Observational evidence linking dietary n-3 PUFA intake and health outcomes is limited by a lack of robust validation of dietary intake using blood n-3 PUFA levels and potential confounding by fish oil supplement (FOS) use. We investigated the relationship between oily fish intake, FOS use and plasma n-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 n-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 n-3 PUFA (OR 6·7 (95 % CI 6·3, 7·1)) and DHA levels (6·6 (6·3, 7·1). FOS use was an independent predictor of high plasma n-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 n-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 n-3 PUFA levels in UKBB participants. If unaccounted for, FOS use may confound the relationship between dietary n-3 PUFA intake, blood levels of n-3 PUFAs and health outcomes.

Cod liver oil is a traditional source of vitamin D in Iceland, and regular intake is recommended partly for the sake of bone health. However, the association between lifelong consumption of cod liver oil and bone mineral density (BMD) in old age is unclear. The present study attempted to assess the associations between intake of cod liver oil in adolescence, midlife, and old age, and hip BMD in old age, as well as associations between cod liver oil intake in old age and serum 25-hydroxyvitamin D (25(OH)D) concentration. Participants of the Age, Gene/Environment Susceptibility–Reykjavik Study (age 66–96 years; n 4798), reported retrospectively cod liver oil intake during adolescence and midlife, as well as the one now in old age, using a validated FFQ. BMD of femoral neck and trochanteric region was measured by volumetric quantitative computed tomography, and serum 25(OH)D concentration was measured by means of a direct, competitive chemiluminescence immunoassay. Associations were assessed using linear regression models. No significant association was seen between retrospective cod liver oil intake and hip BMD in old age. Current intake of aged men was also not associated with hip BMD, while aged women with daily intakes had z-scores on average 0·1 higher, compared with those with an intake of < once/week. Although significant, this difference is small, and its clinical relevance is questionable. Intake of aged participants was positively associated with serum 25(OH)D: individuals with intakes of < once/week, one to six time(s)/week and daily intake had concentrations of approximately 40, 50 and 60 nmol/l respectively (P for trend < 0·001).