FIBER - A DELICATE BALANCE FOR ATHLETES
Updated: Feb 2
By Annie Bothma, November 2022
Fibre intake is associated with many health benefits, but too much of a good thing can actually cause a lot of negative health consequences like GI upset, low-energy availability, reduced absorption and even micronutrient deficiencies. Therefore, the total and timing of dietary fibre intake for athletes becomes a delicate balance.
WHAT IS HEALTHY FOR THE GENERAL POPULATION IS NOT ALWAYS HEALTHY FOR ATHLETES
Governmental guidelines recommend that, generally, a healthy, balanced diet to the general public is one that should remain low in sodium and refined sugar and rich in fibre to reduce the risk of weight gain, developing chronic metabolic, cardiovascular diseases and cancer (Public HealthEngland, 2016).
However, these same recommendations may negatively impact the performance and health of an athlete by not meeting the metabolic and physiological demands of high-training loads and competition (Burke et al. 2019). What is “healthy” for the general public may not be healthy or appropriate for optimal performance in athletes in the context of an athlete’s current training schedule, goals and general health.
For example, a salad may be perfectly fine for inactive people who is trying to lose a bit of weight, but an athlete with a high-energy demand will not be able to meet their energy or carbohydrate requirements if they have a salad for a meal.
WHAT EXACTLY IS FIBRE?
Fibre refers to non-digestible carbohydrates that are intrinsic and intact within plant-based sources, but can also be isolated naturally from plant and animal sources or synthetically manufactured.
A fibre is determined as “good” and “excellent” if it contains 2.5g or 5g, respectively, when listed on the Nutrition Facts panel of a food product. Non-digestible carbohydrates vary in their chemical composition and physiological actions on the body and remain diverse within food supply. With nutrition labelling becoming standardized throughout the world, in 2001, the Institute of Medicine (IOM) proposed the following definitions for fibre to clear up any confusion with what can and cannot be deemed a fibre:
Dietary fibre: Non-digestible carbohydrate and lignin that are intrinsic and intact in plants.
Functional fibre: Isolated, non-digestible carbohydrates that have beneficial effects in humans.
Total fibre: The sum of dietary fibre and functional fibre.
Similar to starch, polysaccharide fibres (primary source of dietary fibres) exhibit long-chains of monosaccharide residues linked together by glycosidic bonds. Whereas starch is linked by alpha-bonds, fibre is linked by beta-bonds. This subtle difference in the chemical structure of glycosidic bonds poses a challenge for the human digestive system. The chemical composition of fibre, once digested, is left intact, with no human digestive enzymes capable of hydrolyzing beta-bonds.
Therefore, the sugars resist absorption in the small intestine with some fibre (insoluble), delivering no nutrients and next to nothing calories. The undigested fibre enters the large intestine and may be completely or partially fermented by gut microbiota; whereby a small amount contributes energy to the body (non-digestible oligosaccharides yielding a calorie value of 1-2 kcal/g) (Livesey, 1992).
This means eating high-fibre carbohydrate foods like bran muffins, cereals or wholewheat grains, will not give you the same amount of carbohydrates like low-fibre carbohydrates such as white rice, pasta or bread.
DIFFERENT TYPES OF FIBRE
Fibres are by no means an independent thing; instead, they vary in their physiochemical properties: water-holding capacity, viscosity, binding ability, bulking ability, and fermentability. However, for simplicity, fibres are broadly characterized and distinguished by their capacity to dissolve in water.
Fibres are classified as soluble or insoluble fibre.
Soluble fibers dissolve in water, whereas insoluble fibres do not. Soluble fibre bypasses the digestion process of the small intestine, undergoing fermentation by microflora in the large intestine. Soluble fibre can be further distinguished by its viscosity, with some forming agelatinous, viscous substance whereas others remain non-viscous. Viscous refers to its fibre thickening effects and increased resistance transit through the digestive tract.
