Objective Consumption of fish and marine n-3 polyunsaturated fatty acids (PUFA) may be associated with a lower risk of atrial fibrillation (AF), but results have been inconsistent. The aim was to investigate this further by measurements of marine n-3 PUFA in adipose tissue.

Design Cohort study.

Setting A total of 57 053 Danish participants 50–64 years of age were enrolled into the Diet, Cancer and Health Cohort Study.

Patients A randomly drawn subcohort of 3440 participants with available data from baseline adipose tissue biopsies.

Interventions Exposure was the adipose tissue content of marine n-3 PUFA, which reflects the endogenous exposure and is also an objective marker of the long-term dietary intake.

Main outcome measures Incident AF during follow-up.

Results 179 cases of AF occurred over 13.6 years. Multivariate, sex-stratified Cox proportional hazards regression analyses using cubic splines showed a monotonic, negative, dose-response trend, but not statistically significant association, between total marine n-3 PUFA in adipose tissue and incident AF. A similar trend towards a lower risk of AF was seen in the second (HR 0.87, 95% CI 0.60 to 1.24) and third tertiles (HR 0.77, 95% CI 0.53 to 1.10) of marine n-3 PUFA compared with the lowest tertile. Similar trends, but also not statistically significant, were found separately for eicosapentaenoic, docosahexaenoic and docosapentaenoic acids.

Conclusions There was no statistically significant association between the content of marine n-3 PUFA in adipose tissue and the development of AF; however, data showed a monotonic, negative dose-response trend suggestive of a negative association.