Fibre: how much, from where, and what happens when it changes

Fibre is not a micronutrient in the strict sense and it belongs in this section because it is one of the most consequential dietary differences and one of the most under-consumed nutrients in most populations.

The recommendation and the reality

Recommendations sit around 25 to 30 grams a day for adults in most countries, with some bodies suggesting more.

Average intakes in many high-income countries fall well short — frequently around half the recommendation.

People eating plant-based diets typically consume considerably more, and this is one of the clearest nutritional advantages of the pattern.

What the evidence supports

Fibre has one of the more robust evidence bases in nutrition.

Large reviews have found dose-response relationships between fibre intake and reduced risk of cardiovascular disease, type 2 diabetes, colorectal cancer and all-cause mortality.

Mechanisms proposed include effects on cholesterol, on glycaemic response, on satiety, on transit time, and on the gut microbiota.

Higher intakes are associated with better outcomes across the range studied, which is unusual — many nutrients show diminishing or reversing benefit at high intakes.

The types

Soluble fibre dissolves in water and forms a gel. Slows gastric emptying, moderates glucose response, and binds bile acids, which is the mechanism behind cholesterol lowering.

Sources: oats, barley, legumes, apples, citrus, psyllium.

Insoluble fibre adds bulk and speeds transit.

Sources: wheat bran, wholegrains, vegetable skins, nuts.

Fermentable fibre and resistant starch, which gut bacteria metabolise, producing short-chain fatty acids that appear to have systemic effects.

Sources: legumes, onions, garlic, bananas, cooled cooked starches.

Most foods contain a mixture. Chasing specific types is less useful than eating a range of plant foods.

The adaptation period

The practical issue for anyone increasing intake.

A sudden large increase causes bloating, wind and discomfort. This is fermentation by gut bacteria that have not yet adapted, and it is normal rather than an intolerance.

It settles over a few weeks as the microbial community adjusts.

To reduce it:

Increase gradually, over weeks rather than overnight.

Drink more water. Fibre without adequate fluid makes constipation worse rather than better, which is the opposite of the intended effect.

Soak and cook legumes thoroughly, discarding the soaking water.

Eat legumes regularly rather than occasionally. Adaptation is what resolves it, and adaptation requires consistent exposure.

Use traditional spices paired with pulses across cuisines — cumin, asafoetida, ginger, fennel.

Cook vegetables rather than eating everything raw, particularly during the transition.

When fibre is a problem

Situations where high fibre is not the goal.

Before and during hard training, where it causes gastrointestinal distress. Reduce fibre around sessions.

Irritable bowel syndrome, where certain fermentable carbohydrates trigger symptoms in a substantial proportion of people.

Structured dietary approaches exist and should be undertaken with a dietitian, since they are restrictive and are intended to be temporary followed by systematic reintroduction.

Inflammatory bowel disease during a flare, where low-residue diets are sometimes advised.

Diverticular disease, where advice has changed — high fibre is now generally recommended for prevention, with low fibre during acute episodes.

Young children, where very high fibre can compromise energy intake and mineral absorption.

Older adults with poor appetite, for the same reason.

Anyone with a stricture or a history of obstruction, where medical advice applies.

The mineral absorption point

Phytates in high-fibre plant foods bind iron, zinc and calcium, reducing absorption.

This is real and it is manageable through preparation — soaking, sprouting, fermenting and leavening all reduce phytate content substantially.

Which is why traditional preparation methods across cultures involve exactly these steps, arrived at empirically long before anyone knew why.

Vitamin C with meals also offsets the effect for iron.

Getting to the target

Straightforward on a plant-based diet.

A portion of legumes: around eight to fifteen grams. Wholegrains through the day: several grams per portion. Vegetables and fruit: two to four grams per portion. Nuts and seeds: a few grams.

Most people eating varied plant food comfortably exceed the recommendation without trying.

The supplement question

Fibre supplements — psyllium, inulin, others — have specific uses, particularly psyllium for constipation and for cholesterol, where the evidence is reasonable.

They do not replace the range of compounds that come with whole plant foods, and for anyone eating a varied plant-based diet they are generally unnecessary.

Inulin and similar prebiotic supplements cause substantial wind in many people and are worth introducing cautiously if at all.

General information rather than individual advice. Anyone with a diagnosed gut condition should discuss fibre intake with a dietitian. Persistent change in bowel habit, blood in stool or unexplained weight loss requires medical assessment.