Dietary Fibre: What It Is, Why You Need It, and How to Get More

What is dietary fibre?

Dietary fibre is a collective term for a range of carbohydrate-like compounds found in plant foods that human digestive enzymes cannot fully break down. Unlike proteins, fats, and most carbohydrates, fibre is not absorbed in the small intestine. Instead, it travels to the large intestine, where it either ferments (is broken down by gut bacteria) or adds bulk to the contents of the bowel.

Fibre is found only in plant-based foods. Fruits, vegetables, whole grains, pulses, nuts, and seeds. Animal-derived foods (meat, fish, poultry, eggs, plain dairy) contain no dietary fibre at all.

The fibre category is broad. It includes:

• Cellulose. The structural material in plant cell walls; insoluble, passes largely intact through the gut.
• Hemicellulose. Found in the cell walls of grains and vegetables; partially fermentable.
• Pectin. Found in the flesh of fruit and some vegetables; soluble, forms a gel in the gut.
• Beta-glucan. Found in oats and barley; soluble, strongly associated with cholesterol reduction.
• Inulin and fructo-oligosaccharides (FOS). Found in onions, garlic, leeks, artichokes, and wheat; fermentable, feed beneficial gut bacteria.
• Resistant starch. Starch that escapes digestion in the small intestine; found in underripe bananas, cooked-and-cooled potatoes and rice, legumes, and whole grains.
• Lignin. A woody structural compound in plant cell walls; insoluble and non-fermentable.

The two most widely used terms are soluble fibre and insoluble fibre, though modern research increasingly classifies fibre by its physical characteristics. How well it dissolves (solubility), how viscous it becomes (viscosity), and how readily gut bacteria ferment it (fermentability). These properties better predict its effects in the body.

Soluble vs insoluble fibre. What is the difference?

Resistant starch, worth knowing about separately

Resistant starch is technically a soluble fibre that behaves like a particularly powerful prebiotic. It is fermented by gut bacteria to produce large amounts of butyrate, the short-chain fatty acid most closely associated with colon health. Resistant starch content increases in foods that have been cooked and then cooled (pasta salad, cold potato, sushi rice), because the cooling process changes the starch structure.

Sources: cooked-and-cooled potatoes, cooked-and-cooled rice, cooked-and-cooled pasta, underripe bananas, lentils, chickpeas, rolled oats.

Prebiotics

Some fibres (particularly inulin, FOS, and resistant starch) selectively feed specific beneficial bacterial species in the gut and are classified as prebiotics. They are found in onions, garlic, leeks, asparagus, Jerusalem artichokes, bananas, oats, and most legumes. A diet rich in these foods promotes the growth of Bifidobacteria and Lactobacillus species, which are associated with better gut health and immune function.

The practical takeaway: You do not need to track soluble vs insoluble fibre separately. Eating a wide variety of whole plant foods (different vegetables, fruits, grains, pulses, nuts, and seeds) naturally provides a mixture of all fibre types. Variety matters more than optimising any one type. See whole foods for the broader case.

How fibre works in the body. The mechanisms

Understanding how fibre produces its effects helps to make sense of why the evidence is so consistent across so many different health outcomes.

1. Fermentation and short-chain fatty acids

When soluble and fermentable fibres reach the large intestine, the gut bacteria that live there ferment them, using them as an energy source and producing short-chain fatty acids (SCFAs) as by-products. The three primary SCFAs are:

• Butyrate. The main fuel source for the cells lining the colon (colonocytes). Butyrate maintains the integrity of the gut barrier, reduces gut permeability ("leaky gut"), has documented anti-inflammatory and anti-cancer properties in the colon, and may influence appetite regulation and insulin sensitivity.
• Propionate. Travels to the liver, where it helps regulate cholesterol synthesis and blood glucose control.
• Acetate. The most abundant SCFA; absorbed into the bloodstream and used as an energy substrate; plays a role in appetite regulation via signalling to the brain.

These SCFAs are central to most of fibre's systemic health benefits. They are why fibre's effects are not limited to the gut.

2. Cholesterol reduction

Soluble fibre, particularly beta-glucan from oats and barley, forms a viscous gel that binds to bile acids in the small intestine. Bile acids are made from cholesterol in the liver. When they are bound by fibre and excreted in faeces rather than reabsorbed, the liver draws more cholesterol from the blood to make new bile acids. This lowers circulating LDL cholesterol. High-quality randomised controlled trials have demonstrated this effect consistently. The British Dietetic Association notes that eating oat bran leads to lower blood pressure and lower total cholesterol.

