Effects of dietary protein-to-lipid ratio on digestive and absorptive processes in sea bass fingerlings

• Feeding with low protein diets down-regulate protease and α-amylase synthesis.
• l-Lys and d-Glc absorption capacities were down-regulated in low protein diets.
• Low energy diets promote higher protease and lipase synthesis.
• l-Met and d-Glc uptake capacity were up-regulated by increasing dietary lipid.
• 52:16 is a good dietary-to-protein ratio for sea bass fingerlings.

Two 8-week trials, one in spring and the other in autumn-winter, were performed with sea bass fingerlings. After each growth period, pancreatic digestive enzyme synthesis in the pyloric caeca (PC), food anticipatory activity adaptation in the proximal intestine (PI) and nutrient absorption capacities in the distal intestine were measured at 24 h post-feeding.In spring, five isoenergetic diets with different protein levels (40, 45, 50, 55 and 59%) were formulated by simultaneously changing lipid and starch content. Animals fed the low-protein diets showed reduced accumulation of alkaline protease (40 to 50% diet), lipase (40 to 45% diet) and α-amylase (40 to 55% diet) in the PC; however, the activities of digestive enzymes in the PI were high in fish fed both low and high dietary protein diets, being significant for alkaline protease (40 and 59% diet) and α-amylase (40 and 45% diet). Zymography of the PC identified four proteases with trypsin-like activity (23, 22, 20 and 17 kDa) and an 18 kDa chymotrypsin activity that were modulated differentially according to diet. The PC/PI ratios were highest for alkaline protease and lowest for α-amylase, showing different patterns of response to changes in dietary composition: higher for protease on 45, 50 and 55% animals and α-amylase on 59% sea bass. l-Lys and d-Glc uptake capacities were down-regulated as dietary protein content declined, while no changes were found in l-Met and l-Trp. Based on the above-mentioned data and the optimum growth of animals fed with 50% protein, a protein-to-energy ratio study was performed.Sea bass were fed with five isoproteic (52%) diets that differed in dietary lipid content (12, 14, 16, 18 and 20%). Alkaline protease and lipase synthesis in the PC was up-regulated in low energy diets (12, 14 and 16% and 12 and 14%, respectively) and no changes were found in α-amylase. PC zymography was consistent with protease measurements. Only food anticipatory activity of lipase was up-regulated in animals fed 12% of lipid. l-Met and d-Glc were up-regulated by increasing dietary lipid and l-Lys and l-Trp were not modified. Dietary lipid content had no effect on final body weight.Finally, seasonal effects on alkaline protease synthesis in the PC were significantly higher in spring and animals showed higher growth in this season than in the autumn-winter. However, adaptation of food anticipatory activity in the PI was higher in autumn-winter. No differences were found in nutrient absorption capacities between seasons.