Low serum concentrations of several
vitamins have been linked to increased risk of diseases including
insulin resistance and
type 2 diabetes (T2D). Fish is a good source of several
vitamins, and the prevalence of T2D is low in populations with high fish intake. The aim of the present study was to investigate the effects of high fish intake on
vitamins in serum from adults in autumn in South-Western Norway at 60° north latitude. In this randomised clinical trial, sixty-three healthy participants with
overweight/
obesity consumed 750 g/week of either cod (n 22) or salmon (n 22) as five weekly dinners or were instructed to continue their normal eating habits but avoid fish intake (Control group, n 19) for 8 weeks. The estimated
vitamin D intake was significantly increased in the Salmon group when compared with the Cod group (P = 6·3 × 10-4) and with the Control group (P = 3·5 × 10-6), with no differences between groups for estimated intake of
vitamins A, B1, B2, B3, B6, B9, C and E. Serum
25-hydroxyvitamin D3 concentration was decreased in all groups after 8 weeks; however, the reduction in the Salmon group was significantly smaller compared with the Cod group (P = 0·013) and the Control group (P = 0·0060). Cod and salmon intake did not affect serum concentrations of the other measured
vitamins. The findings suggest that 750 g/week of salmon was not sufficient to prevent a decrease in serum
25-hydroxyvitamin D3 in autumn in South-Western Norway in adults with
overweight/
obesity.