New Insights into Diarrhea Caused by High-Fat Diet and Fatigue: Gut Microbiota Dysbiosis-Driven Bile Acid Metabolism Disorder

Background: This study investigated the mechanisms underlying diarrhea induced by a high-fat diet (HFD) under a state of fatigue, focusing on gut microbiota dysbiosis, bile acid metabolic disturbance, and gut–liver injury. Methods: Mice were assigned to a normal control diet (NCD) group, a HFD-induced diarrhea under fatigue (HFDM) group, and a HFD-induced diarrhea with aggravated dysbiosis (HFDMA) group. Histopathology, inflammatory factors, intestinal barrier-related proteins, small-intestinal microbiota, and colonic bile acid profiles were assessed, and correlation analyses were performed among gut microbiota, bile acids, and inflammatory factors. Results: Compared with the NCD group, both the HFDM and HFDMA groups showed diarrhea-like and fatigue-like phenotypes, histopathological injury in the small intestine and liver, increased tumor necrosis factor-alpha (TNF-α) and interleukin-6 (IL-6) levels, and impaired intestinal barrier function. No significant differences in inflammatory factors were observed between the HFDM and HFDMA groups. Zonula occludens-1 (ZO-1) expression decreased in both model groups but reached statistical significance only in the HFDMA group, whereas Claudin-1 expression was significantly reduced in both groups. Gut microbiota analysis showed altered community structure, with downward trends in alpha diversity that did not reach statistical significance but clear separation trends in beta diversity. Proteobacteria and Streptococcus increased, whereas Ligilactobacillus decreased. Total bile acid levels did not differ significantly among groups; however, the ratio of secondary to primary bile acids was significantly reduced in both model groups, particularly in the HFDMA group, with decreases in representative secondary bile acids, including hyodeoxycholic acid (HDCA) and isolithocholic acid (isoLCA). Correlation analysis further supported close associations among gut microbial alteration, bile acid disturbance, and intestinal and hepatic inflammation. Conclusions: Gut microbiota dysbiosis may disrupt bile acid metabolism, impair intestinal barrier integrity, and promote intestinal and hepatic inflammatory responses, thereby contributing to diarrhea progression under fatigue and HFD conditions through the gut–liver axis.