Scope Galactomannan and citrus pectin are considered ?super fibers? known for altering gut microbiota composition and improving glucose and lipid metabolism. The study aims to investigate the fiber's effect on a nonalcoholic steatohepatitis (NASH) model. Methods and results Two feeding experiments are carried out using groups of 7?8 week-old male C57BL/6J mice. The diets used are based on a high cholesterol/cholate diet (HCD), such as a nutritional NASH model. Mice are fed a diet with or without 15% fiber-citrus pectin (HCD-CP) or galactomannan (HCD-G) together with the HCD (first experiment), which commenced 3 weeks prior to the HCD (second experiment). Liver damage is evaluated by histological and biochemical parameters. Galactomannan leads to lesser weight gain and improved glucose tolerance, but increased liver damage. This is shown by elevated levels of liver enzymes compared to that with HCD alone. Fibers induce higher steatosis, as evaluated by liver histology. This intriguing result is linked to various changes in the gut microbiota, such as elevated Proteobacteria levels in the galactomannan group, which are correlated with disturbed metabolism and dysbiosis. Conclusions In a NASH mouse model, galactomannan increases liver damage but improves glucose metabolism. Changes in the microbiota composition may answer this enigmatic observation.