Fibre, gently

The mistake almost everybody makes

Somebody is constipated, reads that fibre is the answer, and adds a great deal of it in one determined afternoon. Two days later they are bloated, windy, in more discomfort than before, and convinced that fibre does not work for them.

What actually happened is that they added bulk to a system that is short of water and moving slowly, which is the one combination guaranteed to make things worse. Fibre needs fluid and it needs time. Get those two in the right order and it helps most people; get them wrong and it is actively unpleasant.

So the sequence we suggest in constipation-and-gut is fluid first, then a bit of movement, then fibre added slowly over a fortnight, then a pharmacist if it is still not resolving. Tomás has watched that order work often enough to be firm about it. The constipation guide is the companion page and covers the rest.

The two kinds, and why the difference matters here

Fibre is not one substance, and the distinction is genuinely useful rather than textbook trivia.

	Soluble fibre	Insoluble fibre
What it does	Absorbs water and forms a gel; softens stool	Adds bulk and stimulates movement through the gut
Found in	Oats, psyllium, linseed, barley, apples, pears, citrus, carrots, beans	Wheat bran, wholegrains, nuts, seeds, skins and stalks of vegetables
On these medicines	Usually the better starting point — softening without much extra bulk	Helpful for some, and the more likely of the two to cause bloating and wind on a slow gut

Most real food contains both. But the practical pattern from our circles is clear enough to state: members who started with soluble sources — oats, ground linseed, psyllium, a couple of kiwi fruit — generally got on better than members who went straight for a bowl of bran. That is member experience rather than a trial finding, and it lines up with the mechanism, which is usually a good sign.

How to add it without a miserable fortnight

The whole method, and it is dull.

• Sort the fluid first. A week of drinking properly before you add anything. Fibre without water is the problem, not the solution. See hydration and electrolytes.
• Add one thing at a time. One change, kept for four or five days, before the next. This is also the only way to learn what your own gut objects to.
• Start smaller than feels worth doing. A teaspoon of ground linseed, not three tablespoons. Half a portion of beans, not a bowl.
• Give it a fortnight before judging. Wind and gurgling in the first few days often settle as the gut adjusts.
• Keep it up. Fibre works while you are eating it, not afterwards.
• Drop back a step if it goes badly rather than abandoning the whole idea.

Members who did it this way overwhelmingly report it worked. Members who did it in one go report having decided fibre is not for them, which is a shame and usually not true.

Getting fibre in when there is no room

The real difficulty on these medicines is not knowing what fibre is. It is that the stomach has space for four mouthfuls and you also need protein in there.

What members do about that competition:

• Put it in things rather than beside them. Ground linseed or oat bran stirred into yoghurt or porridge; lentils into soup; grated vegetable into a sauce.
• Psyllium husk in liquid. Takes almost no room and does the softening job. Follow the packet, drink it promptly, and drink water after.
• Kiwi fruit and prunes. Both have a following in our circles out of proportion to their size, and both have some general evidence behind them for constipation.
• Beans and lentils, added over weeks. They bring protein as well, which resolves the competition rather than losing it — see protein without a chore.
• Skins and stalks left on where you would otherwise peel.
• Wholemeal versions of things you already eat, swapped one at a time. No new habits required.

Do not force fibre in a week when you are struggling to eat anything at all. Getting food and fluid in matters more, and eating enough when nothing appeals is the page for that fortnight.

When fibre is the wrong tool

Important, and skipped by most articles on this subject.

If you are constipated and also bloated, in pain, and passing very little, adding bulk can make things worse rather than better. That situation wants advice — from a pharmacist or clinician — before more fibre. The relevant word is impaction and it is not something to manage by eating bran.

If you have a history of bowel obstruction, bowel surgery, inflammatory bowel disease, or a narrowing anywhere in the gut, fibre advice for the general population may not apply to you at all, and some people in that position are told to eat less of it rather than more. Take your instruction from your own team. Our gastro-history circle is full of members who have had to unlearn generic advice.

If you have irritable bowel syndrome, some fibre types reliably make symptoms worse, and psyllium tends to be better tolerated than bran. A dietitian is the right person for this rather than a forum.

And a hard, distended, painful abdomen with vomiting and no wind passing is an emergency, not a fibre question. The red-flag list has it.

The honest evidence position

Fibre and constipation is a reasonably well-studied area in general — psyllium in particular has decent evidence for stool softening, and higher fibre intake is associated with better bowel function across populations. What does not exist is any trial of fibre in people taking GLP-1 medicines specifically.

That matters because the mechanism here is unusual. Constipation in this setting is partly a slow gut, partly reduced volume of food, and partly reduced fluid, and it is not obvious that advice developed for ordinary constipation transfers perfectly. Our practical experience says it mostly does, with the soluble-first caveat above. That is an observation from a few hundred people and we would rather label it than dress it up.

What is not in question: constipation on these medicines is common — it was among the frequently reported adverse events in STEP 1 (Wilding, NEJM, 2021) and across the tirzepatide programme (SURMOUNT-1, Jastreboff, NEJM, 2022) — and it responds to being taken seriously early. Members who dealt with it in week two rather than week eight consistently had an easier first few months, and several found two other symptoms improved at the same time.