People with heart disease who drink, even moderately, may have a slightly increased risk of a common heart rhythm problem, a new study suggests.

The study is not the first to link moderate drinking to the heart arrhythmia, known as atrial fibrillation (AF). But it's still not clear that the habit, itself, is the problem.

Doctors have long known that a drinking binge can trigger an episode of AF, in which the heart's upper chambers begin to quiver chaotically instead of contracting normally.

Things get murky, though, when it comes to moderate drinking.

In general, it's thought that having one or two drinks per day is protective against coronary heart disease - where cholesterol-containing "plaque" builds up in the arteries.

But modest drinking hasn't been linked to a decreased risk of AF - and the new findings suggest that when people already have heart issues, moderate drinking is actually tied to more AF cases.

The study, reported in the Canadian Medical Association Journal, included more than 30,000 older adults who either had clogged arteries, a history of stroke or diabetes complications such as kidney disease. Most had coronary heart disease.

Over about five years, people who drank occasionally or not at all developed AF at a rate of about 1.5 percent each year. For moderate drinkers, the rate was 1.7 percent, and for heavy drinkers, it was 2.1 percent.

The researchers looked at other factors, too - like age, weight and smoking habits. But moderate drinking was still linked to a 14-percent increase in the risk of AF.

"Recommendations about the protective effects of moderate alcohol intake in patients at high risk of cardiovascular disease may need to be tempered with these findings," write the researchers, led by Dr. Yan Liang, of McMaster University in Hamilton, Ontario.

Still, a researcher not involved in the work doubted the link between moderate drinking and AF.

One problem is separating out the effects of binge drinking, according to Dr. Kenneth J. Mukamal, of Harvard University and Beth Israel Deaconess Medical Center in Boston.

Liang and colleagues did do a separate analysis where they excluded people who reported a history of binge drinking - having more than five drinks at a time. And the results were similar.

But, Mukamal said in an email, the study did not repeatedly measure binge drinking habits over the five-year follow-up. So it's impossible to know if moderate drinkers' AF episodes were related to binges.

"The majority of binge-drinking episodes nationwide occur among otherwise moderate drinkers," Mukamal noted.

What's more, he said, the current study included patients who were involved in two clinical trials testing blood pressure drugs.

That's a narrow group of people. "In large studies of general populations - much more representative than these clinical trial participants - AF only appears higher among heavy drinkers," Mukamal said.

Atrial fibrillation arises from a problem in the heart muscle's electrical activity. It's not immediately life-threatening, and in some cases, an AF episode is short-lived and goes away on its own.

But in some people, AF becomes recurrent or permanent, raising their risk of heart failure and blood clots that can travel to the brain and cause a stroke.

The known risk factors for AF include older age, high blood pressure, diabetes, obesity and hyperthyroidism.

According to Mukamal, it's not surprising that moderate drinking seems to offer no protection against AF.

The ways in which alcohol might cut the risk of coronary heart disease - through better "good" cholesterol levels and less blood clotting - don't affect the risk of developing AF.

About 2.7 million Americans have atrial fibrillation, according to the American Heart Association (AHA). But coronary heart disease, the nation's number-one killer, is a much more common cause of death.

In general, experts say that if you're already a moderate drinker (up to one drink a day for women, and two for men), it's probably okay to keep it up.

But for people with certain chronic health problems, the new results may question that guidance, the researchers said.

"Our findings suggest that older individuals with cardiovascular disease or diabetes should probably limit their alcoholic beverages to no more than 1 drink per week," Liang told Reuters Health in an email.

"And binge drinking should be avoided, even if you drink infrequently."