What is dietary fibre?

The term dietary fibre describes a group of carbohydrates that are resistant to digestion in the gut and is essential for health. There are many ways to classify fibre; by its chemical structure, ability to dissolve, ability to thicken liquids once dissolved, fermentability and chain length. All these factors will influence how the fibre behaves in our gut.

Fibre is important for good bowel function and achieving an adequate fibre intake can be a challenge when following a low FODMAP diet as the diet excludes many high fibre foods. For adults, dietary fibre intake should be 30g day. It is recommended that IBS sufferers try to achieve a fibre intake of 25-30g per day. Constipation, which can be due to a low fibre content is a common side-effect of the low FODMAP diet and it may exacerbate IBS symptoms such as abdominal pain, excessive wind, bloating and/or distension.

How can I get enough fibre on a low FODMAP diet?

• Include low FODMAP vegetables with your meals and where possible keep the skins on. For example, carrot (skin on), potato (skin on) aubergines (skin on), courgette (skin on). Other higher fibre vegetables include sweet potato, turnips, and green beans but portion size is still important.
• Snack on low FODMAP fruit such as bananas, grapes, strawberries, rhubarb, kiwifruit, oranges (not juice), passionfruit and blueberries. Remember to weigh the portion sizes carefully.
• Try adding canned lentils/chickpeas/butterbean/red kidney beans/black beans to salads, stews and casseroles. A ‘green’ or low FODMAP serving is around 25-40g so they will need to be weighed. Rinsing them carefully in water before use will also help to reduce the FODMAP content.
• Snacking on nuts such as almonds, around 10 is a good source of fibre and a green FODMAP serving.
• Adding Linseeds, Chia, Pumpkin or sesame seeds to dishes is a good way to boost fibre intake. Chia seeds can also be made in delicious puddings.

The type of fibre is important

Getting a good mix of fibres is also important. The different types of fibre serve different functions in the bowel.

Soluble fibre dissolves in water, making a gel in the gut. This slows down the speed at which food passes through out gut and helps to make us feel full for longer. It works like a sponge, bringing water into the gut and helps to soften our stools making it easier for the stool to pass through the gut and so maintaining regular bowel movements.

Soluble fibre has other health benefits too. It can help lower the bad cholesterol levels, reducing the risk of heart disease. It also helps to stabilise the blood glucose levels in people with diabetes.

Insoluble fibre does not dissolve in the gut, rather it adds bulk to the stools and helps to speed up the passage of waste through the gut. This also helps to ease constipation.

Resistant starch is another source of fibre, and as the name suggests is not digested in the gut. It passes through into the large intestine where it acts as food for the bacteria living there. Unlike FODMAPs, resistant starch is fermented slowly and so the gas that is produced does not cause the distension and discomfort that FODMAPs do.

Finally, there is prebiotic fibre. Prebiotic fibre includes resistant starch and are non-digestible fibres found in food that can stimulate the growth and activity of some bacteria i.e. good bacteria.

Fermentable Fibres

For IBS sufferers the fermentability of fibre is very important in determining how well it is tolerated. Fermentable fibres are less likely to contribute to gastrointestinal discomfort in IBS. The fermentability is all about the rate and extent to which a fibre is broken down by the bacteria in our gut. FODMAPs are short chain carbohydrates whereas fibre is longer chain. Therefore, fibre is fermented at a slower rate and produces gas in a more controlled and steadier rate than FODMAPs. Foods such as oats; barley; green bananas; root vegetables; nuts; citrus fruits and legumes are good sources of fermentable fibres.


In conclusion, a good fibre intake is very important for everyone’s health. It helps to regulate our bowel movements, provides food for the gut bacteria (microbiome) and helps normalise the cholesterol and blood sugar levels. Dietary fibre comes in a variety of forms, and the different types have different effects on the body. When following a low FODMAP diet it can be hard to get a good variety and an adequate intake of fibre but it is still very important to try to achieve an intake of 25-30g per day, from the many different sources.

For individual tailored advice on how to achieve this why not contact us at Sunlight Nutrition enquiries@sunlightnutrition.co.uk or 07414 641026 and book an appointment so that we can help you.

Written by:

Dr Lynda Rigley, Registered Dietitian


BDA Food Fact Sheet Fibre
Manual of Dietetic Practice 5th Edition