Apparent digestion and apparent retention of lipid and fatty acids in Atlantic cod (Gadus morhua) fed increasing dietary lipid levels

The aim of the present study was to investigate how different dietary lipid levels affect growth, liver lipid deposition, apparent digestibility, apparent retention and utilization of total lipid and fatty acids in Atlantic cod. Individually tagged cod, with an average weight of 360 g, were randomly distributed in nine tanks, 49 fish per tank. Five diets with increasing dietary lipid level in the range of 80 to 268 g kg− 1 were fed to the fish for 85 days, in a regression design. Increasing dietary lipid level did not affect growth of Atlantic cod, but the hepatic somatic index (HSI) increased from 7.5 to 11.8%. The liver lipid level increased from 719 to 785 g kg− 1, constituting from 83 to 88% of total body lipid. Apparent lipid digestibility had an optimum level at a dietary lipid content between 138 and 180 g lipid kg− 1 diet with a marked decrease at lipid levels above 180 g kg− 1. Digestibility of saturated fatty acids (SFAs) and monounsaturated fatty acids (MUFAs) responded similar to that of total lipid, while digestibility of the n−3 fatty acids increased with increasing dietary lipid level. Apparent retention of fatty acids increased with increasing dietary lipid level, except for 18:0 and 18:1n−9. Apparent retention of 14:0 and 16:0 was low (0–60%), while the retention of 18:0 was high (150–200%), indicating that cod may elongate 14:0 and 16:0 to 18:0 and also utilize these fatty acids for energy production, particularly in low lipid diets. The low apparent retention of long MUFAs indicated that they were used for energy production. Apparent retentions of 22:6n−3 and 20:5n−3 were intermediate (50–80%) at low dietary lipid inclusion, but increased with increasing lipid level, indicating that also these essential fatty acids may have been utilized for energy production, especially in low lipid diets.