Dietary fiber: classification, functional definition, and its role in modulating nutrient digestibility and gut health in growing pigs

Dietary fiber (DF) was traditionally classified as an anti-nutritional factor due to its negative effects on digesta viscosity, nutrient digestibility, and endogenous nutrient losses. In recent years, however, this perspective has been increasingly re-evaluated and refined in light of emerging evidence underscoring the physiological and health-promoting roles of DF. In response to these emerging trends, DF has been considered as a critical nutritional component in swine diets, particularly in light of advances in molecular nutrition, gut microbiome research, and the characterization of fiber-degrading enzymes. Dietary fiber serves as an essential component in growing pig diets, which maintains normal physiological functions of the gastrointestinal tract (GIT), including mucosal integrity and microbial ecology. Besides, the physiological functions of DF, including its effects on digestibility and gut modulation, are significantly influenced by its chemical composition and structural features. In swine diet, DF modulates the gut microbiota promoting fermentative activity, which in turn stimulates the proliferation of beneficial bacteria in the GIT. However, the efficacy of DF might be constrained by the underdeveloped fiber-degrading capacity observed in young pigs, which is linked to immature gut morphology and reduced microbial diversity compared to adult sow. These physiological limitations, combined with the commercial swine industry’s reliance on traditional cereal-based diets and increasingly restricted regulations on antibiotic use, present significant challenges for optimizing gut health and nutrient utilization in growing pig. In this review, varying types and levels of dietary fiber in feed represent promising candidates as effective alternatives to antibiotics in swine nutrition. But there remains a paucity of systematic investigations into the roles of the physicochemical properties of dietary fiber—namely solubility, viscosity, and fermentability—within swine diets, as well as their impacts on nutrient metabolism and gut microbiota composition in growing pigs. Accordingly, this review summarizes current knowledge on the nutritional digestibility and gut health regulation associated with DF supplementation in growing pigs, with a focus on its physiological effects and limitations during the growing phase of pigs.