In a recent study published in Medicine in Microecology, researchers reviewed how dietary fibers modulate the composition and function of gut microbiota.
Study: The intestinal microbiome: linking dietary fiber with inflammatory diseases. Image credit: Alpha Tauri 3D Graphics/Shutterstock
Diet is considered an important factor in shaping the human gut microbiome. People consume less dietary fiber due to the increase in Western diets (high in simple carbohydrates and fat and low in fiber) associated with industrialization. These diets can affect gut microbial composition and negatively affect host physiology, metabolism, and immunity.
Dietary fibers are complex polymeric carbohydrates that cannot be metabolized by enzymes encoded by the human genome and are metabolized by gut microbes through anaerobic fermentation. Epidemiological studies suggest an increased risk of chronic inflammatory diseases associated with lower dietary fiber intake. The gut microbiome regulates host metabolism and immune homeostasis.
Different dietary fibers and their metabolic products, such as short-chain fatty acids (SCFA), can profoundly affect hosts and modulate gut microbial composition. Therefore, exploring how dietary fibers regulate host microbial communities may enable targeted therapeutic interventions. As such, in the present study, the authors illustrate the effects of dietary fiber interventions on the gut microbiome and inflammatory diseases.
Dietary fibers influence the intestinal microbiota
One study reported that consumption of chicory-derived inulin over a short period increased the proportion of Anaerostipes and Bifidobacterium in healthy adults with mild constipation. Studies have found that inulin and pectin promote the growth of different microbial communities when supplied as the sole energy source to colonic microbes in vitro.
A long-term low-fiber diet can progressively decrease the microbial diversity of mice over several generations, which is irreversible even after re-establishment of a high-fiber diet.
Despite multiple studies investigating the effects of dietary fibers on gut microbial composition in animal models, there are limited studies in humans.
Rural populations and those in less developed countries consume more fiber than urban/industrial populations. A study in the United States concluded that a high-fiber, plant-based diet increased the proportion of Roseburia, Prevotella, Eubacterium, and Ruminococcus, which metabolize plant polysaccharides.
A meta-analysis reported that dietary fiber intervention increased faecal abundance of Lactobacillus and Bifidobacterium spp., but alpha diversity did not change. Consumption of vegetables and whole grains was found to increase microbial diversity in overweight or obese pregnant women.
The breakdown of dietary fiber is regulated by carbohydrate-active enzymes (CAZymes), including carbohydrate esterases (CEs) and glycoside hydrolases (GHs), polysaccharide lyases (PLs), and auxiliary activities. Microbial genes encoding CAZymes govern the ability to utilize fiber. Bacteroidetes are the most common fiber-degrading bacteria, with 18 CE, 17 PL and 269 GH. In vitro studies suggest that bacteria show different preferences for the same dietary fiber.
Effects of dietary fiber on inflammatory diseases
One study observed significantly lower C-reactive protein (CRP) levels in healthy adult men taking dietary fiber. Two Swedish studies indicated that the Mediterranean diet was associated with a reduced risk of late-onset Crohn’s disease, a subtype of inflammatory bowel disease (IBD). In contrast, poor adherence to the Mediterranean diet increased the risk by 12%.
Mediterranean diet improves symptoms and reduces risk and mortality of IBD. Dietary fibers can protect the intestinal barrier; for example, one study found that a high-fiber diet protected mice from colitis. Fiber deprivation can deplete the mucus layer and alter the intestinal wall, increasing its permeability.
High-fiber diets have been shown to reduce the secretion of proinflammatory cytokines and chemokines in patients with rheumatoid arthritis (RA). Additionally, RA patients eating a vegan diet had significantly less swollen and tender joints, pain, lower CRP levels, and erythrocyte sedimentation rate. The mean disease activity score decreased in RA patients after one week of Mediterranean diet consumption.
Clinical applications of dietary fiber
Prebiotics represent one of the key approaches to address gut microbial dysbiosis. Dietary interventions can be classified as low fiber, high fiber, or supplemental fiber. High-fiber dietary interventions significantly increase gut microbial diversity relative to supplemental fiber interventions. One study showed that a Mediterranean-inspired diet reduced inflammatory markers and normalized gut microbiota in patients with Crohn’s disease.
The International Organization for the Study of IBD recommends the intake of vegetables and fruits in patients with Crohn’s disease. According to a systematic review, patients with uncomplicated diverticulitis should follow a liberalized, high-fiber diet; however, there is limited evidence of the benefits of dietary fiber in preventing diverticulitis.
Final remarks
Taken together, numerous studies highlight the beneficial outcomes of dietary interventions with a high fiber content. Thus, dietary fiber interventions can serve as a tool to regulate the gut microbiota. Future research should focus on how personalized diets modulate host responses and the efficacy of small molecule therapies against specific microbial pathways for precision medicine.