A potential mediator of these two contrasting effects of alcohol may be platelet function.
"The contrasting effects of alcohol are similar to the effects of blood thinners like aspirin, which clearly prevent heart attacks but at the expense of some additional bleeding strokes," says author of the study Kenneth J. Mukamal, at Beth Israel Deaconess Medical Center.
Heart attacks are caused by blood clots that form in clogged arteries, and blood thinners can hasten bleeding from injured arteries.
Based on these findings, we speculated that moderate drinking would also act as a blood thinner, adds Mukamal.
A growing body of science underlines the possible protective benefits moderate drinking can have on the heart. Much work has focused on the red wine compound resveratrol, thought to be responsible for the drink's protective effects.
Previous research has shown that moderate drinkers tend to have "less sticky" platelets than abstainers, meaning that fewer blood elements cluster to form blood clots.
"Yet no one before had looked at whether alcohol affects how easily platelets are activated," he says.
"This is important because activated platelets are much stickier than normal ones."
The researchers used data collected from 3,798 participants involved in the Framingham Offspring Study of risk factors for cardiovascular disease.
They analysed data provided by a total of 1,037 participants (460 men and 577 women) for platelet activation and 2,013 participants (879 men and 1,134 women) for platelet aggregation.
"Our findings add to a large body of evidence showing that moderate drinking has effects on blood coagulation, which may have both good and bad effects, but now identify a new avenue by which this effect may occur," said Mukamal.
Among both men and women, an intake of three to six drinks per week or more was linked to lower levels of stickiness measured by aggregability," he adds.
In men the researchers also found that alcohol intake was linked to lower levels of platelet activation.
Together, these findings identify moderate drinking as a potential blood thinner, he concludes.
According to the scientists, minor differences found between the men and women were more likely due to statistical issues than to any clear gender differences.
The next step, Mukamal and his colleagues will evaluate their findings in other populations.
"Heart attacks far outnumber bleeding-type strokes in the United States," says Mukamal, "but in some countries such as Japan, they have much higher rates of bleeding strokes and lower rates of heart attacks than we do, which is perhaps related to dietary differences."