BOSTON, MASSACHUSETTS. Although there is ample evidence that habitual exercise is healthy and reduces the risk of cardiovascular disease, hypertension and diabetes, and can help normalize lipid (cholesterol) levels, there is growing evidence that too vigorous exercise performed for extended periods of time can increase the risk of developing atrial fibrillation (AF), especially lone AF. Researchers at Harvard Medical School and Brigham and Women�s Hospital now report that regular jogging may be particularly detrimental as far as the development of AF is concerned.

Their study included 16,921 male physicians who were enrolled in the Physicians� Health Study in 1982 when they were between the ages of 40 and 84 years. Three and nine years after enrolment, the participants were asked if they regularly engaged in an exercise program vigorous enough to work up a sweat. The majority (62.6%) answered yes and 12.6% reported that they engaged in vigorous exercise 5-7 days a week. The frequency of vigorous exercise was directly associated with alcohol intake, fish consumption, and the use of multivitamin, vitamin C and vitamin E supplements. It is interesting that more than 90% of exercisers reported consuming fish at least once a week, 35% regularly took multivitamins, and 25% supplemented with vitamin C. Non-exercisers, on the other hand, tended to be older, overweight, and smokers. They also had a significantly higher prevalence of diabetes and hypertension.

The physicians participating in the study were asked at 15, 17, 18 and 19 years after enrolment if they had ever been diagnosed with AF � about 10% answered yes. Based on the exercise data obtained 3 years following enrolment, the researchers concluded that men under the age of 50 years who exercised vigorously 5-7 days a week had a substantially higher risk than non-exercisers of developing AF when corrected for all confounding variables. Further analysis of the data showed that regular jogging was associated with a 53% increased risk of AF in all age groups with those jogging in excess of 4 miles a day having the greater risk. No overall association was found between regular cycling, swimming or racquet sports and the development of AF.

The researchers speculate that frequent, vigorous exercise may cause AF through several possible mechanisms including enlargement of the left atrium, development of left ventricular hypertrophy, left ventricular dilation, and an increase in parasympathetic (vagal) tone. They point out that jogging results in greater enhancement of the parasympathetic nervous system than does other types of exercise. They also suggest that the fact that aging tends to decrease parasympathetic activity may account for the finding that older men are less likely to develop AF as a result of vigorous exercise. They conclude that frequent, vigorous exercise is associated with an increased risk of developing AF in young men (under the age of 50 years) and joggers.

Aizer, A, et al. Relation of vigorous exercise to risk of atrial fibrillation. American Journal of Cardiology, Vol. 103, 2009, pp. 1572-77

Editor�s comment: This report adds to an already impressive body of literature concluding that, while regular, moderate exercise is healthy and materially helps in preventing many serious disease conditions, overdoing the exercise, especially by engaging in frequent, long-distance jogging sessions significantly increases the risk of developing afib, particularly in men below the age of 50 years.