Pathogenic effects of excessive intake of dietary fiber on fish and its mechanisms

Due to the needs of environmental protection and cost control, the amount of plant feedstuffs used in aquatic formula feed is increasing. Plant feedstuffs are rich in dietary fibers (DFs), which resulted in over 30% of DFs contained in common commercial aquatic commercial feeds. However, their impacts on fish physiology are still largely overlooked. This article introduces the definition, classification, physicochemical properties, fermentability, and content of DFs in common plant feedstuffs, with a focus on the research findings of the author and team in the fish physiology of DFs. We noticed that hemorrhage, white feces, enteritis, skin-rotting, gill-rotting, hepatic steatosis and fibrosis, green liver, white liver, gallbladder enlargement and even mass deaths could be induced in yellow catfish (Pelteobagrus fulvidraco) when diet included 20%-30% DFs. Similar symptoms had also been observed in largemouth bass (Micropterus salmoides) and grass carp (Ctenopharyngodon idella) fed high DFs diet. The pathogenic effect of DFs was related not only to the content of DFs in the diet but also to the types of DFs. Pectin was much more pathogenic to P. fulvidraco than cellulose. The pathogenic mechanism of DFs is related to their interference with bile acids (BAs) homeostasis and intestinal microbial homeostasis. DFs have the capacity of binding to BAs, which prevents BAs from activating BA receptors such as FXR. FXR negative feedback regulates the synthesis of BAs and inhibits inflammatory reactions. As a result, high levels of DFs caused hypersynthesis of BAs, increasing of the BAs level in tissues and their hydrophobicity, and interfed with the circadian rhythm of BAs, further inducing inflammatory reactions and tissue necrosis due to the cytotoxicity of BAs. Some DFs are fermentable, which caused structural changes in the gut microbiota (might also be duo to the antibacterial effect of BAs). The disorder of intestinal microbial homeostasis further aggravates metabolic disorders, including BAs metabolic disorder. The disease symptoms and progression induced by DFs can also be improved by the other stress factors. Based on the pathogenesis of DFs, the diseases can be prevented and controlled by limiting the intake of DFs, adding BAs and taurine to the diet, and avoiding the superposition effect of DFs with other stress factors. This study for the first time elucidates the pathogenicity of excessive intake of DFs on fish and its potential mechanisms, providing a new perspective for the high-value utilization of plant feedstuffs and optimization of feed formulation, suggesting a new approach for fish disease prevention and control. Further clarification should be provided on the types, levels, and characteristics of DFs in different plant feedstuffs, as well as the tolerance range of aquatic animals to DFs.