Optimal dietary protein level for juvenile Hefang crucian carp ( <i>Carassius auratus</i> ): Balancing growth performance and hepatic health

Abstract A 54‐day feeding trial was conducted to investigate the effects of dietary protein levels on growth performance, digestive capacity, plasma biochemical indexes, and gene expression in Hefang crucian carp ( Carassius auratus ). Six experimental diets were prepared with protein levels of 21.7%, 24.0%, 29.4%, 34.0%, 38.0%, and 43.2%, respectively. Each diet treatment was randomly assigned to triplicate groups of 25 fish, with an initial body weight of (14.60 ± 0.39) g per tank. Fish were fed twice daily to apparent satiation. The results showed that weight gain rate (WGR) and specific growth rate (SGR) exhibited an upward trend with increasing dietary protein levels, and reached the highest value in the 43.2% protein group. Conversely, protein efficiency was significantly higher in the 24.0% protein group compared to the other protein groups ( p &lt; 0.05). As dietary protein levels increased, there was a significant rise in the condition factor (CF), while both the hepatopancreas somatic index (HSI), viscerosomatic index (VSI) and crude lipid of whole body experienced significant decreases ( p &lt; 0.05). The intestinal trypsin activity was significantly elevated in the high‐protein groups (38.0% and 43.2% protein) compared to the low‐protein group (21.7% protein), whereas the plasma triglyceride concentrations exhibited a significant inverse relationship ( p &lt; 0.05). However, at a dietary protein level of 43.2%, the lipid droplet content in the liver of Hefang crucian carp was significantly elevated compared to other experimental groups. Additionally, the group fed the diet with a protein level of 21.7% exhibited significantly higher lipid droplet content than the groups with protein levels of 24.0% and 34.0%. A curvilinear regression analysis indicated that the lipid droplet area in the liver was minimized at a dietary protein concentration of 29.9%. Regarding intestinal gene expression, the asct2 and lat2 genes exhibited the highest expression levels in the group supplemented with 43.2% protein. Conversely, the expression levels of pept1 and cdx2 genes initially decreased and subsequently increased as the dietary protein levels rose, achieving their peak in the low‐protein groups (21.7% and 24.0% protein) ( p &lt; 0.05). Concerning hepatopancreas genes, the expression levels of tor , igf1 , and gh genes were significantly elevated in the 24.0% protein group compared to other treatment groups. Additionally, the expression levels of ghr were significantly higher in the 21.7% and 43.2% protein groups compared to the other treatment groups ( p &lt; 0.05). In conclusion, Hefang crucian carp exhibited optimal growth performance at a dietary protein level of 43.2%, whereas a protein concentration of 29.9% was identified as the critical threshold for maintaining optimal liver health.