Atrial fibrillation is a condition in which the heart's two upper chambers, known as the atriums, do not beat at the same pace as the heart's two lower chambers, resulting in an irregular heartbeat. The condition increases a person's risk of strokes, heart failure and cognitive impairment.
The study found that the strongest overall effects were seen in men and women who ate one ounce of chocolate two to six times per week, said lead author Elizabeth Mostofsky, a researcher at the Harvard T.H. Chan School of Public Health and at Beth Israel Deaconess Medical Center in Boston.
Between 2.7 million and 6.1 million Americans have atrial fibrillation, so it's important to identify effective ways to help prevent the condition from developing, the researchers wrote.
Two previous studies have looked at the connection between chocolate consumption and the risk of developing atrial fibrillation, but their results suggested that any association may have been due to chance.
Unlike those earlier studies, whose participants were only men or only women, the new study included both men and women.
In the latest study, the researchers analyzed data collected from more than 55,000 Danes ages 50 to 64.
The researchers found that more than 3,300 of the participants were diagnosed with atrial fibrillation or atrial flutter (a condition in which the heart beats faster than usual but not irregularly) during a follow-up period that averaged 13.5 years.
When researchers took into consideration other factors that might influence development of atrial fibrillation, such as alcohol intake, smoking, obesity, high blood pressure and cholesterol levels, the study showed an association between people with a moderate intake of chocolate and a lower risk of developing atrial fibrillation.
The study does not prove a cause-and-effect relationship. And although the exact mechanism of how chocolate may prevent atrial fibrillation is not known, it's possible that compounds in chocolate called flavonoids may play a role, the researchers said.
Flavonoids have antioxidant and anti-inflammatory properties, Mostofsky said. They may limit the inflammatory process in the body, reducing the stickiness of the blood and leading to less scarring of connective tissue. All of these factors may help prevent the electrical remodeling of the heart that leads to atrial fibrillation, she explained.
The findings showed that for women, the strongest association was seen in those who ate a one-ounce serving of chocolate once a week: This level of consumption was linked to a 21 percent lower risk of atrial fibrillation compared with those who ate less chocolate. For men, the strongest association was seen in those who ate two to six one-ounce servings of chocolate weekly. These men had a 23 percent lower risk of atrial fibrillation.
Mostofsky cautioned that chocolate sold in Europe generally has a higher cocoa content than chocolate available in the United States.
All in all, the findings suggest that compared with some other snack choices, a moderate intake of chocolate may be a heart-healthy snack, Mostofsky said. But people should choose chocolate with a higher cocoa content, which has more health benefits and protective compounds, she said.
One of the limitations of the study is that people in Denmark are more similar to each other in race and ethnicity than people in other countries, so the findings may not be generalizable to other populations, other researchers wrote in an editorial accompanying the study.
The chocolate consumers in the study were also generally healthier, better educated and had lower rates of hypertension and diabetes than the people who didn't eat chocolate, and all of these factors might lessen their odds for atrial fibrillation, wrote the editorial's authors, Jonathan Piccini and Sean Pokorney, cardiologists at the Duke Center for Atrial Fibrillation in Durham, N.C.