of confirmation or refutation. Several suggested an increased risk of heart attack among coffee drinkers. Others showed a lowered risk of heart attack and stroke. Still others found no connection at all.
Many of these early studies were criticized for being too small or too brief. In response, researchers at the Harvard School of Public Health decided to look at coffee consumption, heart disease and stroke risk among more than 45,000 healthy men enrolled in the school's ongoing Health Professionals Follow-Up Study. Their analysis, published in the New England Journal of Medicine in 1990, found that coffee drinking had no effect on the men's risk of heart attack or stroke.
But in the last few years, a spate of studies has revisited the question, and many of them have found — unexpectedly — that coffee drinking is linked to a decreased stroke risk.
A 2008 study of more than 26,000 male smokers in Finland found that the men who drank eight or more cups of coffee a day had a 23% lower risk of stroke than the men who drank little or no coffee. And a few other reports suggest the effect applies to healthy nonsmokers too. Researchers at UCLA and USC examined data on coffee consumption and stroke prevalence among more than 9,000 participants in the National Health and Nutrition Examination Survey. At a 2009 conference, they reported that the likelihood of stroke was highest among people who didn't drink coffee and lowest among those who drank the most coffee: 5% of people who drank one or two cups a day suffered strokes, whereas 2.9% of people who drank six or more cups suffered strokes. The study will be published in a few months.
Results from an even larger study of coffee drinking and stroke risk were published in the journal Circulation in 2009: Among the 83,000 women enrolled in Harvard's ongoing Nurses' Health Study, those who drank two to four cups of coffee a day had a 19% to 20% lower risk of stroke than women who drank less than one cup a month.
And this year, a study of more than 81,000 men and women in Japan showed that drinking one or two cups of coffee a day reduced the risk of death from cardiovascular disease by up to 23%. The findings were published in the Journal of Epidemiology and Community Health.
Such studies reveal that coffee isn't harmful, as once thought, and might even be beneficial, says Dr. Larry Goldstein, professor of medicine and director of the Duke University Stroke Center. But while they show an association between coffee drinking and lower stroke risk, they still don't prove that coffee is the cause, he says.
"People who drink coffee are different in many ways from those who don't drink coffee," says Dr. Nerses Sanossian, one of the authors of the UCLA-USC study and a professor of neurology at USC.
Any one of those differences, or more than one of them, could be behind the apparently lower stroke risk. Some of the studies that show a link between coffee drinking and reduced stroke risk have also shown that coffee drinkers are more likely to smoke, have lower education levels and have diets higher in potassium. And although it's unlikely that smoking, for instance, is behind their reduced stroke risk, it's possible that something else is. "It may be due to some other factors we haven't even taken into consideration," Sanossian says.
Even though coffee is considered safe, even in large amounts, you shouldn't rush to take up the habit, says Mark Urman, a cardiologist at the Cedars-Sinai Heart Institute. "If you're not a coffee drinker, don't start drinking to prevent a stroke or otherwise," he says. Coffee can cause heart palpitations in some people, and withdrawal symptoms in those who try to skip their daily cups for a day or two. And many people, he adds, like to load their coffees with cream and sugar, which could very well counteract any advantage coffee has for the blood vessels and heart.
Definitive proof that coffee is good for the blood vessels is unlikely to emerge anytime soon, Hemmen says. Such studies would need to randomly select people to drink either a lot of coffee or a little coffee, and then researchers would have to closely monitor their coffee intake and health for decades.
And that, says Hemmen, would be "very difficult, and really expensive."