Fibre supplements, psyllium and prebiotics explained

Dietary fibre is the part of plant food that the body cannot digest. The BDA describes it as being broken down by bacteria in the large intestine, with soluble forms such as oats and fruit dissolving in water and insoluble forms such as wheat bran and nuts staying intact. Fibre supplements concentrate one of these into a powder, granule, capsule or biscuit.

Psyllium comes from the husk of a plantago seed and is sold in the UK as ispaghula husk, for example in Fybogel. The NHS places it among bulk-forming laxatives, supplied as plain or fruit-flavoured granules that you mix with water to make a high-fibre drink. It is available on prescription and from pharmacies and shops.

Ispaghula husk absorbs water in the bowel and swells, which makes stool bulkier and softer. The NHS advises taking it in the morning and evening, preferably after a meal, and notes that it usually takes 2 or 3 days to work.

Adequate fluid is not a detail but the point. Without enough water a bulk-forming product can make stools harder rather than softer and, rarely, can cause a blockage. The NHS says to drink plenty of fluids when taking this type of laxative. Powder should not be inhaled, as this can provoke an allergic reaction in some people.

• Mix the dose into a full glass of water and drink it promptly.

• Keep drinking fluids through the day.

• Give it several days before judging benefit.

• Ask a pharmacist how to space it from other medicines.

The NHS lists wind and bloating as the usual effects. Rarely, mild allergic reactions such as itchy eyes, nose or skin can occur. Starting low and building up reduces gas.

Anyone who has difficulty swallowing, a narrowed bowel, or who has been told they are at risk of an obstruction should not take bulk-forming fibre without medical advice, and neither should anyone with unexplained abdominal pain or vomiting. Fibre supplements also do not suit every kind of constipation, such as constipation caused by some medicines, where other treatments may be recommended.

For irritable bowel syndrome, NHS advice on the constipation-predominant type is to increase soluble fibre from oats, pulses and carrots, with laxatives such as Fybogel as a pharmacist-available option. The BDA IBS advice prefers natural sources such as wholegrains, oats, vegetables, fruit and linseeds, suggests one tablespoon a day of brown or golden linseeds with fluid, and says to avoid adding extra wheat bran.

For general constipation, the NHS suggests fibre-rich foods, fluids, activity and a regular toilet routine first, with pharmacist-recommended laxatives used for a short time only. A GP should assess constipation that persists, blood in the stool, unexplained weight loss, or a sudden change in bowel habit.

Prebiotics are fibres or starches that feed beneficial gut bacteria instead of mainly adding bulk. The BDA lists garlic, onions and asparagus as food examples, and also names resistant starch in foods such as bananas and potatoes, which bacteria ferment into short-chain fatty acids.

Prebiotic supplements are sold on the strength of this idea, but the BDA and NHS pages used for this guide do not recommend a specific product, so evidence for benefit from a given powder is not firmly established in those sources. Fermentable fibres can aggravate bloating in people with IBS, which is why a dietitian may suggest a low FODMAP trial rather than extra prebiotic.

Probiotics are separate again, being live bacteria. The BDA suggests trying one product at a time, taken daily for at least four weeks, to see whether IBS symptoms improve, and the NHS mentions a one-month trial.

Both the NHS and BDA give 30g of fibre a day as the adult aim, noting that most adults eat around 20g. The NHS suggests wholemeal cereal and bread, wholegrain pasta, brown rice, potatoes with skins, beans, lentils, fruit, nuts and seeds. The BDA lists children as needing 15g from age 2 and rising with age, to 25g in secondary school.

Supplements can make a bridge but do not bring the vitamins, minerals and variety of food. A change in diet is therefore the longer-term plan, while a supplement may help with the short term.

See a GP if constipation goes on despite treatment, if you see blood in your stool, lose weight without trying, or have a new, lasting change in bowel habit. Severe abdominal pain with vomiting and no passing of wind or stool is an emergency: use NHS 111 or 999.