3. Blood glucose regulation

Fibre, particularly the viscous gel formed by soluble fibre, slows the rate at which glucose is absorbed from a meal into the bloodstream. This flattens and extends the blood glucose response, reducing post-meal blood sugar spikes and the corresponding demand on the pancreas to produce insulin. Over time, consistently lower glycaemic responses reduce the risk of insulin resistance developing. This mechanism is directly relevant to type 2 diabetes prevention. See sugar and free sugars for more on the blood-glucose side of the picture.

4. Gut transit and bowel health

Insoluble fibre increases stool bulk and weight. A heavier, bulkier stool moves through the colon more quickly, reducing the time that the gut lining is exposed to potentially harmful metabolites and bile acids. This dilution and transit effect is one of the mechanisms proposed for fibre's protective effect against colorectal cancer.

5. Satiety and weight regulation

Fibre contributes to fullness through several routes: it slows gastric emptying (food stays in the stomach longer), it physically increases stomach volume, and the SCFA acetate signals to the brain to reduce appetite. High-fibre foods also typically require more chewing, which slows eating and gives satiety signals more time to register before overconsumption occurs.

6. Carcinogen binding

Insoluble fibre can bind to carcinogens and mutagens present in the digestive tract and carry them through to the faeces for excretion, reducing contact time between these compounds and the gut lining.

7. Microbiome diversity

Different types of fibre feed different species of gut bacteria. A diet rich in diverse plant fibres (from vegetables, fruits, grains, pulses, nuts, seeds, herbs, and spices) supports a more diverse gut microbiome. Gut microbiome diversity is increasingly understood to be a marker of gut health, with low-diversity microbiomes associated with a range of adverse outcomes including inflammatory bowel conditions, metabolic disease, and depression.

What the evidence shows. Health outcomes

A 2025 umbrella review of 33 meta-analyses covering 17,155,277 individuals (ScienceDirect, 2025) is the most comprehensive synthesis of the fibre-health evidence to date. Of 38 health outcomes assessed, 29 (76%) showed significant inverse associations with higher fibre intake. Below is a summary of the evidence by condition.

Cardiovascular disease and heart health

The evidence here is the most mature and consistent. The 2025 umbrella review found convincing evidence (the highest evidence class) for the association between higher fibre intake and lower cardiovascular disease mortality. Highly suggestive evidence was found for coronary heart disease specifically.

The cardiovascular benefits of fibre operate through multiple pathways: LDL cholesterol reduction (via bile acid binding), blood pressure reduction (linked to the SCFA propionate), improved insulin sensitivity, and reduction in systemic inflammation driven by a healthier gut microbiome.

The British Heart Foundation identifies increasing fibre intake as one of the practical steps with the strongest evidence base for cardiovascular risk reduction. A 2019 review published in BMJ Nutrition, Prevention and Health noted that only 13% of men and 4% of women in the UK meet the 30g fibre recommendation, and that the epidemiological evidence for cardiovascular benefit is sufficiently strong to justify treating the fibre gap as a public health priority.

Type 2 diabetes

Higher fibre intake (particularly from whole grains, legumes, and vegetables) is consistently associated with a significantly lower risk of developing type 2 diabetes in large prospective cohort studies. SACN's 2015 Carbohydrates and Health report identified a significant reduction in the risk of type 2 diabetes at 30g of fibre or more per day.

The mechanism is primarily the glycaemic-flattening effect of soluble fibre, improved insulin sensitivity via SCFA production, and the role of a diverse gut microbiome in metabolic regulation. For people already diagnosed with type 2 diabetes, higher fibre intake is associated with better blood glucose control.

Colorectal cancer

This is one of the best-evidenced dietary relationships for any single cancer. The World Cancer Research Fund's Third Expert Report identifies dietary fibre and wholegrain consumption as having strong evidence for protection against colorectal cancer. The 2025 umbrella review found convincing evidence for the association between higher fibre intake and lower risk of colorectal cancer.

Specific figures from the evidence: each 10g per day increase in total dietary fibre intake has been associated with approximately a 10 to 13% reduction in colorectal cancer risk in meta-analyses. The EPIC study (European Prospective Investigation into Cancer and Nutrition), which followed over 500,000 people across 10 European countries, found a significant inverse association between total dietary fibre and colorectal cancer after a mean follow-up of 11 years.

