Postprandial changes in plasma free amino acid concentrations of rainbow trout fed diets containing different protein sources

Postprandial changes in plasma free amino acid concentrations and nitrogen contents of the gastrointestinal digesta of rainbow trout Oncorhynchus mykiss fed different protein sources were examined. Three experimental diets containing either white fish meal (FM), defatted soybean meal (SBM) or malt protein flour (MPF), were fed to triplicate groups (initial mean weight; 63 g) for two weeks at 15 degrees C. After 48 hours of fasting, the fish were fed again the respective diets to satiation. Blood samples were taken at 3, 6, 9, 12, 15, 18, 21, 24, and 48 hours after the last feeding. Plasma concentrations of individual amino acids reached peak levels at 12+/-3 hours post-feeding in the FM group, and at 21+/-3 hours in the SBM and MPF groups. Evacuation of gastric digesta was faster in the FM and SBM groups than the MPF group. However, the 0.9% NaCl insoluble nitrogen levels (%) in the gastric digesta of the SBM and MPF groups seemed relatively stable until 21 hours compared with the decreasing trend observed in the FM group. The insoluble nitrogen contents (mg/100 g BW) in the intestinal digesta were highest in the MPF group, intermediate in the FM group, and unexpectedly lowest in the SBM group throughout the sampling periods. These findings suggest that peak levels of plasma free amino acids in the SBM and MPF groups occurred later than the FM group due in part to different protein digestive processes in the gastrointestinal tract.