| Summary: | The use of dietary protein can be optimized by increasing diet energy, which can be achieved by adding non-protein nutrients such as carbohydrates and lipids. If incorporated in suitable amounts, these items can promote the protein sparing effect, reducing nitrogen excretion and improving the quality of fish farming effluents. The study assessed productive performance, body composition, nutrient and energy retention efficiency and digestibility of the omnivorous fish tambaqui (Colossoma macropomum) fed diets with three carbohydrate (410, 460 and 510 g kg−1) and two lipid levels (40 and 80 g kg−1) in the initial growth phase (juvenile weighing between 10 and 250 g). The experiment was completely randomized, with six treatments and four replicas arranged in a 3 × 2 factorial design. The 1080 tambaqui tested (10.88 ± 0.13 g body weight) were randomly distributed into 24 tanks (500 L; 45 fish/tank) and fed the test diets for 120 days. The highest carbohydrate inclusion (510 g kg−1) reduced food intake and fish growth. A protein sparing effect was observed in the growth of tambaqui fed 460 g kg−1 carbohydrates since they showed higher weight gain, protein efficiency ratio, protein productive value and crude protein participation in weight gain. The increase in lipid levels from 40 g kg−1 to 80 g kg−1 increased body fat deposition and decreased the digestibility coefficients of diet nutrients and diet energy. The results demonstrate that the ideal balanced diet to grow juvenile tambaqui is 460 g kg−1 carbohydrates and 40 g kg−1 lipids. Keywords: Growth, Protein sparing effect, Non-protein energy, Fat, Fish, Protein
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