Colorectal cancer is the fourth most common cancer in the UK and the second most common cause of cancer death. The UK's average fibre deficit, around 12g per day below the 30g target, is a meaningful risk factor at a population level.

Diverticular disease

Diverticular disease, where small pouches develop in the lining of the colon, affects around half of people over 70 in the UK and can cause significant pain, bleeding, and complications. Low fibre intake is the primary dietary risk factor: a low-residue diet leads to harder stools, increased straining during bowel movements, and higher colonic pressure, which drives the development of the pouches. The 2025 umbrella review found convincing evidence for the association between higher fibre intake and lower risk of diverticular disease.

All-cause mortality

The 2025 umbrella review found highly suggestive evidence for an inverse association between fibre intake and all-cause mortality, meaning that people who eat more dietary fibre tend to live longer across a range of populations and study designs. This effect is likely mediated through the accumulated impact of fibre's benefits across cardiovascular, metabolic, and cancer outcomes.

Pancreatic cancer

The 2025 umbrella review found convincing evidence for an inverse association between dietary fibre intake and pancreatic cancer, one of the most difficult cancers to detect early and treat effectively. The mechanism is less well established than for colorectal cancer, but likely involves fibre's role in insulin sensitivity and blood glucose regulation (high insulin levels are a risk factor for pancreatic cancer).

Weight management

Fibre-rich diets are consistently associated with lower body weight and lower BMI in observational studies, and fibre supplementation has been shown to reduce calorie intake and promote weight loss in controlled trials. The mechanisms are well understood: fibre increases satiety per calorie, slows digestion, and reduces the glycaemic response to meals. High-fibre whole foods are also typically lower in energy density than the ultra-processed alternatives that dominate UK diets.

Mental health

The fibre-mental health connection is an indirect but increasingly researched pathway, operating primarily through the gut-brain axis. The SCFAs produced by bacterial fermentation of fibre (particularly butyrate) influence the central nervous system through multiple routes, including reduction of neuroinflammation, production of neurotransmitter precursors, and regulation of the vagus nerve. Higher dietary fibre intake has been associated with lower rates of depression and better cognitive function in several large observational studies. This is an active research area; the mechanistic evidence is promising but causation in humans is not yet firmly established.

Gut health and bowel regularity

Fibre's most immediate and widely experienced effect is on bowel function. Constipation is estimated to affect around 20% of UK adults, and low fibre intake is the leading dietary cause. Both soluble fibre (which softens stool) and insoluble fibre (which bulks it) contribute to comfortable, regular bowel movements. Increasing fibre intake is one of the first recommendations from both the NHS and the British Dietetic Association for anyone with constipation.

Note on IBS: for people with irritable bowel syndrome, fibre is not universally beneficial. Highly fermentable fibres (including some legumes, onions, and garlic) can worsen bloating and gas in people with IBS. Soluble fibre from oats, psyllium, and cooked carrots is generally better tolerated. Anyone with IBS who wants to increase fibre intake should do so gradually and may benefit from guidance from a registered dietitian.

How much fibre are UK adults actually getting?

The gap between the 30g recommendation and UK reality is stark, and it has not improved despite a decade of public health campaigns and industry reformulation.

• Adults: average intake around 18 to 20g per day. The BDA reports 18g, the British Nutrition Foundation reports 20g, reflecting different survey periods. Either way, well below 30g.
• Only 4% of UK adults meet the 30g recommendation (Food & Drink Federation, citing NDNS data).
• Only 4% of children aged 11 to 18 meet age-appropriate fibre recommendations.
• Older adults: mean dietary fibre intake of around 18.3g per day in NDNS data covering 65 to 96 year-olds; only 5.7% met the 30g target.
• The shortfall has been essentially stable since SACN raised the recommendation from 24g to 30g in 2015. A decade of minimal change.

The Food Foundation's analysis of NDNS 2019 to 2023 data identified that cereals and cereal products are actually the largest contributor to fibre in UK diets, providing between 29% and 40% of total fibre intake depending on age group. This reflects the dominance of bread and breakfast cereals in UK eating patterns, and underlines why the type of grain matters. Wholegrain products deliver substantially more fibre than refined equivalents.

