What is Fibre?

Fibre is a type of complex carbohydrate that cannot be digested. It exists in a number of different forms and has a variety of health benefits.

Fibre: Chemistry, Structure, and Properties

Fibre consists of the remnants of plant cells, which resist breakdown and digestion after being eaten. All fibre is derived from plants and is made of chains of carbohydrates (much like starches) but arranged in a manner that cannot be broken down and absorbed. This is because humans and other mammals all lack the enzymes needed to break them down.

Fibre Types: Soluble & Insoluble Fibre

There are two main types of fibre: soluble and insoluble (which refers to whether they are soluble in water or not). They differ in their structure, their effects on the body, and the foods they are found in. Insoluble fibre does not dissolve in water, and is found in most plants and cereal grains, especially wheat bran and oats. The major type of insoluble fibre is cellulose which provides structure to plants and their cells. The main effect and benefit of insoluble fibre is that it passes through the digestive system and adds bulk and softness to stools, reducing the risk of constipation and making visits to the toilet easier. It is also partly fermented by gut bacteria. Soluble fibre dissolves in water, making a gel. Its main form – pectin – is found in in many fruits and vegetables, especially beans, seeds and oats. Soluble fibre makes its way through the digestive system but is mostly fermented by gut bacteria, meaning it does not contribute very much to softer stools.

What is prebiotic fibre?

Most fibre is fermented by the bacteria in the gut, making it a ‘prebiotic’. Prebiotics are indigestible foods that can be fermented by bacteria and contribute to the health of the host. High fibre intakes – especially insoluble fibre – can increase the number of ‘good’ bacteria in the gut, such as Lactobacilli and Bifidobacteria. These bacteria are thought to improve health by producing molecules called short-chain fatty acids (SFCAs).

What are the health benefits and recommended daily fibre intake?

A high-fibre diet provides a number of health benefits, and the goal should be to eat 30g or more of fibre each day, but the average intake in the US and UK is only around 50-60% of that.

Reduce Risk of Cardiovascular Disease

High-fibre diets can reduce the risk of cardiovascular disease by reducing the levels of low-density lipoprotein (LDL), often referred to as the ‘bad cholesterol’. This benefit primarily comes from soluble fibre, and certain foods like oats or psyllium use health claims for the lowering of blood lipids (such as cholesterol).

Protect Against Colorectal (Bowel) Cancer

A high-fibre diet is also thought to protect against colorectal (bowel) cancer, which may be due to gut bacteria making short chain fatty acids when they ferment fibre. However, the research is not conclusive on fibre’s effect on cancers.

The goal should be to eat 30g or more of fibre each day, but the average intake in the US and UK is only about 50-60% of that.

Fibre Recommendations for Sport

Because fibre is predominantly found in high carbohydrate foods, it is easy to incorporate into the diet of athletes, who generally need fairly large amounts of carbohydrate to fuel training and competition. Increases in fibre can be achieved simply by replacing low-fibre carbohydrate sources such as white rice or processed cereals with brown rice, whole grain bread, pasta, or oats. Fruit can also easily be incorporated into a high carbohydrate diet, providing helpful amounts of fibre. However, sometimes the recommendation is to reduce fibre intake! Reducing fibre intake in the days before a competition can help to reduce gastro-intestinal problems. It can also help a little with reducing body weight by reducing the amount of undigested food in the system, this should only be a short-term practice used before competitions or races.

Try to choose low-fibre alternatives in the days before competition in order to reduce the amount of undigested food in your system.

Bottom Line on Fibre

Dietary fibre is an important component of our diet and on most training days we must ensure fibre intake is relatively high. However, there are also occasions (especially around competition) where reducing fibre intake for short periods can be advantageous.

What is a Carbohydrate?

What are the Main Causes of Gastrointestinal Problems?

References

Dhingra D, Michael M, Rajput H, Patil RT. Dietary fibre in foods: a review. J Food Sci Technol. 49(3):255-66, 2012
Slavin J. fibre and prebiotics: mechanisms and health benefits. Nutrients. 5(4):1417-35, 2013
Lupton J. Microbial degradation products influence colon cancer risk: the butyrate controversy. J Nutr. 134(2):479-82, 2004
Jeukendrup and Gleeson. Sport nutrition. Human Kinetics, Champaign IL, 2018

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