Characterisation of lipid transport in Atlantic cod (Gadus morhua) when fasted and fed high or low fat diets

Atlantic cod with an average initial weight of 413 g were fed for 112 days diets containing either 11.4 or 30.5% lipid (of dry matter) until they reached a final weight of approximately 750 g. Samples of intestine, liver and blood were taken at the end of the feeding period, and then after 3 weeks of fasting. The lipid level in the diet did not significantly affect fish growth. The hepatosomatic index (HSI), however, increased by a factor of 1.5, and the liver lipid index by a factor of 1.7, when fish were fed the high-fat diet. The increase in HSI was primarily due to a larger average size of liver cells, due to a higher level of lipid deposition than in the livers of fish fed the low-fat diet, which had more and smaller liver cells with less lipid. Vesicles with the size of very low density lipoproteins (VLDLs) were found in liver cells of fish fed the high-fat diet. In fish from the same dietary group, vesicles with the size of chylomicrons were found in intestinal enterocytes. These particles were however not present in tissues from fasted fish. The cholesterol and protein rich high density lipoprotein (HDL) was the predominant lipoprotein class in cod serum, with the triacylglycerol (TAG) rich low density lipoprotein (LDL) as the next most common class. Some TAG-rich VLDLs and chylomicrons were also present in the serum. The apo-lipoprotein compositions of the lipoprotein classes in cod were similar to those of other species: the apoB was the major protein in chylomicrons, VLDLs and LDLs, while apoA predominated in HDLs. There was a tendency to reduced liver lipid index in fasted fish, indicating lipid mobilisation from the liver. The fatty acid (FA) serum profile showed an increased percentage of saturated FAs, while there was a reduction in the percentage of saturated FAs in the liver with fasting, indicating a higher mobilisation of saturated FAs relative to other FAs. The increased degree of blood capillary vascularisation (angiogenesis) found after fasting is likely to improve the supply of nutrients and FAs from the liver via serum to other organs and tissues. Mobilised lipids were mainly transported esterified in TAG, primarily in HDL, less in LDL.