Prebiotics and diets low in fermentable residues (low-FODMAP diet) might reduce symptoms in patients with functional gastrointestinal disorders, despite reports that some non-absorbable, fermentable meal products (prebiotics) provide substrates for colonic bacteria and thereby increase gas production. We performed a randomized, parallel, double-blind study of patients with functional gastrointestinal disorders with flatulence. We compared the effects of a prebiotic supplement (2.8 g/day Bimuno containing 1.37g B-GOS) plus a placebo (Mediterranean-type diet; prebiotic group, n=19) vs a placebo supplement (2.8 g glucose) plus a diet low in fermentable oligo-, di-, mono-saccharides and polyols (low-FODMAP group, n=21) for 4 weeks; patients were then followed for 2 weeks.