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Intestinal health in salmon fed a plant-based diet

Access to marine raw materials for fish feed production is limited. Any future increase in the numbers of farmed salmon will therefore necessitate an increased use of feeds of plant origin. If salmon are to grow satisfactorily and remain healthy, it is important to obtain knowledge of just how plant-based feed affects the health of fish.

For his doctorate, Einar Lilleeng studied how plant ingredients in fish feeds affect the health and immune defence of fish. It is important to gain more knowledge of this field so that we can develop feeds rich in plants that are not detrimental to fish health.

Lilleeng's work has increased our understanding of how plant-rich feedstuffs, which contain a series of anti-nutrients, affect the digestive processes and immune system of the salmon intestine. Simultaneously, the research has contributed to a basic understanding of how fish digestive and immune systems function.

Lilleeng utilised soya for his plant ingredient and used molecular-biological methods to study what happens in the fish intestine. The research comprised a series of feeding trials with salmon, of both short and long durations, so that both acute and more chronic reactions in the intestine could be studied.

As mentioned above, any increase in the Norwegian production of salmon will necessitate an increased use of plant-based feeds. Replacing marine ingredients with plant-based ingredients exposes fish to a series of "foreign" components, for example, starch and anti-nutrients that may upset natural processes occurring in the intestine. Plant components such as lectins, saponins, phyto-oestrogens, phytic acid, tannins and others, which do not exist in the natural feed of wild fish, may disturb digestive processes and affect health. Plant ingredients also introduce proteins that may stress the immune system of the intestine.

Lilleeng used soya meal as the source of his ingredients, which is known to contain a series of anti-nutrients and to disturb the intestinal function of salmon. Lilleeng showed that intestinal immune defences become activated immediately feeding with soya commences. He also showed that enzymes normally associated with protein digestion have abnormally high levels of activity in the intestines of salmon with enteritis as a result of soya feeding. It appears that the intestinal mucous membrane, which previously has not been considered to be a source of these enzymes, also contributes to the high levels.

Lilleeng and his colleagues have increased our knowledge of the receptors of the fish intestine, so-called PAR2-receptors, which may be activated by such digestive enzymes. Activation of these receptors is very likely a key factor in the development of soya-induced enteritis. This work has been an important contribution to the understanding of how the fish intestine defends itself against harmful substances in the feed and against disease generally.

The use of plant ingredients in the feed may expose the salmon to too much starch. The nutritive value of starch is limited, since salmon digest starch very poorly. By cloning and studying amylase, the enzyme that digests starch, Lilleeng and his colleagues have shown that the enzyme is missing an important part, a kind of "hook" that binds the starch so the enzyme can digest it. They also showed that a molecule that is important for the secretion of the enzyme into the intestine differs in salmon and mammals. this may explain why there is little amylase to be found in the fish intestine. These characteristics of salmon amylase may explain why salmon digest starch so poorly.

Lilleeng's doctorate is an important part of the research program of The Gut and Health Section, which is a part of The Aquaculture Protein Centre (APC, This Centre of Excellence is the only one within the field of aquaculture in Norway. The Norwegian Research Council, together with the three mother institutions UMB, NVH and Nofima Marin, finances APC.

Cand. med. vet. Einar Lilleeng defended his PhD thesis, entitled "Molecular responses in exocrine pancreas and intestinal immune apparatus of Atlantic salmon. Effects of diets continuing soybean meal", at the Norwegian College of Veterinary Science on October 3, 2008.

Magnhild Jenssen | alfa
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