It is well-known by now that gut microbiota plays maybe the most important role in our health, helping control digestion (1), empowering our immune system (2) and brain health (3). What wasn’t exactly known till recently was the type of dietary fibre that appear to be more beneficial. We were aware that prebiotic fibre were surely helpful, but apart from that, we didn’t have a clear image of which are more helpful than others. Until this systematic review and meta-analysis (4) from Bond University in Australia, by Dr. Katrina Campbell, gave us a new perspective on how some fibre can be more beneficial than others.
More specifically, 64 randomized clinical trials with 2099 participant were included in the study analyzed by the researchers. The goal was to compare dietary fiber interventions achieved through food or supplementation with placebo/low-fiber comparators.
The outcome was what most of us can imagine: all healthy adults who received increased fiber intake presented higher abundance of Bifidobacterium και Lactobacillus which are probiotics (good bacteria) that live in our intestines and stomach, as well as higher fecal levels of butyrate compared with the placebo/low-fiber group. Other bacteria, like Faecalibacterium prausnitzii, Roseburia spp., Eubacterium rectale and Ruminococcus bromii weren’t affected in terms of adequacy by fibre intake, when the researchers compared the outcomes of the two groups.
As far as fibre type is concerned and what effect each type has on gut microbiota, there is the following categorization among accepted prebiotic fibers, candidate prebiotic fibers and general fibers –the last type are the fibres that can’t be classified neither as accepted, nor as candidate. According to the researchers, fructans and galactooligosaccharides, which were the accepted fibre type, led to greater abundance of Bifidobacterium spp. and Lactobacillus spp., while the candidate prebiotic fibres affected only Bifidobacterium. General fibres didn’t affect neither of the bacteria. In this whole process, fermentability pf fibres seems to play an important role. Furthermore, there were no changes detected in α-diversity and abundances of other bacteria apart from those two, which shows how stable can the gut microbiota be, although past studies have shown otherwise.
What must be noted is that there were differences in intervention effects in subgroup analyses depending upon trial design. Dietary fiber led to significantly lower α-diversity compared with placebo/low-fiber comparators in crossover design trials. Also, subgroup analysis separating the effects of food and supplement interventions showed that food interventions had no effect on Bifidobacterium and Lactobacillus spp. The authors think that this might be owed to the relatively low sample size studies of food interventions and also to the fact that most of the trials using food interventions, supplemented with grain and cereal foods to increase fiber intake and not a diverse range of fibrous foods.
Note that, interestingly, there were no differences in the effect of dietary fiber interventions on Bifidobacterium spp. abundance with varying doses of fiber. While this requires further clarification, it seems that lower-dose supplementation may offer an advantage in patients who experience GI symptoms with higher fiber loads and it shows that even less than 5gr of fibre may be enough for gut microbiota fermentation.
As we can understand, further research is surely required for researchers and health professionals to understand the role of individual fiber types on gut microbiota, but this meta-analysis gives us some important clues.
https://www.bmj.com/content/361/bmj.k2179
https://www.ncbi.nlm.nih.gov/pubmed/28260787
