Dietary fibre in irritable bowel syndrome (IBS)

Dietary fibres are a variety of non-digestible plant-based carbohydrates that are not absorbed by the small intestine and differentially impact the digestive system depending on interaction with colonic microbiota

• insoluble fibres are found in peelings of fruits and vegetables, seeds, whole grains, and wheat bran
  • insoluble fibres increase stool bulk while stimulating colonic motility and mucus production (1)
  • can contribute to common IBS symptoms such as bloating and abdominal discomfort (1)

Soluble fibre is found in psyllium (synonymous with ispaghula husk), corn fibre, calcium polycarbophil, methylcellulose, oat bran, and the flesh of fruits and vegetables:

• soluble fibre mixes with and holds water in the intestine
  • soluble fibre comes in many forms including powder, gummies, wafers, and capsules to improve palatability
• can be categorized into viscous or non-viscous, short or long-chain carbohydrates, and highly or minimally fermentable types:
  • short-chained and highly fermentable fibres (eg, oligosaccharides) can trigger IBS symptoms through bacterial byproducts, but might also have beneficial effects as prebiotics

Psyllium

• is viscous and minimally fermentable
• is an arabinoxylan polymer that resists digestion in the human upper gastrointestinal tract (2)
• holds water in the lumen of the intestine and improves colonic transit without worsening IBS symptoms, especially in those with IBS with constipation
• psyllium has been shown in randomised placebo-controlled trials to reduce IBS symptoms (2)

Based on this evidence, most international guidelines recommend soluble but not insoluble fibre for the treatment of global IBS symptoms:

• it is suggested that a fibre intake of about 25 to 35 grams per day with slow increase can help prevent gas and bloating (1)

Guidelines from the British Dietetic Associate (BDA) with respect to the dietary management of constipation in adults note (3):

Fibre supplements: Good Practice statements

• fibre supplement doses above 10 g/d are optimal for increasing the number of people with constipation who have a clinical benefit, improving stool output and reducing the severity of straining (evidence-based recommendation)
• consuming fibre supplements for a minimum duration of 4 weeks is optimal for increasing stool frequency and improving global constipation symptoms of constipation (evidence-based recommendation)
• in people with constipation who experience tolerance issues with fibre, fibre supplement intake may be increased gradually with weekly increments to avoid adverse effects, such as bloating and flatulence (expert opinion recommendation)
• when advising the use of inulin-type fructan supplements in constipation, the possibility of increased flatulence should be discussed (evidence-based recommendation)
• fibre supplements should be accompanied by additional fluid intake where clinically appropriate (expert opinion recommendation)

Reference:

1. Wang X J, Thakur E, Shapiro J. Non-pharmaceutical treatments for irritable bowel syndrome. BMJ 2024; 387 :e075777.
2. Gunn D et al. Psyllium reduces inulin-induced colonic gas production in IBS: MRI and in vitro fermentation studies. Gut. 2022 May;71(5):919-927.
3. Dimidi E, van der Schoot A, Barrett K, Farmer AD, Lomer MC, Scott SM, Whelan K. British Dietetic Association Guidelines for the Dietary Management of Chronic Constipation in Adults. Neurogastroenterol Motil. 2025 Oct 13:e70173.