Dietary Fiber Hierarchical Specificity: the Missing Link for Predictable and Strong Shifts in Gut Bacterial Communities.

Most dietary fibers used to shape the gut microbiota present different and unpredictable responses, presumably due to the diverse microbial communities of people. Recently, we proposed that fibers can be classified in a hierarchical way where fibers of high specificity (i.e., structurally complex and utilized by a narrow group of gut bacteria) could have more similar interindividual responses than those of low specificity (i.e., structurally simple and utilized by many gut bacteria). To test this hypothesis, we evaluated microbiota fermentation of fibers tentatively classified as low (fructooligosaccharides), low-to-intermediate (type 2 resistant starch), intermediate (pectin), and high (insoluble β-1,3-glucan) specificity, utilizing fecal inoculum from distinct subjects, regarding interindividual similarity/dissimilarity in fiber responses. Individual shifts in target bacteria (as determined by linear discriminant analysis) confirmed that divergent fiber responses occur when utilizing both of the low-specificity dietary fibers, but fibers of intermediate and high specificity lead to more similar responses across subjects in support of targeted bacteria. The high-specificity insoluble β-glucan promoted a large increase of the target bacteria (from 0.3 to 16.5% average for sp. and 2.5 to 17.9% average for Bacteroides uniformis), which were associated with increases in ratios of related metabolites (butyrate and propionate, respectively) in every microbial community in which these bacteria were present. Also, high-specificity dietary fibers promoted more dramatic changes in microbial community structure than low-specificity ones relative to the initial microbial communities. In the face of interindividual variability and complexity of gut microbial communities, prediction of outcomes from a given fiber utilized by many microbes would require a sophisticated comprehension of all competitive interactions that occur in the gut. Results presented here suggest that high-specificity fibers potentially circumvent the competitive scope in the gut for fiber utilization, providing a promising path to targeted and predictable microbial shifts in different individuals. These findings are the first to indicate that fiber specificity is related to similarity and intensity of response in distinct human gut microbiota communities.