Introduction Docosahexaenoic acid (DHA) is an n-3 polyunsaturated fatty acid, present in fish oils, which possess anti-inflammatory properties, including biological effects on genetic expression, immune cell function and the production of inflammatory eicosonoids. The aim of this investigation was to conduct the first prospective cohort study to determine if low dietary intakes of this nutrient were associated with the development of incident Crohn's disease.

Methods A total of 229 702 healthy participants aged 30–74 years of age were recruited in a prospective cohort study (EPIC–European Prospective Investigation Into Cancer) who were resident in either: Sweden, Denmark, The Netherlands, Germany or The UK. At baseline, participants completed food frequency questionnaires from which their intakes of DHA and other dietary fatty acids were determined. The cohort was followed-up between 1992 and 2004 to identify those who developed incident Crohn's disease, with each diagnosis medically confirmed by a review of the medical notes. Each case was matched with four controls for gender, age at recruitment and centre, and the analysis performed using conditional logistic regression. Adjustments were made for: smoking, total energy, fat and fatty acids, which affect the inflammatory process.

Results In the cohort, 73 participants developed incident Crohn's disease (64% women) at a mean age of 56.3 years (SD=11.1 years). Of these, 51% had disease affecting the terminal ileum and 19% had a pancolitis. The four higher quintiles of DHA intake were all inversely associated with the development of Crohn's disease, with the highest (>310 mg/day) having the largest effect (OR 0.07, 95% CI 0.01 to 0.80, p=0.03), with a dose-response across categories (OR trend 0.54, 95% CI 0.30 to 0.98, p=0.04). The attributable fraction, namely the percentage of cases which may be due to the lowest dietary intake of DHA was 40%.

Conclusion The data suggest a potential dose-dependent protective effect for increasing dietary DHA in the aetiology of Crohn's disease. DHA should be measured in future aetiological studies of this disease and could be assessed as a dietary treatment in clinical trials of patients.

Competing interests None declared.