Fibre: how much, from what, and how fast to add it
Field notes for the Gut Trail3 min readUpdated 21 September 2026
Fibre is having a moment online, which is fair, because it is the one change in the gut world with a wide base of evidence behind it. Most adults get around fifteen to eighteen grams a day. The target is closer to thirty. The gap is real, the fix is mostly food, and the way people go wrong is speed.
What fibre does
Fibre is the part of plants your gut cannot digest. Soluble fibre forms a gel that slows things down and feeds the bacteria in your large intestine; insoluble fibre adds bulk and keeps things moving. You want both, and most whole plant foods carry both.
The bacteria matter. They turn fibre into short-chain fatty acids that the lining of the gut uses for fuel. A gut that gets more fibre tends to be more regular, less bloated once it has adjusted, and steadier after meals.
How much
About thirty grams a day for adults is the common target, slightly less for women in some guidelines and a little more for men. For a sense of scale: a bowl of porridge is about four grams, an apple with its skin about four, a serving of lentils about eight, a slice of wholemeal bread two or three.
You do not need to count for long. Count for a week to see where you are, then work by habit: a fibre-rich breakfast, beans or lentils in one meal a day, fruit and vegetables with the skin on, wholegrain versions of what you already eat.
From what, in order
Oats, beans, lentils and chickpeas do most of the work: they are cheap, filling, and high in the soluble kind. Ground flaxseed on porridge or yoghurt adds a spoonful of both kinds and some plant omega-3 fats. Berries, pears and apples with skin, broccoli, carrots, and any wholegrain you will actually eat fill the rest.
A sourcing note on oats: most oats are conventionally grown, and the weed killer glyphosate is sometimes used on the crop before harvest. Tests have found traces in many oat products, well under the US legal limit but above a stricter benchmark set by one advocacy group, and regulators and that group disagree on what level matters. You do not have to drop oats. If oats are a daily staple, choose organic where the price allows, and rotate in other wholegrains such as barley, brown rice or buckwheat.
Psyllium husk is the one supplement with reasonably good evidence for regularity. It is a soluble, gel-forming fibre that clinical guidelines list first among fibre supplements. It is a top-up, not a replacement for food, and it needs a full glass of water every time.
How fast, and the mistake to avoid
Add about five grams a week, not thirty in a day. The bacteria that handle fibre need time to grow into the new supply; skip that and the extra fibre ferments into gas and bloating, and most people quit right there.
Drink more water as you add fibre. Fibre without water can make things slower, not faster.
Expect a week or two of adjustment. Mild extra gas early on is the bacteria catching up, and it settles.
When to see someone
Bloating or a change in your gut that has lasted more than about four weeks, or is getting worse, is a doctor's question, and so is any blood in your stool, losing weight without trying, or pain that came on suddenly and severely. The Gut Trail asks about those first, before anything about your fibre.
The specific version, for you
A two-minute trail guide for your digestion. A few questions, a focus area, and a few things worth trying. It starts with a short safety check.
Questions people ask
Do I need a fibre supplement?
Most people can reach the target with food. Psyllium is a reasonable top-up if food alone is hard, taken with plenty of water and at least two hours apart from any medication.
Why did more fibre make me more bloated?
Almost always speed. Adding a lot at once outpaces the bacteria that digest it. Drop back, then add about five grams a week.
Is bread and pasta fibre?
Wholegrain versions, yes, a little. White versions, very little. Swapping to wholegrain is one of the easiest gains.
Sources
Educational content, not medical advice. If this has lasted more than a few weeks or is getting worse, see a doctor.