Deficiency of dietary niacin decreases digestion and absorption capacities via declining the digestive and brush border enzyme activities and downregulating those enzyme gene transcription related to TOR pathway of the hepatopancreas and intestine in young grass carp ( C tenopharyngodon idella )

To investigate the effects of niacin on growth, digestion and absorption capacity, and the potential mechanism for digestive and brush border enzyme activities, grass carp ( C tenopharyngodon idella ) (256 ± 0.41 g) were fed diets containing 3.95 (basal diet group), 14.92, 24.98, 35.03, 44.97 and 55.01 mg niacin kg −1 diet for 8 weeks. Results indicated that percentage weight gain ( PWG ), feed intake and feed efficiency were the lowest in basal group ( P  <   0.05). Similarly, niacin deficiency decreased hepatopancreas trypsin, chymotrypsin, lipase and amylase activities ( P  <   0.05), intestinal N a + , K + ‐ ATP ase, alkaline phosphatase, γ‐glutamyl transpeptidase and creatine kinase ( CK ) activities, the cholecystokinin (CCK) content in proximal intestine ( PI ) and growth hormone content in serum ( P  <   0.05). Furthermore, niacin deficiency downregulated gene expression of hepatopancreas trypsinogen 1, trypsinogen 2, chymotrypsinogen and amylase, intestinal N a + , K + ‐ ATP ase alpha subunit isoform 1, N a + , K + ‐ ATP ase alpha subunit isoform 8 and CK , and target of rapamycin ( TOR ) and S 6 kinase 1 ( S 6 K 1) of hepatopancreas and intestine ( P  <   0.05), whereas upregulated eIF 4 E ‐binding protein (4 EBP ) gene expression ( P  <   0.05). The niacin requirement for young grass carp (256–689 g) based on PWG , hepatopancreas trypsin activity and N a + , K + ‐ ATP ase in PI was 34.01, 35.10 and 42.08 mg kg −1 diet, respectively.