Different levels of dietary fiber have varying effects on the bile acid profile, liver and gut histology, and stress tolerance of the largemouth bass (Micropterus salmoides)

It is generally believed that dietary fiber (DF) is beneficial to health, but some studies have also suggested that DF may cause bile acid (BA) metabolism disorders and tissue damage. This discrepancy might be attributed to differences in DF dosage. Here, juvenile largemouth bass ( Micropterus salmoides ) were used to test this hypothesis and to explore the potential mediating role of the gut microbiota. A DF mixture was incorporated into experimental diets at 0 % (CON), 5 % (DF5), 10 % (DF10), and 15 % (DF15), and fish were fed these diets for eight weeks. Histological analysis revealed liver and gut tissue damage in the DF15 group. With increasing DF level, total BA (TBA) content in the body and liver showed an initial decline followed by an increase; the proportion of taurocholic acid (TCA) in bile decreased, whereas the proportion of taurodeoxycholic acid (TDCA) and the hydrophobicity index of BAs increased. The relative abundance of predicted microbial functional potential related to BA metabolism in the DF15 group was higher than that in the CON group ( P < 0.2). The abundance of Pseudomonas in the gut was negatively correlated with the TCA proportion in both bile and gut ( P < 0.05) and positively correlated with the TDCA proportion ( P < 0.05). In a separate experiment, fish were fed the same diets and subjected to transport stress on day 7. By day 6 post-stress, lesion incidence and the green-liver and green-bile rates increased with DF level. The magnitude of changes in liver and serum TBA content from 6 h to 24 h increased with increasing DF ( P < 0.05). Collectively, these results suggest that the physiological effects of DF are dose dependent; high DF amplifies BA oscillations, causes liver and gut damage, and reduces tolerance to transport stress, with involvement of the gut microbiota. • The effects of different levels of dietary fiber on bile acid pool size may be opposite. • High dietary fiber increased the amplitude of bile acid oscillations. • High dietary fiber promoted microbiota-mediated biotransformation of bile acids. • High dietary fiber induced liver and gut damage and reduced tolerance to transport stress.