Soluble and insoluble fibre: how they differ, how each type behaves in the digestive tract, which foods contain them and why your diet needs both.
Soluble and insoluble fibre are the two main groups of dietary fibre. Both come from plants, and human enzymes digest them only partly or not at all, yet they behave differently in water. Soluble fibre forms a gel or a thick mass with water, while insoluble fibre stays almost unchanged and adds bulk to stool. Most plant foods contain both types, so a varied diet is the simplest way to get enough of each.
This article draws on recommendations from the European Food Safety Authority (EFSA), the EU list of authorised health claims, information from the U.S. National Library of Medicine (MedlinePlus), Mayo Clinic and the Harvard T.H. Chan School of Public Health, and peer-reviewed research. You will find the sources at the end of the article, and they are marked with numbers in the text, for example [1].
If fibre is a new topic for you, start with our guide “Fibre: what it is and where to find it”. We explain how much you need each day in “How much fibre do you need per day?”.
What are soluble and insoluble fibre?
Dietary fibre is made up of plant carbohydrates and similar substances that human digestive enzymes do not break down in the small intestine. So they reach the large intestine, where bacteria break down some of them [1]. EFSA considers 25 g of fibre a day to be adequate for normal laxation in adults [1].
Depending on how fibre behaves in water, it is divided into two groups:
- Soluble fibre attracts water and forms a gel during digestion, which slows digestion down [2, 3].
- Insoluble fibre does not dissolve in water, adds bulk to stool and appears to help food pass more quickly through the stomach and intestines [2, 3].
Mayo Clinic points out that most fibre-rich plant foods contain both types, so there is no need to worry about the ratio: eating a variety of foods is enough [3].
Soluble fibre: what it is and how it works
Soluble fibre swells in water and becomes viscous. This gel moves more slowly through the digestive tract, so digestion slows down [2]. It is because of this property that soluble fibre is most often mentioned in connection with cholesterol, carbohydrate absorption and feeling full.
Soluble fibre and cholesterol
MedlinePlus states that soluble fibre is linked to lower cholesterol [2], and Harvard researchers write that it can help lower blood glucose and cholesterol [4]. One of the best-studied examples is beta-glucan from oats and barley. The EU allows the claim that beta-glucans contribute to the maintenance of normal blood cholesterol levels, but only when 3 g of beta-glucan from oats or barley is eaten per day [5]. This shows that the effect depends on the specific fibre and its amount, not on the word “soluble”.
If you have been diagnosed with high cholesterol, diet matters, but it does not replace the treatment your doctor has prescribed. Never stop taking medicines without talking to your doctor.
Soluble fibre and carbohydrate absorption
Because the gel slows digestion, soluble fibre can slow the absorption of carbohydrates. Research links this to steadier blood glucose after meals [4, 6]. Researchers stress that this effect is typical of fibres that form a viscous gel, such as psyllium and beta-glucans [6].
Fibre and feeling full
Fibre-rich food is usually more filling, because it takes longer to chew and soluble fibre takes up volume in the stomach. Harvard researchers write that fibre helps keep hunger and blood sugar in check [4]. This does not mean fibre changes body weight on its own: what matters most is your overall diet and lifestyle.
Fermentation and gut bacteria
Some soluble fibres, such as inulin, are fermented by bacteria in the large intestine. This is a natural process that also produces gas [6]. So if you increase these fibres suddenly, you may get bloating or wind [3, 7]. We write more about inulin in “Inulin: what it is”.
Where to find soluble fibre
Soluble fibre is plentiful in oat bran, oats, barley, beans, lentils, peas, nuts and seeds, and in some fruit and vegetables, such as apples, citrus fruit, bananas and carrots [2, 3]. One of the most popular soluble fibres in supplements is psyllium [2].
Insoluble fibre: what it is and how it works
Insoluble fibre does not dissolve in water and does not form a gel. It holds water in the bowel, adds bulk to stool and helps food move through the digestive tract [2, 3]. Harvard researchers link it to more regular bowel movements [4].
