Effects of diet composition on apparent nutrient absorption along the intestinal tract and of subsequent fasting on mucosal disaccharidase activities and plasma nutrient concentration in Atlantic salmon Salmo salar L.

Atlantic salmon (1.9 kg), were fed a low carbohydrate/high protein or a high carbohydrate/low protein diet for 20 days prior to a fasting period of 60 days. At the end of the feeding period chyme was collected from the gastro‐intestinal tract divided into six sections, for studies of starch, lipid and amino acid absorption. Blood and gut wall samples were taken on days 0, 5, 12, 33 and 60 of fasting. The results showed that 50% or more of amino acid, lipid and starch absorption took place in the pyloric region. The distal intestine was also of quantitative importance for lipid absorption. Diet composition affected only starch absorption. The effect was apparent in the pyloric region with no compensatory absorption in the more distal regions. Brush‐border disaccharidase activities were negligible in the stomach, high in mid‐intestinal regions and decreased towards the distal regions. High carbohydrate diet caused reduced trehalase activity. Fasting caused immediate decreases in disaccharidase activities as well as in plasma glucose, triacylglycerols, cholesterol and insulin. During the second week of the fasting period, temporary increases were seen in mucosal maltase, sucrase and trehalase activities and in plasma insulin level. Similar changes were observed for several plasma amino acids. To regain efficient nutrient digestion after a fasting period, a restoration period may therefore be needed.