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The effects of combining soyasaponins with plant ingredients on intestinal function and fish health were investigated in an 80 d study with Atlantic salmon (270 g) distributed thirty each into twenty-four tanks with seawater. Soyasaponins were supplemented (2 g/kg) to diets with maize gluten (MG), pea protein concentrate (PPC) and sunflower (SFM), rapeseed (RSM) or horsebean meals. A diet with soyabean meal (SBM) and another with wheat gluten and soyasaponins served as reference diets. Marked soyasaponin effects were observed when combined with PPC. This combination induced inflammation in the distal intestine (DI) similar to SBM, reduced feed intake, apparent digestibility of lipid, most amino acids and ash, decreased bile salt levels in intestinal chyme and decreased leucine aminopeptidase (LAP) activity but increased trypsin activity in the DI. No enteritis was observed in other diet groups, but small consistent negative soyasaponin effects were seen on lipid and fatty acid digestibility, faecal DM and LAP activity of the DI. Soyasaponin combination with RSM reduced digestibility of all nutrients including minerals. The mineral effect was also seen for SFM, whereas with MG and SFM a positive soyasaponin effect on feed intake was observed. Caution should be exercised to avoid ingredient combinations giving high saponin levels, a condition that appears to be a key factor in diet-induced enteritis together with certain plant ingredients.Key words: Soyasaponins: Plant protein ingredients: Antinutritional factors: Fish feed: Gastrointestinal tract Alternative dietary protein sources to supplement and replace limited marine ingredients in fish feeds are important for the future of the fish farming industry. Plant ingredients such as soyabeans hold promise with their good amino acid profiles that can easily be improved for fish requirements by supplementation with deficient amino acids. However, soyabean meal (SBM) inclusion has been demonstrated to induce enteritis and reduce performance in salmonids (1 -3) and carp (4) . The factors responsible for the disorders have not been conclusively identified, but soyasaponins, and possibly other bioactive antinutritional factors (ANF) in SBM, are implicated in the aetiology (5 -7) .Saponins are heat-stable glycosides present in soyabean and other legumes such as pea and lupin (8,9) . Saponins, with their membrane-active nature and affinity to cholesterol and bile salts (10,11) , possess a number of potential biological effects compatible with the negative effects observed in fish fed diets containing SBM. There have been conflicting findings from studies on dietary effects of saponins to teleost fish. In one study, a saponin-rich extract from SBM and Quillaja saponins, both at a 0·3 % dietary inclusion rate of saponin, greatly reduced feed intake and growth in Chinook salmon and depressed growth in rainbow trout (12) . Furthermore, the Quillaja saponin diets (0·15 and 0·3 % dietary inclusion) both induced substantial damage to the intestinal mucosa for both Chino...

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In Atlantic salmon (Salmo salar L.), and also in other fish species, certain plant protein ingredients can increase fecal water content creating a diarrhea-like condition which may impair gut function and reduce fish growth. The present study aimed to strengthen understanding of the underlying mechanisms by observing effects of various alternative plant protein sources when replacing fish meal on expression of genes encoding proteins playing key roles in regulation of water transport across the mucosa of the distal intestine (DI). A 48-day feeding trial was conducted with five diets: A reference diet (FM) in which fish meal (72%) was the only protein source; Diet SBMWG with a mix of soybean meal (30%) and wheat gluten (22%); Diet SPCPM with a mix of soy protein concentrate (30%) and poultry meal (6%); Diet GMWG with guar meal (30%) and wheat gluten (14.5%); Diet PM with 58% poultry meal. Compared to fish fed the FM reference diet, fish fed the soybean meal containing diet (SBMWG) showed signs of enteritis in the DI, increased fecal water content of DI chyme and higher plasma osmolality. Altered DI expression of a battery of genes encoding aquaporins, ion transporters, tight junction and adherens junction proteins suggested reduced transcellular transport of water as well as a tightening of the junction barrier in fish fed the SBMWG diet, which may explain the observed higher fecal water content and plasma osmolality. DI structure was not altered for fish fed the other experimental diets but alterations in target gene expression and fecal water content were observed, indicating that alterations in water transport components may take place without clear effects on intestinal structure.