An emerging role of vitamin D in amino acid absorption in different intestinal segments of on-growing grass carp ().

Vitamin D (VD), an essential nutrient for animals, has been demonstrated to stimulate the uptake of certain amino acids. However, the role of VD in the intestine, the primary site for digestion and absorption of nutrients, remains poorly characterized. Here, the grass carp () was studied to assess the influence of different doses of VD (15.2, 364.3, 782.5, 1,167.9, 1,573.8, and 1,980.1 IU/kg) on growth performance, intestinal morphology, digestive absorption, amino acid transport, and potential signaling molecule levels in a feeding experiment. As a result, dietary VD improved growth performance, intestinal structure, and digestive and brush border enzyme activities. Additionally, most intestinal free amino acids and their transporters were upregulated after VD intake, except for Ala, Lys, Asp, Leu, solute carrier () , , and mRNA in different segments, Leu and 4 mRNA in the proximal intestine, and mRNA in the mid and distal intestine. In the crucial target of rapamycin (TOR) signal pathway of amino acid transport, the gene and protein expression of TOR, S6 kinase 1, and activating transcription factor 4 were elevated, whereas 4E-binding protein 1 was decreased, further suggesting an advanced amino acid absorption capacity in the fish due to VD supplementation. Based on percentage weight gain, feed efficiency, and trypsin activity, the VD requirements of on-growing grass carp were estimated to be 968.33, 1,005.00, and 1,166.67 IU/kg, respectively. Our findings provide novel recommendations for VD supplementation to promote digestion and absorption capacities of fish, contributing to the overall productivity of aquaculture.