Why the gap is not closing

The fibre gap in the UK is primarily a consequence of:

1. Insufficient fruit and vegetable intake. Only 9% of 11 to 18 year-olds meet the five-a-day target.
2. High consumption of ultra-processed foods, which are low in fibre.
3. Preference for refined grains (white bread, white rice, white pasta) over wholegrain equivalents.
4. Low consumption of pulses. Lentils, beans, and chickpeas remain significantly underused in UK cooking despite being cheap, versatile, and among the highest-fibre foods available.

Building towards 30g. A practical day plan

One common response to hearing the 30g target is that it sounds impossible. It is not, but it requires intentional choices across the day rather than relying on one high-fibre food to do all the work. Here is an example of how 30g+ can be reached on an ordinary day using foods available in any UK supermarket:

The day plan above uses ordinary, inexpensive foods. It does not require specialist products, supplements, or dramatic cooking changes. The key moves are: porridge instead of boxed cereal, wholemeal instead of white bread, a tin of lentils or chickpeas in the evening meal, a piece of fruit as a snack.

How to increase fibre. Practical steps

1. Switch grains from white to whole. Wholemeal bread, wholemeal pasta, and brown rice deliver roughly double the fibre of their white equivalents, with little difference in cooking time or taste once you adjust. This single swap, applied consistently, closes a meaningful share of the daily gap.
2. Make pulses a regular feature. Lentils, chickpeas, kidney beans, and split peas are the most fibre-dense practical foods available at UK supermarkets, and they are cheap. A tin of lentils or chickpeas costs around 50 to 70p and provides a full day's contribution to the fibre target. Adding a tin to soups, curries, stews, and salads requires no additional cooking.
3. Eat the skin. The skin of potatoes, apples, pears, courgettes, and carrots contains a significant share of the total fibre. Leaving it on when washing and cooking requires no extra work and meaningfully increases the fibre content of the meal.
4. Start breakfast with oats. Plain rolled oats are one of the most fibre-dense common breakfast foods, particularly for soluble fibre. A 40g portion of dry oats provides around 4g of fibre, including beta-glucan, the specific type with the strongest evidence for LDL cholesterol reduction. Adding fruit and seeds to oats can take a single breakfast to 10 to 15g of fibre.
5. Add seeds. Chia seeds and ground flaxseeds are among the most concentrated fibre sources per gram available. A tablespoon of chia seeds (around 15g) adds 5g of fibre with almost no taste impact. They can be stirred into porridge, yogurt, soup, or smoothies. Ground flaxseeds can be added to bread, baked goods, or sprinkled on salads.
6. Snack on nuts and fruit. A handful of almonds plus a piece of whole fruit provides 5 to 8g of fibre and is nutritionally far superior to almost any packaged snack alternative.
7. Include vegetables at every meal. Frozen vegetables (peas, sweetcorn, edamame, spinach, broccoli) are NOVA Group 1 whole foods, cheap, quick, nutritionally excellent, and meaningfully fibrous. Keeping a stocked freezer removes the main practical barrier.
8. Increase gradually. This is important. If you currently eat around 15 to 18g of fibre per day and you try to jump to 30g overnight, you are likely to experience bloating, gas, and discomfort as your gut bacteria adjust. Increase fibre by around 3 to 5g per week. Drink plenty of water. Fibre draws water into the bowel, and increasing fibre without increasing fluid intake can worsen constipation rather than help it. Most people find that after two to four weeks their gut adjusts and the symptoms settle.

What about fibre supplements?

Fibre supplements (including psyllium husk, sold as Fibogel in the UK, methylcellulose, inulin, and various bran products) can be useful for people who struggle to meet their fibre needs through food, or who need relief from constipation. Psyllium husk in particular has a reasonable evidence base for LDL cholesterol reduction and bowel regularity.

However, supplements do not replicate the full benefit of dietary fibre from whole foods. They provide isolated fibre types without the accompanying vitamins, minerals, phytonutrients, and the variety of different fibre types that feed a diverse microbiome. They are a useful bridge or supplement (in the literal sense) to a diet, not a replacement for the food sources.

The consistent message from SACN, the NHS, the British Dietetic Association, and the British Nutrition Foundation is to close the fibre gap through food first. The practical food changes above are achievable, affordable, and come with nutritional co-benefits that supplements cannot match.

Key statistics at a glance