Protein sparing by dietary lipids in the diets of Pangasianodon hypophthalmus: an appraisal of growth, body composition, digestive enzymes, and metabolic responses.

Lipid-induced protein sparing conserves protein for growth, reducing feed costs and nitrogenous waste in the environment. In a 90-day experiment, the protein-sparing effects of lipids on the growth, nutrient content, amino acid profile, hematobiochemical parameters, digestive enzymes, and antioxidant status of Pangasianodon hypophthalmus were evaluated. Four isoenergetic diets (3223.26 kcal/kg) with varying protein (P) and lipid (L) contents were formulated: D1 (P39/L10, control), D2 (P35/L12), D3 (P31/L15), and D4 (P27/L18). Acclimatized fingerlings (N = 240; initial weight, 12.75 ± 0.2 g) were randomly allocated to four dietary treatment groups (20 fish/aquarium) in triplicate and fed to satiation twice daily. Reducing dietary protein from 39 to 35% with increasing lipids from 10 to 12% maintained optimal growth, indicating a protein-sparing effect. However, further reductions (31-27% protein) in lipids (15-18%) impaired growth and increased biological indices. The body and muscle protein contents remained stable between 39 and 35% protein but declined with increasing reduction, whereas the lipid content increased. Essential amino acids (lysine, methionine, isoleucine), antioxidant status, hepatic enzymes, plasma metabolites (glucose, cholesterol, triglycerides), and protease were stable at 39 to 35% protein but decreased beyond that point, whereas the lipase and malondialdehyde contents showed opposite trends. Intestinal morphology (villus height and width, muscle thickness) was unaffected by 39 to 35% protein but was reduced with further protein reduction. These findings highlight the protein-sparing effect of lipids in P. hypophthalmus when lipids increase from 10 to 12% and when proteins decrease from 39 to 35%.