High dietary carbohydrate inclusion by both protein and lipid replacement in gilthead sea bream. Changes in digestive and absorptive processes

Abstract

The effect of high amounts of dietary carbohydrates on digestive and absorptive processes was studied in juvenile gilthead sea bream. Two trials were conducted over 14 weeks. In the first trial, sea bream were fed with isoproteic diets with lipid replaced with starch and in the second with isolipidic diets with protein replaced with starch; in both wheat starch was included at above the established maximum levels for this fish species. At the end of each trial, intestinal alkaline protease, α-amylase and lipase activities were measured post-feeding, and alkaline phosphatase and leucine aminopeptidase activities and nutrient absorption capacities were measured under basal conditions.When lipid (23 to 17%) was replaced with starch (12 to 28%) in the 46% protein diet, a decrease in trypsin and chymotrypsin activities was found without modifying the trypsin/chymotrypsin ratio as dietary starch increases. Moreover, the pH of the intestinal content in the pyloric caeca increased and alkaline phosphatase activity and d-glucose absorption capacity decreased. Thus, compensatory mechanisms in buffer capacity, pancreatic enzyme activities and nutrient absorption capacities took place, allowing the maintenance of feed utilization and growth.Diets for the second trial were formulated with the lowest lipid content tested previously (17%) and protein (46 to 40%) was substituted with starch (19 to 39%). Lipase and trypsin activities increased as dietary starch raise, chymotrypsin activity and l-lysine absorption were not modified and amylase activity and d-glucose and L-alanine absorption decreased, but these digestive compensatory mechanisms were not sufficient, and there was a significant increase in FCR and a decrease in voluntary feed intake and SGR. This study suggests that sea bream are able to modulate the digestive process when protein requirements are met.