Fish oil supplement use modifies the relationship between dietary oily fish intake and plasma omega-3 polyunsaturated fatty acid levels: An analysis of the UK Biobank.

Observational evidence linking dietary omega-3 polyunsaturated fatty acid (PUFA) intake and health outcomes is limited by a lack of robust validation of dietary intake using blood omega-3 PUFA levels and potential confounding by fish oil supplement (FOS) use. We investigated the relationship between oily fish intake, FOS use, and plasma omega-3 PUFA levels in 121,650 UK Biobank (UKBB) participants. Ordinal logistic regression models, adjusted for clinical and lifestyle factors, were used to quantify the contribution of dietary oily fish intake and FOS use to plasma omega-3 PUFA levels (measured by NMR spectroscopy). Oily fish intake and FOS use were reported by 38% and 31% of participants, respectively. Increasing oily fish intake was associated with a higher likelihood of FOS use (P<0.001). Oily fish intake ≥twice a week was the strongest predictor of high total omega-3 PUFAs (odds ratio [OR] 6.7 [95% confidence interval 6.3-7.1]) and DHA levels (6.6 [6.3-7.1). FOS use was an independent predictor of high plasma omega-3 PUFA levels (2.0 [2.0-2.1]) with a similar OR to that associated with eating oily fish <once a week (1.9 [1.8-2.0]). FOS use was associated with plasma omega-3 PUFA levels that were similar to individuals in the next highest oily fish intake category. In conclusion, FOS use is more common in frequent fish consumers and modifies the relationship between oily fish intake and plasma omega-3 PUFA levels in UKBB participants. If unaccounted for, FOS use may confound the relationship between dietary omega-3 PUFA intake, blood levels of omega-3 PUFAs, and health outcomes.