Insoluble fibre and bowel movements
The best-studied example is wheat bran. The EU allows the claims that wheat bran fibre contributes to an increase in faecal bulk, and that it contributes to an acceleration of intestinal transit when at least 10 g of wheat bran fibre is eaten per day [5]. Research shows that particle size also matters: coarse bran particles act differently from finely ground ones [6].
Can you have too much insoluble fibre?
Increasing any kind of fibre suddenly can cause bloating, wind or cramps [3, 4]. Fibre draws water into the bowel, so if you drink too little you may run short of fluids [7]. That is why it is best to increase fibre gradually over a few weeks and drink more fluids at the same time [3, 7]. We write more about this in “Bloating: common causes and everyday habits”.
Where to find insoluble fibre
Insoluble fibre is plentiful in wheat bran, wholemeal flour and bread, brown rice, quinoa, nuts, beans, cauliflower, green beans, potatoes, leafy greens and fruit eaten with the skin, such as pears and apples [2, 3, 4].
Soluble and insoluble fibre: comparison table
| Feature | Soluble fibre | Insoluble fibre |
|---|---|---|
| What happens in water | Attracts water and forms a gel or thick mass | Does not dissolve, holds water, does not form a gel |
| Where it is found | Pulses, oats and barley, the flesh of fruit, some vegetables | Cereal bran, wholegrain products, vegetables, fruit skins, nuts |
| How it works | Slows digestion; some types are linked to normal cholesterol levels and steadier blood glucose after meals [2, 4, 5] | Adds bulk to stool, helps food move through the digestive tract [2, 3, 5] |
| Food examples | Oat bran, oats, barley, lentils, beans, peas, apples, citrus fruit, carrots, psyllium | Wheat bran, wholemeal bread, brown rice, cauliflower, green beans, potatoes, pears with the skin |
| What to keep in mind | Some of it is fermented by bacteria, which can produce gas [6] | Needs enough fluid [7] |
Why does your diet need both soluble and insoluble fibre?
The two types of fibre do different jobs, so they complement each other. However, researchers note that splitting fibre into soluble and insoluble does not tell the whole story. It also matters whether a fibre forms a viscous gel, how much water it holds and how much of it gut bacteria ferment [6]. For example, psyllium and inulin are both soluble fibres, but psyllium forms a gel and inulin does not [6].
So the most practical approach is not to count percentages, but to eat a variety of plant foods. Mayo Clinic and Harvard researchers advise getting fibre from food first, because food also provides vitamins, minerals and other useful nutrients [3, 4].
How to get more soluble and insoluble fibre every day
- Choose oats for breakfast. Porridge with an apple or berries gives you both types of fibre.
- Swap white bread for wholegrain. Wholewheat products, especially bran, are rich in insoluble fibre [4].
- Eat pulses several times a week. Lentils, beans and chickpeas contain both types of fibre [2, 3].
- Eat fruit with the skin on. Edible skins are rich in insoluble fibre [4].
- Add vegetables to every meal. The NHS advises eating a variety of fruit and vegetables every day [8].
- Increase fibre gradually and drink water. This makes it easier for your gut to adjust [3, 7].
Important: this research is about fibre and nutrition in general. It does not describe the effect of any fruttberry product and does not replace medical advice.
When should you see a doctor?
Fibre is part of your diet, not a treatment. If you have been diagnosed with high cholesterol, diabetes or a digestive disease, discuss changes to your diet with your doctor and do not stop prescribed medicines. The NHS lists signs that should always be checked by a doctor [9]:
| Sign | What to do |
|---|---|
| Blood in your stool | See your GP |
| Unusual changes in bowel habits that continue, such as going more often or looser stools | See your GP |
| Losing weight without trying | See your GP |
| Persistent tummy pain or a lump in your tummy | See your GP |
| Ongoing tiredness without a clear reason | See your GP |
Also talk to your doctor if constipation keeps coming back even though you eat enough fibre and drink enough water.
