Prebiotics (PHGG, GOS, inulin)

Prebiotics are carbohydrates that pass undigested to the colon and are fermented there by bacteria, with the intent of increasing species such as Bifidobacterium. The common ones sold for gut symptoms are partially hydrolysed guar gum (PHGG), galacto-oligosaccharides (GOS), inulin, and fructo-oligosaccharides. The awkward fact for IBS is that GOS and fructans are themselves FODMAPs, the carbohydrates that a low FODMAP diet removes, so the same fermentation that feeds bacteria also produces gas and can provoke bloating PMID 30949662. The 2018 systematic review of prebiotics, probiotics, synbiotics, and antibiotics in IBS, which informed the ACG and BSG guidelines, described the prebiotic data as sparse PMID 30294792.

The most complete pooled analysis is a 2019 meta-analysis of 11 randomised trials in 729 adults with IBS or other functional bowel disorders. Response rates were 54% with prebiotics and 63% with placebo, no difference, and there were no differences in abdominal pain, bloating, flatulence, or quality of life. The detail that matters is the subgroup analysis: flatulence improved with doses of 6 g per day or less and with non-inulin-type prebiotics, whereas inulin-type fructans made flatulence worse. Prebiotics did raise faecal bifidobacteria, and no trial was at low risk of bias across all domains PMID 30949662.

Individual trials fit that pattern. A UK trial of 44 patients found that 3.5 g per day of a trans-galacto-oligosaccharide improved stool consistency, flatulence, bloating, and a composite symptom score, while the higher 7 g dose improved only global assessment and anxiety scores PMID 19053980. An Israeli placebo-controlled trial randomised 121 patients to 6 g of PHGG or placebo for 12 weeks; PHGG improved diary bloating and gas scores but had no effect on overall severity or quality of life, and dropout was higher in the placebo group PMID 26855665. An older open Italian trial compared 5 g of PHGG with 30 g of wheat bran in 188 patients; both improved pain and bowel habit per protocol, PHGG was better tolerated, and far more patients switched from bran to PHGG than the reverse PMID 12184518.

Points clinicians commonly raise: the effect is dose- and type-dependent, with low doses of GOS or PHGG the better-tolerated options and inulin the most likely to cause gas; a prebiotic is hard to combine with the restriction phase of a low FODMAP diet, since it adds back the carbohydrates the diet removes; changes in bacterial counts have not translated into reliable symptom change; and products are regulated as supplements, so dose and composition vary.

Talk to a clinician before starting or stopping anything.