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Dietary vegetable lipids and intestinal integrity - effects on fatty acid composition, barrier functions, pathogen translocation, microbiota and disease resistance

Lipid composition of TAG changes rapidly in liver and anterior intestine when fish were fed diets enriched in one particular fatty acid; 18:1, 18:2 or 18:3. TAG fatty acid composition rapidly approached the level of the selected fatty acid in the diet. In the posterior intestine and muscle the response was slower, indicating that these tissues do not use neutral lipids directly from the diet. The polar lipids also changed in accordance to the diet but only to about one third of the effect seen for non-polar lipids, suggesting that the fish are able to keep the fatty acid composition of the cell membranes at a fairly stable level irrespective of the dietary fatty acid supply. Mixed vegetable diets, all containing a small amount of marine oil resident in the un-extracted fishmeal, showed less pronounced changes in tissue fatty acid composition. When fed the extreme diets, the fish encountered problems in maintaining the levels of 20:5n-3 and 22:6n-3 in their polar lipids, a pattern that was not obvious in the fish fed mixed diets. Even fish fed only the small amounts of marine lipids that were retained in the fish meal (2% of total lipids) were relatively successful in maintaining their levels of 20:5n-3 and 22:6n-3 in a similar range as the fish receiving 40% and even 100% fish oil (FO). This suggests that the fatty acids, 20:5n-3 and 22:6n-3, are dietary requirements for the fish but that even small amounts of marine fatty acids are almost sufficient to meet this requirement.

Fish fed sunflower oil (SO) were distinguished from the other dietary groups in that these fish were not fully capable of maintaining their levels of 20:5n-3, instead they seem to increase their levels of 20:4n-6 in the membranes which probably is due to the ample supply of 18:2n-6 from the SO and may lead to altered membrane characteristics. Fish fed the extreme diets showed severe accumulation of lipid droplets in the enterocytes. This situation sometimes led to damaged cells and cell junctions. In the fish fed mixed diets, there was a substantial accumulation of lipid droplets but not to a damaging degree. A higher amount of the fatty acid 16:0 in the mixed diets might be a reason for lower amount of droplets. Another possibility is that he higher amounts of PC in the mixed diets diminish the formation of lipid droplets. Even though the lipid droplet formation and the cell damage was observed in all dietary groups, only SO and rapeseed oil (RO) physiologically affected the intestinal barrier functions as judged by paracellular permeability and epithelial resistance. SO affected the barrier functions of Atlantic salmon, whereas RO was the main effector in rainbow trout.

Exposure of the intestinal epithelium to the pathogen bacteria Aeromonas salmonicida created ultra structural changes with loosened junctional complexes and loosened whole enterocytes. This was mainly apparent in the anterior intestine of Atlantic salmon and in the posterior intestine of rainbow trout. Exposure of the intestinal epithelium is also followed by a decrease in intestinal permeability, suggesting that the pathogen bacteria are able to affect the barrier properties of the epithelium. A similar effect is seen when exposing the epithelium to the bacterial endotoxin, LPS, as well as to culture medium from live A. salmonicida. This clearly indicates that the bacteria itself can secrete virulent factors of which a probable candidate is LPS. Thus, if fish fed a diet that negatively affects the barrier functions, i.e. one containing SO and are orally infected with a pathogen bacteria, this may lead to an increased risk for systemic infections. However, in the present project no effect of different mixed dietary vegetable oils on the actual translocation rate of A. salmonicida could be demonstrated. Rather, Atlantic salmon parr fed SO showed a lower translocation rate of pathogen bacteria then corresponding groups fed FO. This can be due to several possibilities: The change in composition of membrane lipids may affect the transcellular translocation routes, decreasing those even though the paracellular routes are increased. The increase in the paracellular pathway may increase the transfer of bacterial components and virulent factor that act as antigens and stimulate the immune system, which might give a decreased actual translocation rate.

Also, the present study reveals that the only two diets demonstrated the presence of Carnobacterium mobile-like strains, which are endogenous bacteria with antagonistic activity against A. salmonicida, was SO and RO.

Thus, even though some of the extreme and mixed vegetable diets changed the fatty acid composition, affected the ultra structure of the epithelium and the intestinal barrier properties, the overall growth, performance and intestinal bacterial translocation of the fish was unaffected in the short term perspective, but it may lead to health effects in the long term perspective.

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