Metabolic Fate Is Defined by Amino Acid Nature in Gilthead Seabream Fed Different Diet Formulations

SIMPLE SUMMARY: The maximisation of fish growth depends on amino acids’ availability in tissues at an optimum ratio since imbalances may lead to their utilisation for energy rather than growth. Amino acids may be catabolised in multiple pathways and be classified according to their metabolic fate: ketogenic and glucogenic. Ketogenic amino acids (e.g., lysine) are precursors of ketone bodies or long chain fatty acids and can be used in lipogenesis. Glucogenic amino acids (e.g., methionine) can be converted into glucose through gluconeogenesis. Some amino acids, such as tryptophan, can be ketogenic and glucogenic. This study aimed to evaluate how fish discriminate among different amino acids when they are fed different diets to attain the best utilisation of the feed. This trial was carried out in gilthead seabream juveniles fed experimental diets containing different levels of protein and/or distinct lipid levels. The metabolic fate and bioavailability of the indispensable amino acids lysine, methionine, and tryptophan were defined by their ketogenic and/or glucogenic nature rather than diet formulations. The optimisation of diets that consider the amino acids’ bioavailability will maximise protein retention in fish and is a viable solution to develop cost-effective fish diets. ABSTRACT: The sustainability of the Aquaculture industry relies on optimising diets to promote nitrogen retention and maximise fish growth. The aim of this study was to assess how different dietary formulations influence the bioavailability and metabolic fate of distinct amino acids in gilthead seabream juveniles. Amino acids (lysine, tryptophan, and methionine) were selected based on their ketogenic and/or glucogenic nature. Seabream were fed practical diets with different protein (44 and 40%) and lipid contents (21 and 18%): 44P21L, 44P18L, 40P21L, and 40P18L. After three weeks of feeding, the fish were tube-fed the correspondent diet labelled with (14)C-lysine, (14)C-tryptophan, or (14)C-methionine. The amino acid utilisation was determined based on the evacuation, retention in gut, liver, and muscle, and the catabolism of the tracer. The metabolic fate of amino acids was mainly determined by their nature. Tryptophan was significantly more evacuated than lysine or methionine, indicating a lower availability for metabolic purposes. Methionine was more retained in muscle, indicating its higher availability. Lysine was mainly catabolised, suggesting that catabolism is preferentially ketogenic, even when this amino acid is deficient in diets. This study underpins the importance of optimising diets considering the amino acids’ bioavailability and metabolic fate to maximise protein retention in fish.