Dietary xylitol supplementation improves salinity tolerance in Nile tilapia (Oreochromis niloticus) associated with changes in carbohydrate metabolism and the gut microbiota

The development of saline aquaculture is crucial for the advancement of the aquaculture industry and global food supply. Xylitol is a natural pentitol and osmolyte that may effectively improve the salinity tolerance of fish. In this study, Nile tilapia (Oreochromis niloticus) were fed diets supplemented with 0 (control), 0.5, 1.0, 1.5 and 2.0 g/kg xylitol under 20 salinity stress conditions. The effects on growth performance, body composition, carbohydrate metabolism, antioxidant capacity, osmoregulatory capacity, tissue histology and intestinal microbiota were evaluated. Salinity stress significantly reduced weight gain (WG) and the specific growth rate (SGR) and increased the feed conversion ratio (FCR) in tilapia. It also increased the malondialdehyde (MDA) content and decreased the total antioxidant capacity (T-AOC). In addition, salinity stress significantly decreased intestinal microbial richness and altered the gut microbial community structure. Interestingly, dietary xylitol supplementation significantly enhanced the growth performance of tilapia and increased the whole-body crude protein content. Moreover, dietary xylitol altered the expression of genes associated with the glucuronate-xylulose pathway and the pentose phosphate pathway (PPP), which was associated with changes in carbohydrate metabolism and antioxidant capacity. Additionally, dietary xylitol supplementation was associated with improved hepatic and gill tissue morphology and enhanced osmoregulatory function. Furthermore, microbiota analysis in the SW-1.5 group revealed alterations in microbial community composition, including changes in the relative abundance of Actinobacteria and Proteobacteria, increased microbial diversity, and enhanced community network complexity under salinity stress. In addition, quadratic polynomial regression analysis based on weight gain and specific growth rate indicated that the estimated optimal dietary xylitol supplementation range for tilapia under salinity stress was approximately 1.40–1.51 g/kg under the present experimental conditions. Overall, this study demonstrated that an appropriate level of dietary xylitol supplementation can significantly alleviate the adverse effects of salinity stress on tilapia. This study provides a theoretical basis and technical support for the development of functional feeds for saline water aquaculture. This study is the first to systematically evaluate the potential association of dietary xylitol supplementation with carbohydrate metabolism, antioxidant capacity and gut microbiota modulation in tilapia under salinity stress.