SOURCES OF SOLUBLE FIBRE INCLUDE:
Viscous fibres increase the transit time through the human digestive tract, causing a delay in gastric emptying, resulting in a sensation of fullness (Slavin & Green, 2007). A delay in gastric emptying may also reduce a rise in blood glucose after a meal and potentially have beneficial effects on insulin sensitivity (Slavin, 2013). Furthermore, viscous fibres can impinge on the absorption of dietary fat and cholesterol, along with the hepatic metabolism of cholesterol and bile acids, leading to a reduction in plasma cholesterol concentrations (Samaan, 2017).
Insoluble fibres do not form gel-like substances, but they can act as a sponge for water. Insoluble fibres are less prone to fermentation in the large intestine compared to soluble fibres. Insoluble fibres are distinguished by their low density and high porosity. These properties increase fecal bulk, the excretion of bile acids and softening of stool, reducing the transit time through the intestinal tract. The fermentable properties indicates the extent at which dietary fibre is degraded by gut microbiota within the colon into short-chain fatty acids.
SOURCES OF INSOLUBLE FIBRE:
Whole-grains and Whole-wheat flour
Cereals such as Fiber One and All-Bran
Wheat bran rusks
Vegetables, such as cauliflower, green beans and potatoes
HOW MUCH FIBRE DO YOU NEED PER DAY?
Fibre is commonly regarded as an overlooked nutrient, due to no deficiency state being determined. At present, the Adequate Intake (AI) level for dietary fibre of 14g of fibre per 1000 calories of energy consumed or 25 g for adult women and 38 g for adult men is based on the median amount shown to achieve the lowest risk to coronary heart disease (Dahl & Stewart, 2015).
As athletes, if you consume mostly homemade whole-foods instead of packaged and highly processed take-outs you will likely meet your daily fibre requirements. That is why it is fine for athletes to sometimes eat refined carbohydrates - you don't always have to pick the whole grain option!
In fact, most athletes do not have to go out of their way to try consume more fibrous foods; simply by consuming adequate calories to meet your daily energy-requirements will allow you to meet your daily dietary fiber intake.
THE FIBRE PARADOX
The health benefits of a diet rich in fibre transcend beyond its role in easing digestion through increased laxation. Additionally, it is associated with a reduction in the risk to cardiovascular disease, type II diabetes, improved glucose management, body weight regulation, immune function and improved gut microflora (Slavin 2013). However, at certain times fibre may be detrimental to an athlete's performance and even health.
Athletes who consume a high-fibre diet in close-proximity to training or competition may impair sports performance through:
Accelerated Fluid Losses due to increased bowel movements.
Gastrointestinal Discomfort such as increased gas production, cramping and bloating.
Reduced Energy Availability due to insufficient calorie intake.
Lower carbohydrate Intake which may lead to reduced muscle glycogen re-synthesis and delayed recovery.
Inhibit the absorption of minerals (such as iron, calcium, magnesium and zinc), which can each independently impact the performance and health status of an athlete.
Total fibre intake and fibre timing is something that all athletes need to consider, but those on a plant-based diet that are consuming high volumes of fibrous foods, may be at an increased risk of excessive fibre intake.
Despite the far-reaching beneficial health effects of fibre, diets containing large amounts may provide gastrointestinal symptoms such as bloating and flatulence (in particular, non-digestible oligosaccharides and resistant starch), increase fluid losses due to increased bowel regularity, reduce the energy density of a diet leading to low energy availability and inhibit the absorption of minerals (iron, calcium, magnesium and zinc); which can each independently impact the performance and health status of an athlete during competition or phases of a macrocycle with high training loads.
Nutrition plays a decisive role in determining the health status of an athlete. The maintenance of a well-functioning immune system, avoiding illness, injury, and fatigue are all by-products of a well-tailored nutrition plan which support the maintenance of athletic performance. This is why athletes should consider working with a sports nutritionist who can help them strategically periodize their fibre intake in accordance with the demands of their sport event.
Remember, you can get too much of a good thing!
Sport Nutrition, Third Edition. (2019) by Asker Jeukendrup, Micheal Gleeson