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What is Fiber?

Fiber is an important part of daily athlete nutrition, but athletes should use caution in the days before the big race.

Fiber 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.


Fibers Role in an Athletes Diet


Fiber: Chemistry, Structure, and Properties

Fiber consists of the remnants of plant cells, which resist breakdown and digestion after being eaten. All fiber 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.


Fiber Types: Soluble & Insoluble Fiber

There are two main types of fiber: 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 fiber does not dissolve in water, and is found in most plants and cereal grains, especially wheat bran and oats. The major type of insoluble fiber is cellulose which provides structure to plants and their cells. The main effect and benefit of insoluble fiber 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 fiber dissolves in water, making a gel. Its main form – pectin – is found in in many fruits and vegetables, especially beans, seeds and oats. Soluble fiber 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 fiber?

Most fiber 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 fiber intakes – especially insoluble fiber – 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 fiber intake?

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


Reduce Risk of Cardiovascular Disease

High fiber 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 fiber, 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 fiber 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 fiber. However, the research is not conclusive on fiber’s effect on cancers.


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


Fiber Recommendations for Sport

Because fiber 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 fiber can be achieved simply by replacing low fiber 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 fiber. However, sometimes the recommendation is to reduce fibre intake! Reducing fiber 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.

High fiber foods
Try to choose low fiber alternatives in the days before competition in order to reduce the amount of undigested food in your system.

Bottom Line on Fiber

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


Related Articles

What is a Carbohydrate?

What are the Main Causes of Gastrointestinal Problems?



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  • Jeukendrup and Gleeson. Sport nutrition. Human Kinetics, Champaign IL, 2018



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