High dietary 18:2n-6/18:3n-3 ratio does not inhibit elongation and desaturation of 18:3n-3 to EPA and DHA in Atlantic salmon (Salmo salar L.)

This study investigated whether retention of n-3 fatty acids (FAs) is influenced by the levels of dietary monounsaturated (MUFA)-, saturated (SFA)- and n-6 FAs. The feeding trial used a mixture design, with rapeseed-, palm (PO)- and soybean oil (SO), providing high levels of MUFA, SFA and n-6 FAs, respectively, while 18:3n-3, eicosapentaenoic acid (20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3) were kept constant. Furthermore, a diet group with high SO and reduced 18:3n-3 was included in addition to the mixture design. There were no differences in growth or proximate composition, but the PO diet gave reduced FA digestibility and increased feed conversion ratio. All diet groups had a net production of DHA, shown by retention values >100% (133%-177%). High dietary n-6 FAs of up to 43% of FAs (18:2n-6/18:3n-3 ratio 8.6) did not negatively affect DHA retention, but rather had a small positive impact. High levels of substrate (dietary 18:3n-3 at 10% of FAs) did not enhance DHA retention compared to 5% 18:3n-3. This is a highly significant finding for the aquaculture industry, allowing for greater flexibility in the choice of lipid sources to replace fish oil in salmon feeds.