Food manufacturers seeking health benefit claims often use inulin, a storage fructan polymer from chicory root, to increase the fibre content of processed foods. However, its simple linear structure means that compared with other sources of fibre it is rapidly fermented, meaning it often causes negative gut symptoms, particularly for people with IBS.
Despite the apparent clinical benefits of a low-fermentable oligoaccharide, disaccharide, mono-saccharide and polyols (FODMAP) diet in IBS, it inevitably also reduces intake of substances which act as prebiotics, leading to a reduction in beneficial gut bacteria, which has its own potential negative consequences. Alternative approaches to control colonic fermentation in IBS are therefore being pursued.
Psyllium, a form of fiber made from the husks of the Plantago ovata seeds, has been found to improve symptoms in IBS although the exact mechanisms are unclear.
The authors of the current study recently used MRI to show that psyllium’s open network of polymers traps water in the small bowel and increases ascending and descending colon water content, creating a laxative effect. They also previously used radio-isotopic imaging to demonstrate that psyllium 3.5g three times a day substantially reduced lactulose-associated acceleration of proximal colonic transit in healthy volunteers, but limitations of the method used means the mechanisms of action was unclear.
The current study used MRI to test the hypothesis that psyllium can reduce the increase in colonic gas induced in IBS by inulin. There, the researchers hypothesise that it is rapidly fermented producing large amounts of gas which correlates with symptoms of flatulence, bloating, discomfort and pain in patients with IBS. The researchers also further investigated the potential mechanisms of action by assessing the viscosity of test substances, known to affect fibre functionality, and their in vitro gas production by microbiota derived from stool donated from the participants with IBS.
Human MRI study
This was a single-centre, four-period, four-treatment, placebocontrolled, crossover trial. IBS patients were recruited through primary care via the NIHR Clinical Research Network in England and through general advertisement.
Participants were provided with a low-fibre, low-FODMAP meal to eat at home the evening prior to and during the study day in order to standardise intake of foods likely to increase colonic gas. The test drinks contained (a) 20g inulin powder, (b) 20g psyllium powder, (c) 20g inulin powder and 20g psyllium powder and (d) 20g dextrose powder as a placebo control.
MRI scans were performed immediately after the drink and then hourly for six hours, while hydrogen breath tests and symptom scoring occurred every half hour. After 210 minutes, participants received a 338 calorie meal of rice pudding, jam and orange juice and completed a hydrogen breath test and symptom score. A washout period of at least six days was used between study days.
Subjects collected their own stool samples at home for analysis in the in vitro study.
A total of 19 participants completed the study (15 female and 4 male, aged 19-65).
The results indicate that inulin had little effect in the small bowel but was rapidly fermented in the colon causing a rise in colonic gas and breath hydrogen. The results also indicated that both rise in colonic gas and breath hydrogen after inulin could be largely inhibited by concurrent ingestion of psyllium.
Surprisingly, the in vitro study found that combining inulin and psyllium actually enhanced gas production. Thus, the effect observed in vivo was not due to any direct inhibition of colonic fermentation by psyllium. Furthermore, this study showed that the inulin and psyllium combination had the same high viscosity seen with psyllium alone which did not per se inhibit fermentation. Therefore the researchers hypothesise that "the in vivo effect is due to the increase in chyme viscosity leading to both a reduction in the rate of delivery of inulin to the colon together with restriction of mixing of colonic microbiota with the inulin bolus".
They conclude: "Our small mechanistic study has demonstrated that adding psyllium to inulin reduces gas production in patients with IBS and suggests that by choosing diets with adequate amounts of viscous fibre, patients may be able to obtain the prebiotic health benefits of high-fibre diets without exacerbating their IBS symptoms, particularly flatulence. Larger clinical trials are now indicated to confirm the clinical value of such mechanistic insights."
Gunn D, Abbas Z, Harris HC, et al
"Psyllium reduces inulin-induced colonic gas production in IBS: MRI and in vitro fermentation studies"