which in turn reduces proinflammatory signaling through activation of the farnesoid X receptor (FXR), shows a study recently published in Cellular and Molecular Gastroenterology and Hepatology
“Our results support the notion that pharmacologic FXR activation might be useful in managing IBD [inflammatory bowel disease], and thus, further investigation of its mechanisms of action are warranted,” wrote the authors, led by Andrew Gewirtz, of the Center for Inflammation, Immunity and Infection, Institute for Biomedical Sciences, Georgia State University, Atlanta.
Dr. Andrew Gewirtz
Dr. Andrew Gewirtz
Dietary fiber has long been understood to be a key component to a healthy diet by promoting intestinal and metabolic health, but it is unclear whether dietary fiber benefits IBD, specifically Crohn’s disease and ulcerative colitis, and if so, what fiber types are best for these conditions. Some studies have suggested an association between fiber-rich diets and reduced incidence of IBD, but some IBD patients experience intolerance to fiber-rich foods and associated fiber-rich foods with disease flares. In mouse models with colitis, semi-purified fibers have been associated with both the easing and exacerbation of IBD symptoms, with soluble/fermentable fibers like inulin and pectin generally worsening colitis.
The study had two goals: Identify specific fibers that might ameliorate two models of experimental colitis in mice models and to better understand the mechanism by which fiber(s) might suppress inflammation.
Mice were fed high-fiber grain-based chow or diets enriched with semi-purified fibers that included inulin, cellulose, pectin, glucomannan, and psyllium, but only psyllium, a semi-soluble derived from Plantago seeds, improved colitis, and metabolic syndrome. The other fibers often protected against obesity but worsened colitis.
Consuming diets enriched with psyllium were found to “markedly” protected against both dextran sulfate sodium– and T-cell transfer–induced colitis. The protection was independent of fermentation and occurred in animals with minimal microbiota. The animals had increased expression of genes that influence bile acid secretion, and the researchers noted increased levels of both fecal and serum BA.
The increased serum levels prompted the researchers to investigate psyllium’s role in signaling activation through BA receptors, especially FXR. An FXR agonist also reduced colitis severity, while an FXR antagonist worsened it. FXR-deficient mice gained little benefit from psyllium supplementation, further suggesting that FXR mediates psyllium’s effect.
All soluble fibers impacted gut microbiota composition, but none more than psyllium which protected mice from developing dextran sulfate sodium colitis. While other soluble fibers increased in the abundance of bacteria (that is, fecal/luminal bacterial density), psyllium decreased in this area, which may explain why some fermentable fibers, including inulin, exacerbate colitis.
“These results indicate that psyllium’s protection against colitis involves its ability to increase circulating bile acid levels, thus activating FXR signaling,” the authors wrote.
Researchers found some evidence that prolonged psyllium supplementation could lead to mild elevations in AST and ALT, suggesting that the ability of psyllium to chelate BA could lead to lipid deficiency, especially in the presence of a low-fat diet.
“We suggest that future studies of psyllium in humans measure serum BA and consider roles for FXR activation in mediating impacts of this fiber,” the authors wrote.
The study was supported by the National Institutes of Health and the Crohn’s and Colitis Foundation. The authors disclosed no conflicts.