The Study

A high fiber diet is believed to fuel the gut microbiota, which is believed to be responsible for a multitude of health effects. But how exactly fiber deprivation impacts the gut microbiota and the health in general is largely unknown. In a recent study, Desai et al. and colleagues from the University of Michigan Medical School, USA studied the effects of intermittent and chronic fiber deprivation on the behavior of the gut microbiome.  The researchers used a gnotobiotic or a germ-free strain of mice to assess the fiber-microbiota-gut barrier relationship.

The researchers inoculated the mice with 14 representative strains of bacteria, which are commonly found in the human gut. Initially, the mice were fed on a high fiber diet for 14 days.  The gut microbiota flourished over the two weeks.  Afterward, the mice were divided into three groups and were fed on different diets: fiber-rich, fiber-free, and prebiotic-based diet. Unlike the fiber-rich diet, which had intact fiber participles, the prebiotic-based diet had purified glycans in them and lacked complex polysaccharides.

Fiber Deprivation and Gut Barrier

The groups eating a fiber-free diet experienced significant changes in the balance of their gut microbiota. Researchers found that the balance of the bacterial population in these mice shifted in the favor of the strains that could easily digest the mucin O-glycans (MOGs), which serve a barrier function in the gut.  Furthermore, the researchers measured the thickness of the gut mucosa (the inner gut layer) in the mice to determine whether the growth of MOGs degrading bacteria actually led to a breakdown of the gut wall.  Interestingly, the thickness of the gut mucosa barrier was significantly higher in all the groups except in the fiber-free group.

Finally, since the gut mucosa barrier serves to keep the disease causing bacteria at bay, the researchers postulated that a decrease in the mucosa thickness will predispose the mice to the growth of pathogenic bacteria. To test this hypothesis, they infected two more groups of similar mice (high fiber vs. fiber free groups) and studied them for 4 weeks. At the end of 4 weeks, researchers found that the growth of Citrobacter rodentium (a gut pathogen) and subsequently the incidence of colitis (gut infection) were higher in the fiber-free group.
In summary, fiber helps to fuel the gut microbiota and prevents them turning against the gut mucosa for nutrition. This way, a diet rich in complex prebiotics functions to uphold the integrity of the gut mucosa barrier and prevent the growth of pathogenic bacteria.

References

A Dietary Fiber-Deprived Gut Microbiota Degrades the Colonic Mucus Barrier and Enhances Pathogen Susceptibility
http://www.cell.com/cell/abstract/S0092-8674(16)31464-7