Plasma amino acids in rainbow trout ( Oncorhynchus mykiss) fed intact protein or a crystalline amino acid diet

Abstract

Plasma-free amino acids were studied in two groups of adult rainbow trout (346 ± 107 g mean body weight) fed for 6 weeks a control diet containing intact protein (fish meal, poultry meal, feather meal, corn), or a diet based on a crystalline amino acid mixture. After 2 days starvation, those fish which received intact protein were force-fed with wheat gluten; trout given the amino acid diet were force-fed with the same amino acid diet. Both groups were fed at a rate of 0.32% crude protein of body weight. Blood samples were taken 0, 3, 6, 9, 12, 15, 18, 24, 32, 40, 48 and 72h after force-feeding. Amino acid concentrations began to rise within the first 3h post-feeding in both groups; the amino acid increase was significantly higher ( P < 0.01 and 0.05) at 3h and 48h (but not at other sampling times) in the amino acid diet than in the wheat gluten diet. Accordingly, in trout given wheat gluten, peak levels occurred considerably later (18h post-feeding) when compared to those trout receiving the amino acid diet (9h after force-feeding). Individual amino acid increases in the peripheral plasma were closely correlated ( P < 0.05) 9, 15, 18 and 32h post-feeding to the amino acid profile of wheat gluten, but at no time to the amino acid content of the crystalline amino acid diet. Ranking the average increase of the individual amino acids until peak level and, correspondingly the average decrease until starvation level, indicated lysine to be highly deficient in wheat gluten and to be the first limiting amino acid in the amino acid diet.