| Summary: | Marine resources are today limited, and due to this there is an increasing inclusion of non-marine lipids in the diet of farmed salmonids. An optimal diet for farmed Atlantic salmon (Salmo salar L.) should not only promote fast growth, but also keep the fish at good health, making it robust to face changes and stressors from the surrounding environment. There is, however, still knowledge lacking about the nutritional needs of Atlantic salmon. Lipid sources vary in their fatty acid (FA) composition, as well as in content of other lipid soluble compounds, such as sterols and environmental contaminants. The focus of this thesis has been to study the effect of FAs and lipid soluble compounds on Atlantic salmon lipid metabolism and metabolic health. Very long-chained n-3 polyunsaturated fatty acids (VLC n-3 PUFAs), typically found in the marine environment, have on several occasions shown to have lipid-lowering properties. The lower dietary and tissue concentrations of these FAs in Atlantic salmon due to the substitution of marine oils with terrestrial oils, may thus have effects on the lipid metabolism. This was studied by performing four dietary trials using feeds with low concentrations of marine oils (0 – 86 g marine oils kg-1 feed), and thus also low dietary concentrations of VLC n-3 PUFAs (1.5 - 8.5 % eicosapentaenoic acid (EPA) + docosahexaenoic acid (DHA) of total FAs and 0.5 – 6.6 % of diet). Lower dietary concentrations of VLC n-3 PUFAs tended to increase liver lipids in all the dietary trials. However, the observed changes in liver lipid content could not solely be explained by dietary concentrations of VLC n-3 PUFAs. Thus, dietary concentrations of saturated fats or plant sterols, commonly known as phytosterols, are hypothesised to affect liver lipid stores. Phytosterols are well-known to lower the uptake and synthesis of cholesterol, and might therefore have caused a cholesterol deficiency in the fish fed the low-marine diets containing low cholesterol levels. The transcription factors affected by ...
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