Types of fibre

Soluble vs. insoluble fibre

Fibre is often classified by its solubility in water.

• Soluble fibre dissolves in water to form a gel‑like substance.  It slows the movement of food through the gut, helps lower blood cholesterol and glucose levels, promotes satiety and can serve as a substrate for beneficial gut microbes.  Good sources include oats, barley, legumes, fruit (apples, bananas, citrus), psyllium and some vegetables.

• Insoluble fibre does not dissolve in water and passes through the digestive tract largely unchanged.  It adds bulk to stool, stimulates intestinal motility and helps prevent constipation.  Sources include whole‑grain products (wheat bran, whole‑wheat bread and pasta), nuts, seeds and fruits and vegetables with edible skins.

Most plant foods contain both types, but relative amounts vary.  Consuming a variety of fibre‑rich foods ensures a balance of soluble and insoluble fibre.  Emerging guidance suggests aiming for roughly a 2:1 ratio of insoluble to soluble fibre, e.g., 20 g insoluble and 10 g soluble if targeting 30 g per day.

Further classifications

Beyond solubility, fibres can also be described by their viscosity (ability to form gels) and fermentability (whether gut bacteria break them down).  Viscous fibres such as beta‑glucans and pectins form gels that slow gastric emptying and can help lower cholesterol and glucose.  Fermentable fibres (e.g., inulin, resistant starch, beta‑glucans) are metabolised by gut bacteria into short‑chain fatty acids, supporting microbiome diversity and producing anti‑inflammatory effects.  Non‑fermentable fibres (e.g., cellulose, lignin) pass through the colon intact and increase stool bulk .

There are also dietary fibres, which occur naturally in plant foods, and functional fibres, which are extracted or synthesised and added to foods or supplements.  Examples of dietary fibres include cellulose, hemicellulose, lignin, beta‑glucans and pectin, while functional fibres include psyllium, polydextrose and modified inulin.

How much fibre do we need?

UK fibre recommendation (SACN/NHS)

• Adults (16–18+): 30 g/day .

• Children: 2–5 yrs: 15 g, 5–11 yrs: 20 g, 11–16 yrs: 25 g.

• Pregnancy & breastfeeding: same adult target (30 g/day).

Sources of dietary fibre

Fibre comes exclusively from plants.  The table below highlights common foods and the approximate amount of fibre per standard serving.  Values are general estimates; actual content varies by variety and preparation.

Whole fruits and vegetables, legumes (beans, lentils, chickpeas), nuts, seeds and whole grains are the richest sources.  Processing (e.g., milling grain or peeling fruit) removes fibre; therefore, choose foods in their whole form when possible .  Fibre supplements and fortified foods can help if diet alone is insufficient, but they lack the full range of micronutrients found in whole foods.

Physiological effects of fibre

Digestive health

Fibre adds bulk and softness to stool.  Insoluble fibres increase stool weight and speed up transit through the colon, helping to prevent constipation .  Soluble fibres absorb water, forming gels that may ease diarrhea by solidifying loose stools or relieve constipation by softening hard stools.  Eating enough fibre helps maintain bowel regularity and reduces the risk of haemorrhoids and diverticulitis.  Some fibres (e.g., psyllium) are used therapeutically to treat constipation.

Cardiovascular health

Soluble fibres such as beta‑glucans, pectins and guar gum can bind bile acids and reduce cholesterol absorption.  Meta‑analyses of controlled trials show modest reductions in total and LDL cholesterol when soluble fibres are consumed .  High‑fibre diets are linked to lower blood pressure, lower levels of inflammation and reduced risk of heart disease .  Observational cohorts consistently show that higher intakes of cereal fibres (e.g., oats, barley, wheat bran) are associated with lower incidence of cardiovascular disease and mortality.

Blood sugar regulation and diabetes

Fibre slows digestion and the absorption of carbohydrates, leading to smaller post‑meal rises in blood glucose.  This mechanism is particularly important for people with or at risk of diabetes.  Soluble fibre forms gels that delay gastric emptying and carbohydrate absorption, helping to stabilise blood sugar levels.  High‑fibre diets (especially those rich in cereal fibres) are associated with a lower risk of type 2 diabetes.  In people with diabetes, increasing fibre intake improves glycaemic control and may reduce the need for medication.

