Background: Physical activity (PA) is well known to reduce the risk of cardiovascular disease. We hypothesized that regular PA, possibly acting through reductions in blood pressure and body mass index (BMI), would reduce the risk of incident atrial fibrillation (AF) in women.
Methods and results: We prospectively followed 34 759 women who reported their leisure-time PA levels for the occurrence of AF. We estimated energy expenditure in metabolic equivalent (MET)-h/wk and validated self-reported AF with medical records. The mean (SD) age of the 34 759 participants was 54.6 (7.0) years, the mean BMI was 26.0 (5.0) kg/m(2), 26.5% had hypertension, and the median (IQR) PA was 8.4 (2.8, 20.4) MET-h/wk. After a median of 14.4 years of observation, 968 women had development of AF. In age-, cholesterol-, smoking-, alcohol-, diabetes-, and race-adjusted models, increasing quintiles of PA were associated with reduced risks of AF (hazard ratio for extreme quintiles, 0.82; 0.66 to 1.01; P trend=0.007 over quintiles). Although this association was not substantially different after adjusting for hypertension (0.87; 0.70 to 1.07; P trend 0.02), it was attenuated after adjustment for BMI (0.99; 0.80 to 1.23; P trend=0.22). Women who achieved the federal government's recommendation of 7.5 MET-h/wk of PA were at reduced risk of AF compared with those who did not (0.86; 0.75 to 0.98; P=0.03). This association was also attenuated by BMI (0.96; 0.84 to 1.10; P=0.57).
Conclusions: In middle-aged women, physical activity was associated with a modestly reduced risk of AF. However, this relationship was no longer significant after controlling for body mass index.