The gut microbiome is currently considered to be a potential target for preventing conditions that have been associated with imbalances in gut microbial communities. In addition to medication, diet is a major modulator of gut microbiota composition, and this is explained by the way some fibers (containing microbiota-accessible carbohydrates) can be selectively utilized by commensal microbes, thus conferring a health benefit. However, studies have not yet explored which dietary fibers, apart from the subset of prebiotic fibers, may be more beneficial to gut health.
The researchers analyzed 64 randomized clinical trials involving 2099 healthy participants (of which 58 studies were included for meta-analysis). During the study, healthy adults with increased fiber intake achieved through food or supplementation (different fibers and at different doses) were compared with placebo/low-fiber comparator groups.
Between-group differences in a-diversity of fecal microbiota at the end of the interventions was the primary outcome, whereas between-group differences in abundances of common bacterial groups, including Bifidobacterium and Lactobacillus species, and in fecal short-chain fatty acids were both secondary outcomes.
Healthy adults with an increased fiber intake had a higher abundance of Bifidobacterium spp. and Lactobacillus spp., together with higher fecal butyrate levels compared with placebo group/lower-fiber consumers. The selective increase in both Bifidobacterium spp. and Lactobacillus spp. supports the well known selective use of prebiotic fibers for the gut microbiota. In contrast, dietary fiber did not affect the relative abundance of other bacteria—including Faecalibacterium prausnitzii, Roseburia spp., Eubacterium rectale and Ruminococcus bromii—compared with placebo/low-fiber groups.
Studies were also subgrouped to disentangle the effect of fiber type (accepted prebiotic fibers, candidate prebiotic fibers or general fibers) on the gut microbiota. It was shown that fructans and galactooligosaccharides (accepted prebiotic fibers) led to a significantly greater abundance of Bifidobacterium spp. and Lactobacillus spp. By way of contrast, candidate prebiotic fibers only affected Bifidobacterium genus, while general fibers (fibers not classified as accepted or candidate prebiotics) did not affect the abundance of Bifidobacterium and Lactobacillus spp. This supports the idea that the degree of fermentability is a crucial feature of the dietary fiber to be used as a substrate for the gut microbiota (for instance, although resistant starch is an insoluble fiber, it is highly fermentable with the microbiota).
Food interventions, on the other hand, had no effect on Bifidobacterium and Lactobacillus spp. when compared with supplement interventions. The authors explain that this may be due to the small sample size and the use of mostly grains and cereals as a source of fiber, which did not allow for studying the impact of other fiber-rich foods on gut microbiota composition.
Meanwhile, no differences in Bifidobacterium spp. abundance were identified based on the doses of fiber administered. This may well imply that even low doses of fiber (less than 5 g) are enough for gut microbiota fermentation to occur.
The assessed fiber interventions were not accompanied by changes in a-diversity and abundances of other bacteria apart from Bifidobacterium and Lactobacillus, and this supports the stability of gut microbiome structure and physiology. However, these data on a-diversity are in contrast with previous observational studies that have found positive correlations between ingested fiber and microbiota diversity, highlighting that the study’s design may lead to bias when looking at the way dietary fiber has an impact on gut microbiota composition.
On the whole, data from this new meta-analysis shows that different fibers have a different impact on the gut microbiome. We look forward to more, well-designed studies that assess the impact of different dietary fiber types and at different doses to get a more complete view of the effects of dietary fiber on gut microbiota composition and metabolism.
So D, Whelan K, Rossi M, et al. Dietary fiber intervention on gut microbiota composition in healthy adults: a systematic review and meta-analysis. Am J Clin Nutr. 2018; 107(6):965-83. doi: 10.1093/ajcn/nqy041.