| Summary: | The shift of farmed Atlantic salmon from a marine- to plant-based diet the last decades has revealed challenges concerning the dietary requirements necessary to ensure fish health. Of the nutrients lacking in plant feeds, choline has been identified as essential to guarantee efficient lipid transport and metabolism. The distinctive sign of choline deficiency is an excessive presence of unabsorbed fat accumulating within the intestinal enterocytes, a condition known as steatosis. The results of the first attempts to define choline requirement for post-smolt Atlantic salmon were published recently. However, it can be expected that choline requirements vary with life stages and environmental and dietary conditions, but information on such relationships is lacking. Besides being fundamental for lipid transport, choline has gained particular interest due to its important role in DNA methylation processes. So far, very little is known regarding the effects of variation in DNA methylation in fish, but it is likely that disturbances in methylation processes may lead to increased disease risk. In this thesis, two studies, resulting in three papers, were conducted to investigate how choline requirement in farmed Atlantic salmon may be influenced by different production conditions, and how dietary choline level may interfere with DNA methylation. However, our understanding of the cellular mechanisms responsible for nutrient interactions with fish digestive physiology is still fragmentary. To address this, the third study in this thesis focused on in vitro studies of lipid metabolism. This approach is an important tool to facilitate further research into the basic functions of the digestive tract. In the first study, we investigated the effects of increasing dietary lipid levels on steatosis symptoms. This study involved Atlantic salmon parr that were fed choline-deficient diets. The fish were raised at two different environmental temperatures: 8 and 15 °C. The results showed that fish raised at 15 °C had higher feed intake ...
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