Carbohydrate content in the diet of red-bellied pacu (Piaractus brachypomus) and its effects on growth performance, nutrient deposition, digestive enzymes, lipogenesis, and glycolysis.

This study evaluated the effects of varying dietary carbohydrate (CBH) levels on growth performance, body composition, hematological parameters, metabolism, and intestinal and hepatic enzyme activity in juvenile red-bellied pacu (Piaractus brachypomus). Fish (initial weight 70.25 g) were fed three times daily to apparent satiety for 45 days with diets containing 371.80-378.20 g kg-1 crude protein and 17.00-17.42 MJ kg-1 gross energy, and supplemented with 230, 260, 290, 320, or 350 g CBH kg-1. Fish were distributed among 15 tanks (practical volume of 800 L; stocking density of 0.878 g L-1), with treatments replicated 3 times. Optimal growth performance, feed conversion ratio, and whole-body nutrient deposition were observed in fish fed diets containing 260-290 g CBH kg-1. Fish fed the highest CBH level (350 g kg-1) exhibited reduced erythrocyte counts but increased hemoglobin concentration. Increasing dietary CBH levels resulted in higher hepatosomatic index, plasma triglyceride concentration, and liver and muscle glycogen stores, while reducing the viscerosomatic index and hepatic aspartate aminotransferase activity, with no effect on glycemia. Intermediate CBH levels (260-320 g kg-1) increased intestinal amylase and lipase activities but reduced alkaline protease activity. In the liver, high-CBH diets stimulated the activity of key enzymes involved in glycolysis (6-phosphofructo-1-kinase, PFK1; and pyruvate kinase, PK), the pentose phosphate pathway (6-phosphogluconate dehydrogenase, 6PGDH), and the tricarboxylic acid cycle (isocitrate dehydrogenase, ICDH), while decreasing the activity of rate-limiting enzymes in gluconeogenesis (fructose-1,6-bisphosphatase 1, FBPase1) and amino acid deamination (glutamate dehydrogenase, GDH). In conclusion, dietary CBH levels between 260 and 290 g kg-1 are recommended to optimize growth performance and enhance digestive and hematometabolical efficiency in juvenile red-bellied pacu. Exceeding this range may impair performance and welfare, highlighting the importance of precise dietary formulation in aquaculture.