The virtues of fibre as part of a healthy diet are commended. Now, in two new studies, researchers have determined how a fermentable fibre (inulin) contributes to gut health and barrier function, protecting against diet-induced obesity and colonic mucus degradation in mouse models.

In the first study, Andrew Gewirtz and colleagues examined the role of dietary fibre as a possible means of promoting healthy host–microbe interactions to prevent metabolic syndrome. “A peaceful relationship between the gut microbiota and the intestine promotes metabolic health,” explains Gewirtz, noting that disturbances in this relationship can promote low-grade inflammation that can impair metabolic signalling and promote metabolic syndrome.

Mice were fed either standard chow or a high-fat diet (HFD, which induces obesity and features of metabolic syndrome) supplemented with either fermentable (inulin) or insoluble (cellulose) fibre. Compared with standard chow, supplementation with inulin protected the mice against diet-induced obesity and also reduced levels of hepatosteatosis.

Looking into the underlying processes, a HFD enriched with inulin, but not that enriched with cellulose, had beneficial effects on the intestinal barrier, increasing gut epithelial cell proliferation and preventing atrophy of the colon and the intestinal crypts. Moreover, supplementation with inulin (a well-known prebiotic) restored the gut microbiota levels and bacterial growth that had become depleted with a HFD (HFD resulted in ∼10-fold reduction in total faecal bacterial loads). Interesingly, inulin also corrected some of the changes to the composition of the gut microbiota as a result of a HFD and restored microbiota–mucosa separation.

Credit: Laura Marshall/Macmillan Publishers Limited

The beneficial effects of inulin for colonic and metabolic health were dependent on the microbiota and IL-22, but not short-chain fatty acids. “Enriching the diet with inulin but not cellulose largely restores many of the beneficial bacteria and re-energizes host defences, thus preventing microbiota encroachment and protecting against HFD-induced metabolic syndrome,” says Gewirtz.

In the second study, Gunnar C. Hansson and colleagues focused on the effects of diet on the protective mucus layer in the mouse intestine and the crucial separation of bacteria from the intestinal epithelium. “Mucus penetrability is dynamic and varies with bacterial composition,” explains Hansson. Given evidence that susceptibility to diabetes increases with a low-fibre diet, Hansson asks: “Could the link be that a low-fibre diet renders the inner mucus layer more penetrable, allowing bacteria to come closer to the epithelium, and by this trigger low-grade inflammation?”

Mice were fed either standard chow or a Western style diet (WSD, rich in saturated fats and simple sugars, but low in dietary fibre), as well as drinking water supplemented with either inulin, the probiotic Bifidobacterium longum NCC2705, or a combination of the two before analysis of colonic mucus properties.

a fermentable fibre (inulin) contributes to gut health and barrier function

A WSD profoundly affected the inner colonic mucus layer in mice, influencing key functions (growth rate was impaired with increased penetrability). Moreover, the composition of both luminal microbiota and mucosal microbiota in the distal colon were altered by a WSD, and distinct shifts in the microbiota composition paralleled changes in mucus penetrability and growth.

Faecal microbiota transplantation from chow-fed mice prevented the deterioration of the colonic mucus on a WSD. Crucially, treatment with B. longum NCC2705 or inulin prevented the development of mucus defects in mice fed a WSD.

“Only 3 days of Western diet, low in fibre, make the inner mucus more penetrable to bacteria,” notes Hansson. “The molecular mechanism behind the effect of diet on the formation of bacterial metabolites and the way they signal to the host and control mucus properties is poorly understood ... something that will require major efforts to understand better,” he adds.