Effects of dietary protein levels on lipid metabolism and intestinal health of hybrid grouper (♀ Epinephelus fuscoguttatus × ♂ Epinephelus lanceolatus) fed high-lipid diets

The study aims to investigate the effects of dietary protein levels in high-lipid diets (16 %) on lipid metabolism and intestinal health of hybrid grouper ( ♀ Epinephelus fuscoguttatus × ♂ Epinephelus lanceolatus ). Six experimental diets with equal lipid content (CL 16 %), but protein levels of 51 % (D1, control group), 48 % (D2), 45 % (D3), 42 % (D4), 39 % (D5), and 36 % (D6) were designed. The results showed that reducing dietary protein levels significantly influenced physiological responses. Body indices were modulated, with the viscerosomatic index (VSI) increasing at the lowest protein level and condition factor (CF) peaking at a moderate reduction. Serum lipid profile improved, marked by decreased low-density lipoprotein (LDL) and increased high-density lipoprotein (HDL), alongside enhanced lipoprotein lipase (LPL) activity and reduced liver triglycerides (TG). Liver gene expression shifted toward lipid catabolism, with downregulation of acetyl-CoA carboxylase ( acc ) and fatty acid synthase ( fas ) and upregulation of hormone-sensitive lipase ( hsl ), peroxisome proliferator-activated receptor α ( pparα ), and lipoprotein lipase ( lpl ). Intestinal health improved, reflected in increased digestive enzyme (lipase, trypsin, amylase) activities, improved morphology, and a favorable microbial shift characterized by reduced Proteobacteria and increased Firmicutes. The above results suggest that appropriate protein levels in high-lipid diets improve grouper lipid metabolism, enhance lipolysis, as well as improve intestinal health in grouper. • Appropriate protein levels in high-lipid diets can regulate lipid metabolism and intestinal health in groupers. • Reducing dietary protein to 42 %-45 % optimizes hepatic lipid metabolism by raising pparα, hsl, lpl and lowering acc , fas . • The study shows protein levels modulate intestinal health by reducing Vibrioceae and increasing Streptococcaceae. • The optimal dietary protein level for grouper fed a high-lipid (16 %) diet, balancing growth, liver and intestinal health is42 %-45 %.