Weight management and satiety

High‑fibre foods are generally more filling and less energy‑dense than low‑fibre foods.  Fibre increases chewing time, triggers satiety signals and slows gastric emptying, which can reduce total energy intake .  Epidemiological studies suggest that higher fibre intake is associated with lower body weight and a lower risk of obesity.  Viscous soluble fibres (e.g., beta‑glucans and pectins) appear to enhance satiety and may help with weight management.

Gut microbiome and inflammation

Fermentable fibres serve as substrates for gut bacteria, producing short‑chain fatty acids (SCFAs) such as acetate, propionate and butyrate.  SCFAs nourish colonocytes, lower colonic pH, enhance mineral absorption and exhibit anti‑inflammatory effects.  A diverse, fibre‑rich diet supports a healthy gut microbiome, which may in turn modulate immune function and reduce chronic inflammation.  Preliminary research links adequate fibre intake with reduced risk of dementia and improved immune responses.

Cancer and longevity

High fibre intake is linked with a lower risk of colorectal cancer and may reduce risk of breast cancer.  Large cohort studies show inverse associations between fibre consumption and all‑cause mortality; each additional 10 g/day of fibre has been associated with incremental reductions in cardiovascular mortality in some analyses.  The protective effect may be mediated through reduced inflammation, improved lipid profiles and healthier body weight.

Practical strategies to increase fibre intake

Switch to whole grains: Replace white bread, pasta and rice with whole‑wheat bread, brown rice, quinoa, barley or oats.  Choose cereals with at least 5 g of fibre per serving and read labels to ensure whole grains are listed first.

Include legumes daily: Add beans, lentils, chickpeas or peas to soups, salads, curries or dips.  Legumes provide both soluble and insoluble fibre and contribute plant protein.

Eat a variety of fruits and vegetables: Aim for at least five portions per day.  Leave edible skins on when possible; skins contain insoluble fibre.

Snack smart: Choose fresh fruit, raw vegetables, nuts, seeds or air‑popped popcorn instead of processed snacks.  A handful of nuts or dried fruit adds fibre but also calories, so be mindful of portion sizes.

Bulk up baked goods: Substitute whole‑grain flour for half or all of white flour when baking; add bran, rolled oats or ground flaxseed to muffins and breads .

Hydrate: Increase fluid intake as you increase fibre to avoid gastrointestinal discomfort.

Increase gradually: Introduce fibre slowly over a few weeks to allow the gut microbiota to adapt and to minimise bloating or gas.

Consider fibre supplements judiciously: Supplements (psyllium, methylcellulose, inulin) can help meet requirements when dietary changes are insufficient .  Consult a healthcare professional before use, especially for individuals with digestive disorders.

Conclusion

Dietary fibre, once viewed primarily as a remedy for constipation, is now recognised as an essential component of a healthy diet.  It encompasses a diverse group of plant carbohydrates with varying solubility, viscosity and fermentability.  Consuming a variety of fibre‑rich foods ensures a balance of soluble and insoluble fibres and provides fermentable substrates that nourish the gut microbiome.  Health authorities worldwide recommend intakes ranging from about 25 g to over 30 g per day, yet most people consume far less.  Elevating fibre intake through whole grains, legumes, fruits, vegetables, nuts and seeds can improve digestive function, lower cholesterol, regulate blood sugar, promote satiety, support a healthy microbiome and reduce the risk of chronic diseases including heart disease, diabetes, obesity and certain cancers.  Simple dietary shifts and gradual increases can help individuals achieve fibre goals and reap the broad health benefits of this overlooked nutrient.

Reference list:

• Geissler, C. & Powers, H. (eds) (2023) Human Nutrition. 14th edn. Oxford: Oxford University Press.

• Gropper, S.S., Smith, J.L. & Carr, T.P. (2021) Advanced Nutrition and Human Metabolism. 8th edn. Boston, MA: Cengage Learning.

• Rezende, E.S.V., Lima, G.C. and Naves, M.M.V. (2021) ‘Dietary fibers as beneficial microbiota modulators: A proposed classification by prebiotic categories’, Nutrition, 89, p. 111217. Available at: https://doi.org/10.1016/j.nut.2021.111217.

• Stephen, A.M. et al. (2017) ‘Dietary fibre in Europe: current state of knowledge on definitions, sources, recommendations, intakes and relationships to health’, Nutrition Research Reviews, 30(2), pp. 149–190. Available at: https://doi.org/10.1017/S095442241700004X.