Fiber

Why does fiber cause gas and bloating?

Updated 4 Nov 2026

Short answer

Bacteria in the large intestine ferment fiber, and this makes gas.[1] The steps are:

  1. Fiber reaches the large intestine undigested.[2]
  2. Bacteria ferment it and make carbon dioxide, hydrogen and methane.[1]
  3. Gas stretches the bowel, and the belly feels full and uncomfortable.[3][4]

The effect is strongest with highly fermentable fibers such as inulin, resistant starch and some oligosaccharides, and when intake rises too quickly.[5] People with irritable bowel syndrome feel it more. In one study, 13 of 29 people with the condition reached a set level of symptoms after 40 g of inulin. Their gas levels were similar to those of healthy people.[3]

This problem often goes away when the gut bacteria get used to the increase, so sources suggest a slow start over a few weeks.[6] One review suggests adding 2 to 3 g every few days.[5]

Sources (6)

  1. Community herbal monograph on Plantago ovata Forssk., seminis tegumentum. European Medicines Agency, HMPC, 2013. www.ema.europa.eu
  2. Scientific Opinion on Dietary Reference Values for carbohydrates and dietary fibre. European Food Safety Authority, 2010. www.efsa.europa.eu
  3. Colon Hypersensitivity to Distension, Rather Than Excessive Gas Production, Produces Carbohydrate-Related Symptoms in Individuals With Irritable Bowel Syndrome. Major G, Pritchard S, Murray K, Alappadan JP, Hoad CL, Marciani L, et al. Gastroenterology, 2017. PMID 27746233
  4. Bloating. NHS, 2026. www.nhs.uk
  5. The Role of Dietary Fiber in Health Promotion and Disease Prevention: A Practical Guide for Clinicians. Daley SF, Vadakekut ES StatPearls, 2026. PMID 32644459
  6. Fiber: MedlinePlus Medical Encyclopedia. MedlinePlus, US National Library of Medicine, 2026. medlineplus.gov

More about fiber

What is fiber?How does fiber lower cholesterol?How much fiber do you need per day?What is fiber used for?How long does fiber take to lower cholesterol?Is fiber safe?

For education; not medical advice. Talk to a doctor before you start, stop or change a supplement.