Amino Acid Metabolism in Gilthead Seabream Is Affected by the Dietary Protein to Energy Ratios

Abstract

The dietary protein to energy ratio (P/E) has proven to influence protein utilization and/or growth in several fish species. This study intended to unravel the bioavailability and metabolic fate of lysine and methionine in gilthead seabream (Sparus aurata) juveniles fed plant diets with different P/E ratios. Seabream juveniles were fed two isonitrogenous diets (45% crude protein) differing in crude lipids (20 and 14%): LowP/E ( P / E ratio = 20.0 mg protein k J − 1 ) and HighP/E ( P / E ratio = 21.4 mg protein k J − 1 ). After three weeks, fish ( 11.6 ± 4.3 g ) were tube-fed the respective diet labelled with 14C-protein (L-amino acid mixture), 14C-lysine, or 14C-methionine. Protein, lysine, and methionine utilization were determined based on the proportion of 14C-amino acid evacuated, retained in the free or protein-bound fraction of liver and muscle, or catabolized. This study revealed that a decrease in P/E ratio resulted in lower amino acid evacuation (p <0.05), contributing to a more efficient amino acid uptake. Results indicate that amino acids are retained as protein in the liver and not only temporarily available in the free pool. The amount of free amino acids retained in the muscle of LowP/E fed fish was significantly higher than in HighP/E fish ( p < 0.05 ) due to a simultaneous higher retention of lysine and methionine, without affecting the overall protein retention. Methionine catabolism was significantly lower than lysine or protein independently of the P/E ratio ( p < 0.05 ), reinforcing that this amino acid is preferentially spared for metabolic functions and not used as energy source. In contrast, increasing the dietary P/E ratio decreased lysine catabolism and increased its availability for growth. The bioavailability and metabolism of individual amino acids should be considered when optimizing P/E ratios in diets for gilthead seabream juveniles. Formulating diets with optimum P/E ratios will improve diet utilization and fish performance.