Dietary Fibre and the Microbiome
It is well known that fibre, in appropriate quantities, is an important part of a healthy diet. There are different types of fibre, each benefiting the body in different ways. Soluble fibre mixes with water in the intestine to form a gel-like substance, which acts as a trap to collect certain body wastes and then move them out of the body. Insoluble fibre absorbs and holds water, producing uniform stool and helping to push gut contents through the digestive system quickly. Insoluble fibre in appropriate amounts and with additional fluid intake can promote bowel regularity and improve constipation.
The Consortium for Spinal Cord Medicine (2020) recommends an initial diet with no less than 15 grams of fibre daily, and the Multidisciplinary Association of Spinal Cord Injury Professionals (MASCIP) (2009) group recommends an average intake of 18 grams, however, they acknowledge that adjustments should be made if problems arise with stool consistency. It is currently not recommended to uniformly place individuals with SCI on high fibre diets due to individual differences and tolerances (Consortium for Spinal Cord Medicine, 2020).
Discussion
Though many people with SCI report that adjusting their diet improves bowel function (Coggrave et al. 2006b), there is little evidence to support this in general. Cameron et al. (1996) looked at increasing dietary fibre and found that this does not have the same effect in people with SCI as has been previously demonstrated in people without neurogenic bowel dysfunction. Rather, the effect may be the opposite of the desired result. Therefore, adding more fibre alone does not improve bowel function; for individuals with low fibre intake and constipation, fibre in the diet may be increased gradually and the effect on bowel function carefully observed. More evidence is required to assess the effectiveness of adding fibre to the diet of individuals with SCI.
Kim et al. (2016a) in a pre-post study, investigated the effect of the dried immature fruit of Poncirus trifoliata, known as Poncirus fructus which is widely used as a traditional medicine in Eastern Asia for the treatment of gastrointestinal disorders but had only been studied in animal models or participants with intact gastrointestinal tracts. Although statistically significant improvements in 25 individuals with SCI were noted in constipation, Bristol stool scale, CTT and stool retention, 28% of participants had adverse events particularly loose stool or diarrhea. The authors note that the mechanism of action, extraction methods and dosing requirements are neither clear nor standardized.
There is evidence that suggests people with SCI have different microbiome diversity and structural composition compared to people without SCI (Faber et al. 2021; Gungor et al. 2016; Zhang et al. 2018a). Specifically, Gungor et al. (2016)reported that butyrate-producing bacteria are reduced in people with SCI and that bacteria genera significantly differ between UMN and LMN groups. Furthermore, Yu et al. (2021) reported that people with complete SCI had larger reductions and composition variation than those with incomplete SCI in comparison to people without SCI. However, another study reports there are no significant differences in diversity, but rather structure and quantity (p<0.05) (Lin et al. 2020; N=46).
Conclusion
There is level 2 evidence (Gungor et al. 2016; Zhang et al. 2018a), level 3 evidence (Yu et al. 2021) and one systematic review (Faber et al. 2021; N=14 studies) that indicate people with SCI have reduced diversity of gut microbiota compared to people without SCI. The systematic review reports that individuals with complete thoracic SCI exhibit larger composition variation and reductions in microbiome diversity from healthy individuals than those who have incomplete thoracic SCI.
There is level 2 evidence (Lin et al. 2020) that people with SCI show microbiome dysbiosis due to the structure and quantity of microbiota, whereas alpha diversity did not significantly differ in comparison to healthy individuals.
There is level 4 evidence from one pre-post test (Kim et al. 2016a) that dosages of the aqueous extract Poncirus fructus (PF) may improve stool retention, colon motility and constipation symptoms of neurogenic bowel.
There is level 4 evidence (from one case series; Cameron et al. 1996) that indicates high fibre diets may lengthen colonic transit time in individuals with SCI.
