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 nitr ogen contents of the gastrointestinal digesta of rainbow trout Oncorhy nchus mykiss fed different protein sources were examined. Three experi mental 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 ho urs of fasting, the fish were fed again the respective diets to satiat ion. Blood samples were taken at 3, 6, 9, 12, 15, 18, 21, 24, and 48 h ours after the last feeding. Plasma concentrations of individual amino acids reached peak levels at 12+/-3 hours post-feeding in the FM grou p, and at 21+/-3 hours in the SBM and MPF groups. Evacuation of gastri c digesta was faster in the FM and SBM groups than the MPF group. Howe ver, the 0.9% NaCl insoluble nitrogen levels (%) in the gastric digest a of the SBM and MPF groups seemed relatively stable until 21 hours co mpared with the decreasing trend observed in the FM group. The insolub le nitrogen contents (mg/100 g BW) in the intestinal digesta were high est in the MPF group, intermediate in the FM group, and unexpectedly l owest in the SBM group throughout the sampling periods. These findings suggest that peak levels of plasma free amino acids in the SBM and MP F groups occurred later than the FM group due in part to different pro tein digestive processes in the gastrointestinal tract.