Effects of fasting and feeding on plasma amino acid levels in brown trout

Plasma amino acid (AA) levels were analysed at various intervals during fasting and after feeding in juvenile brown trout, Salmo trutta. After 3 days of food deprivation, total plasma AAs decreased significantly (–26% vs. control) due to a fall in the most abundant essential AAs (EAA): branched-chain amino acid (BCAA) and Thr, and there was also a decrease in non-essential AA (NEAA) levels, especially Ala and Asn. After 15 days of fasting, BCAA increased, possibly related to muscle protein mobilization. After 50 days of food deprivation, a significant increase in Gln levels (40% vs. 15 days of fasting) and a decrease in BCAA (–25% vs. 15 days of fasting) was noted. Feeding a commercial trout pellet caused a clear and progressive increase in plasma AA levels. Both EAA and NEAA concentrations peaked at 11 h post-feeding (4068 ± 160 and 4261 ± 239 µM NEAA, respectively) with a greater increase in EAA (+62%) than NEAA (40%) with respect to control values. These data show that changes in the plasma AA pool clearly reflect the nutritional state of fish and help us to understand the complex AA metabolism.