Physically active postmenopausal women had a 10 percent lower risk of developing atrial fibrillation, compared to women who were more sedentary, researchers report in the Aug. 20 issue of the Journal of the American Heart Association.
Active women enjoyed this protection against the heart rhythm disorder even if they were obese, the study found. Obesity is an important risk factor for atrial fibrillation.
"We pretty clearly show that in this older population, the more they exercised, they less likely they were to develop atrial fibrillation -- and the obese women were the ones who benefited most from this exercise," said study author Dr. Marco Perez, director of the Inherited Arrhythmia Clinic at the Stanford University School of Medicine in California.
This study should clear up concerns that physical exercise might contribute to atrial fibrillation, said Perez and Dr. Gordon Tomaselli, a professor of cardiology at Johns Hopkins Medical School and a spokesman for the American Heart Association.
"The older population is much more vulnerable, and there has been a question in our field whether we should be recommending more exercise in these older folks," Perez said.
Atrial fibrillation is an electrical disorder of the heart that causes it to beat in a rapid and disorganized way. The condition increases a person's risk of stroke and heart failure.
Some studies have shown that elite athletes may develop atrial fibrillation as a result of their regular strenuous efforts. Those findings caused doctors to wonder if exercise might be bad for regular folks at risk for the condition, Tomaselli explained.
"This study shows that moving, keeping a body in motion, is a good thing even if you have risk factors for heart disease," he said. "You can't use atrial fibrillation as an excuse for not being physically active, if you're an average guy or gal."
About 1.1 million U.S. women currently have atrial fibrillation, and prevalence of the disorder is expected to increase 2.5-fold during the next 50 years, researchers said in background information.
This study involved more than 80,000 participants with the Women's Health Initiative, an observational health study involving women aged 50 to 79. Observational studies can only show if there is an association between factors and cannot prove cause-and-effect relationships.
At the start of the study, researchers asked the women how often they walked outside for more than 10 minutes daily or how often they engaged in physical activity hard enough to sweat.
After 11 years, the researchers found that the most physically active women had a 10 percent lower risk of developing atrial fibrillation compared to those who didn't walk outside for 10 minutes at least once each week.
The women with the highest protection took part in physical activity equivalent to walking briskly for 30 minutes six days a week, or bicycling at a leisurely pace for an hour twice a week, researchers said.
Moderately physically active women had at least a 6 percent lower risk of developing atrial fibrillation. Walking briskly for 30 minutes twice a week would provide this benefit, the study authors said.
Strenuous exercise also reduced risk of atrial fibrillation. Women who undertook activity equivalent to running a couple of hours a week had a 9 percent lower risk, the study found.
Obesity still was linked to an overall increased risk of atrial fibrillation, but the investigators found that obese women who exercised a lot cut their risk in half.
Active obese women had a 17 percent increased risk of the disorder, compared to a 44 percent increased risk for obese women who took part in little to no physical activity, the study found.
Physical activity likely decreases risk of the heart rhythm disorder by lowering blood pressure and reducing inflammation in the body, Tomaselli and Perez said.
Exercise also might help limit physical changes that occur in the heart as a result of aging or obesity, which in turn increase the risk of atrial fibrillation and heart disease, Perez said.
If obese women start losing weight as a result of their exercise, the benefits likely will be even greater, Tomaselli said.
Obesity causes a number of physical changes that have been linked to the development of atrial fibrillation, including inflammation, enlarged heart and altered heart rate, the researchers noted.
"On average, if somebody lost weight and were more physically active, their risk of atrial fibrillation would be lower," Tomaselli said.
For more about atrial fibrillation, visit the U.S. National Library of Medicine.