Different protein to energy ratio diets for gilthead sea bream (Sparus aurata): Effects on digestive and absorptive processes

Sea bream juveniles were fed for a 12-week period with 7 isoenergetic diets with different levels of protein (35, 38, 41, 44, 47, 50 and 53%). The diets were formulated by changing simultaneously the levels of lipid (from 27 to 19%) and starch (from 21 to 10%). After the growth period, we studied the digestive enzyme activities at 5 h post-feeding and the nutrient absorption capacity at 24 h post-feeding. A progressive increase in the total protease activity (TPA) was found as the content of diet protein rose (from 35 to 41%) and a diminution of TPA activity was detected in sea bream fed high soybean concentrate content diets (50 and 53% dietary protein). Lipase activity did not change by dietary composition. An up-regulation of α-amylase activity, and d-glucose and l-Ala absorption capacities was found in fish fed with the lowest carbohydrate diets, corresponding to 50 and 53% dietary protein. Moreover, the l-lysine absorption capacity was upregulated in fish fed with low protein diets (35, 38, 41 and 44%). Thus, changes in diet composition can modulate enzymatic activities and nutrient absorption capacity, to improve food use and assure growth performance. In this sense, only sea bream fed with the lowest protein diet (35%) had a significantly lower SGR (< 1), although they had the highest voluntary feed intake. Sea bream fed with P44 and P47 diets need the minimal adaptive changes to diet and showed good growth.