Dietary intakes of vitamin D promote growth performance and disease resistance in juvenile grass carp (Ctenopharyngodon idella).

Vitamin D (VD) is an essential nutrient for fish and participates in a variety of physiological activities. Notably, both insufficient and excessive supplementation of VD severely impede fish growth, and the requirements of VD for fish vary considerably in different species and growth periods. The present study aimed to evaluate the appropriate requirements of VD for juvenile grass carp (Ctenopharyngodon idella) according to growth performance and disease prevention capacity. In this study, diets containing six supplemental levels of VD (0, 300, 600, 1200, 2400, and 4800 IU/kg diet) were formulated to investigate the effect(s) of VD on the growth performance, antioxidant enzyme activities, and antimicrobial ability in juvenile grass carp. Compared with the VD deficiency group (0 IU/kg), the supplementation of 300-2400 IU/kg VD significantly enhanced growth performance and increased antioxidant enzyme activities in the fish liver. Moreover, dietary supplementation of VD significantly improved the intestinal health by manipulating the composition of intestinal microbiota in juvenile grass carp. In agreement with this notion, the mortality of juvenile grass carp fed with dietary VD was much lower than that in VD deficient group upon infection with Aeromonas hydrophila. Meanwhile, dietary supplementation of 300-2400 IU/kg VD reduced bacterial load in the spleen and head kidney of the infected fish, and 1200 IU/kg VD supplementation could decrease enteritis morbidity and increase lysozyme activities in the intestine. These findings strengthened the essential role of dietary VD in managing fish growth and antimicrobial capacity. Additionally, based on weight gain ratio and lysozyme activities, the appropriate VD requirements for juvenile grass carp were estimated to be 1994.80 and 2321.80 IU/kg diet, respectively.