What is in the fruttberry fibre blend?
Many people only manage to get part of the recommended amount of fibre from food. In that case, a fibre blend can help top up your diet. We created fruttberry out of a personal need: we wanted a fibre blend that would be pleasant to drink every day.
Ingredients
- Psyllium husk is the base of the blend.
- Agave inulin is a soluble fibre that is fermented in the gut.
- Freeze-dried strawberries (12%) and blackcurrants (8%) provide taste and colour from real berries, not from flavourings.
- Glucomannan from konjac tubers is another soluble fibre. We write more about it in “Glucomannan: what it is”.
How much fibre is in one teaspoon?
100 g of the blend contains 77 g of fibre. In an independent test by the KTU Food Institute, a sample of our blend contained as much as 84.4 g of fibre per 100 g. One teaspoon (5 g) provides about 3.86 g of fibre and only 11 kcal. The blend contains no added sugar, gluten, lactose or preservatives, and it is suitable for vegans.
Stir one teaspoon into 200–250 ml of water and drink it straight away, then increase the amount every 3–4 days. Psyllium and glucomannan swell, so always take the blend with enough water: taking it with too little fluid carries a risk of choking. If you have trouble swallowing, do not take this kind of fibre.
For home, the 250 g jar is the most practical, and for work and travel there are 30 single servings of 5 g.
All fruttberry dietary fibre products in one place.
Frequently asked questions about soluble and insoluble fibre
What is the difference between soluble and insoluble fibre?
Soluble fibre forms a gel in water and slows digestion, while insoluble fibre does not dissolve, adds bulk to stool and helps food move through the digestive tract [2, 3].
Is “soluble fibre” the same as “water-soluble fibre”?
Yes. Both terms mean the same thing. Likewise, insoluble and water-insoluble fibre are the same.
Which foods contain both types of fibre?
Most plant foods: pulses, oats, wholegrain products, vegetables, fruit and nuts [3]. That is why a varied diet is the simplest way to get both.
Do I need to count the ratio of soluble to insoluble fibre?
No. Mayo Clinic advises simply eating a variety of fibre-rich foods [3]. The most important thing is to get enough fibre each day: EFSA gives 25 g for adults [1].
Why can fibre make you bloated?
Most often because fibre was increased too quickly or you drink too little fluid. Some soluble fibre is fermented by bacteria, which produces gas [6]. Increase fibre gradually and drink enough water [3, 7].
Sources
- EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific Opinion on Dietary Reference Values for carbohydrates and dietary fibre. EFSA Journal. 2010;8(3):1462.
- MedlinePlus, U.S. National Library of Medicine. Soluble vs. insoluble fiber. Reviewed 2026.
- Mayo Clinic. Dietary fiber: Essential for a healthy diet. Reviewed 2026.
- Harvard T.H. Chan School of Public Health. Fiber. The Nutrition Source. Reviewed 2022.
- European Commission. Commission Regulation (EU) No 432/2012 establishing a list of permitted health claims made on foods, other than those referring to the reduction of disease risk and to children's development and health. OJ L 136, 25.5.2012, p. 1–40.
- Lambeau KV, McRorie JW Jr. Fiber supplements and clinically proven health benefits: How to recognize and recommend an effective fiber therapy. Journal of the American Association of Nurse Practitioners. 2017;29(4):216–223.
- British Dietetic Association. Fibre: Food Fact Sheet. 2021.
- NHS. How to get more fibre into your diet. Reviewed 2026.
- NHS. Symptoms of bowel cancer. Reviewed 2026.
The information provided is general in nature and does not replace advice from a doctor or dietitian. The scientific data refer to fibre and nutrition in general and do not describe the properties of any specific product. If you have a long-term condition, take medicines, are pregnant or breastfeeding, talk to a healthcare professional before you start taking food supplements. A food supplement is not a substitute for a varied and balanced diet.
