Fibre (or fiber, in the US) is a complex carbohydrate (type of sugar). Most carbohydrates are broken down by the body to provide fuel in the form of glucose. However, unlike other carbohydrates, fibre cannot be digested by the human body: we lack the necessary enzymes for this process. Instead, fibre passes through our digestive systems largely unchanged until it reaches the microbes in our colons, which digest it on our behalf.

We therefore gain no energy from fibre. It provides us with no calories, sugars, protein or vitamins, because it is not digested. However, fibre or ‘roughage’ provides many important functions in the body. This is both through its action on our digestive systems, and also through the actions of our gut microbes. Read on to find out more about this essential dietary factor.

Fibre in the Diet

Fibre comes exclusively from plants and fungi (mushrooms). It is found in more or less every plant that we eat.

However, there is no fibre in meat, fish or animal products (including dairy).

The Benefits of Fibre

We have known for years that fibre was helpful to support bowel function, and to ‘keep you regular’. However, more recent studies have shown that eating fibre provides benefits far beyond that. These include:

• People who eat more fibre are less likely to be diagnosed with several serious illnesses and chronic conditions, including heart problems, several types of cancer, stroke and diabetes.

• People who eat more dietary fibre tend to have lower blood pressure, cholesterol and body weight. These all help to reduce your risk of heart disease.

• High fibre foods help to fill you up. Fibre is bulky and takes up space in our stomachs, making us feel full and leaving less room for other foods containing calories. This can be helpful if you are trying to reduce your calorie intake and lose or maintain weight.

• Foods that are high in fibre keep you feeling fuller for longer. It takes the body longer to process fibre and move it through the digestive system. This means your stomach stays fuller for longer after eating high fibre foods.

• Fibre also improves your body’s control over your blood sugar and blood lipids, meaning that people who eat more fibre are less likely to have sugar ‘spikes’ and lows, and high circulating blood lipids—all of which are generally bad for your health.

• Fibre seems to have a role in your immune system, and therefore your general health, ability to resist infections, and allergic reactions.

• Fibre is well known for sustaining regular bowel movements and helping to prevent constipation, because it stimulates the digestive system.

How Fibre Works

If fibre simply passes through your body, how does it have so many beneficial effects?

The answer is our gut bacteria, or microbiome.

Fibre moves through the stomach and small intestine without being changed, because we lack the enzymes needed to digest it. However, when it reaches our colons, it comes into contact with tens of thousands of bacteria that do, indeed, have the necessary enzymes for its digestion.

However, these bacteria do not just digest the fibre (as in, break it down into its smallest molecules). Instead, they transform it into short-chain fatty acids, and particularly butyrate.

Butyrate is valuable because it has strong anti-inflammatory properties. It also supports the growth of ‘good’ microbes in your gut, and suppresses ‘bad’ ones. It repairs the barriers and membranes in the gut, contributing directly to a healthier digestive system. However, butyrate also spreads throughout the body, providing these anti-inflammatory and healing effects elsewhere too.

This explains why more fibre is better. More fibre equals more butyrate produced, equals more anti-inflammatory molecules in the body, equals faster healing of any damage.

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Types of Fibre

There are two main types of fibre, soluble and insoluble. Both are beneficial to health.

Soluble Fibre

Soluble fibre dissolves in the stomach creating a sticky gel-like substance, almost like a type of glue. This ‘glue’ traps certain components of food, fats and sugars, making them more difficult for the body to absorb.

This means that sugars (carbohydrates) are absorbed more slowly and blood sugar levels are kept steadier for longer. Foods high in fibre tend to have lower glycaemic index (GI) scores, meaning that sugars in them are released more slowly. The speed of release varies between individuals, but the GI index provides a reasonable guide.

See the section about Glycaemic Index (GI) on our Carbohydrates page for more information.

When soluble fibre dissolves, it can also bind to certain fats in our stomachs. People who have high fibre diets are less likely to suffer from high cholesterol. Fibre can bind to and absorb cholesterol in the intestine before it can enter the bloodstream. This is especially the case for low-density lipoproteins (LDL), the form of cholesterol that, at high levels, can lead to serious health problems.

Foods that contain good levels of soluble fibre include:

• Oats and barley
• Most beans and peas
• Whole grains – as found in some cereals and brown bread
• Nuts and seeds
• Fruit and vegetables

Insoluble Fibre

Insoluble fibre does not dissolve in water or your stomach.

Instead, it absorbs water and increases in size. As insoluble fibre passes through your digestive system, it provides bulk and moisture to stools, and has a natural laxative effect. It therefore reduces the symptoms of constipation. Bulkier stools are also helpful in cleaning the wall of the intestine, removing wastes and promoting a healthy colon.

Foods that contain good levels of insoluble fibre include:

• Whole grain wheat, because it includes bran (see below)
• Corn (including popcorn)
• Oats and oat bran
• Nuts
• Fruit and vegetables (especially the skins).

Bran

Bran is the hard outer layer of cereals such as wheat, barley, oats and rice.

As the name indicates, ‘whole grain’ foods contain the whole grain, including the bran. Processed foods often include grains that have had the outer casing (bran) removed. Such foods, like white bread, have less nutritional value than whole-wheat or wholemeal alternatives because they contain less fibre. To increase your fibre, choose brown, granary or seed breads, brown rice and brown pasta.

Bran is also known to contain many essential fatty acids that are vital to health and can aid digestion.

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In Summary

Fibre plays an important part in a healthy diet—and in keeping you healthy.

We gain no energy or nutrients directly from fibre, because we cannot digest it ourselves. However, our gut bacteria can digest it for us, and this helps to keep our digestive systems healthy. It also has other beneficial effects, such as reducing the risk of heart disease and certain cancers. High fibre foods also make us feel fuller for longer and reduce the ‘spikes’ from eating carbohydrates, improving our general health.

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