TUESDAY, Dec. 2 (HealthDay News) -- A healthy middle-aged woman can have up to two drinks a day without increasing her risk of the abnormal heartbeat called atrial fibrillation, a new study finds.

A group including Dr. Christine M. Albert, a professor of medicine at Harvard Medical School, mined the data of the large-scale Women's Health study, looking for a relationship between alcohol intake and atrial fibrillation, the most common abnormal heart rhythm.

"There have been previous studies in men which suggested that at higher intakes of alcohol, there may be an elevation in the risk for atrial fibrillation," said Albert, who is also director for arrhythmia prevention at Brigham and Women's Hospital, in Boston. "But we did this study because most previous studies did not include a lot of women and women generally drink less than men."

The nearly 35,000 women in the study were all over 45. None had atrial fibrillation or any other heart condition at the start of the study. They described their alcohol intake when the study began, and again 48 months later. Over an average follow-up period of 12.4 years, 1.9 percent of the women who had one drink or less day developed atrial fibrillation, compared to 1.8 percent of those having one to two drinks a day and 2.9 percent of those having two or more drinks a day.

"So there is a 40 to 50 percent increase in the incidence of atrial fibrillation at about three drinks a day," Albert said. Still, he said, " the absolute risks [for any one person] are pretty low," with only 2.25 people out of a thousand suffering such an event each year.

In atrial fibrillation, the two upper chambers of the heart (atria) beat irregularly and faster than they should. Blood can pool in the atria, leading to formation of clots that can block a major artery to the brain, causing a stroke.

"Atrial fibrillation is becoming more common," said Albert. "It occurs in about 1 percent of people up to the age of 80 and it can cause significant symptoms in those who have it."

The link between alcohol consumption and atrial fibrillation seen in the study applies only to the group of women in that study, Albert noted. "It may not apply to African-Americans, who were not generally represented in the study or to women with heart disease, who were not represented in the study," she said.

The new study "confirms what we saw before, that for most people drinking reasonable amounts of alcohol per day, alcohol consumption is not related to the risk of atrial fibrillation," said Dr. Kenneth J. Mukamal, an associate professor of medicine at Harvard Medical School who was not involved with the research.

"It is noteworthy that the risk was concentrated in women drinking the very heaviest amounts of alcohol," Mukamal said.

He was a member of a group that studied the relationship in a large number of Danish adults, publishing the results in 2005. "We looked at both men and women and found an elevated incidence in men only at the highest risk category, five or more a day," Mukamal said. "We never saw a higher risk in women. What this study indicates is that it may be that if women drink enough, you find a higher risk."

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• Arrhythmia
• Women's Health