White MN, et al. - In a randomized, double-blind, controlled trial, researchers assessed the impact of fish oil supplementation on the levels of urinary and rectal eicosanoids in subjects with a history of colorectal adenomas. Randomization was stratified based on the gene variant rs174535 in the fatty acid desaturase 1 enzyme (FADS1), which influences tissue levels of arachidonic acid. In all, 141 participants received 2.5 g of fish oil per day or olive oil supplementation for 6 months. Urinary prostaglandin E₂ metabolite (PGE-M) production was reduced with fish oil supplementation vs olive oil. With fish oil supplementation, no attenuation was detected in rectal prostaglandin E₂ (PGE₂) overall; however, in the subgroup of participants not using aspirin or nonsteroidal anti-inflammatory drugs (NSAIDs), a significant attenuation in PGE₂ was noted with fish oil. The impact of fish oil on PGE₂ production did not appear to be modified by FADS1 genotype. Overall, a modest but beneficial effect of fish oil supplementation on eicosanoids related to colorectal carcinogenesis was evident, especially in those not on aspirin or NSAIDs.