Key takeaways
- Everyday dietary needs are not targets to reproduce in every hour of racing.
- Fibres have different properties; one number cannot describe all their effects.
- There is no universal g/h limit that predicts your digestive comfort.
What is fibre?
Fibre includes food components that are not digested like available carbohydrate in the small intestine. It occurs in cereals, fruit, vegetables and pulses, and can also be added as an ingredient. It contributes to bowel function in everyday eating. EFSA’s dietary references concern that general setting, not an hourly target during a race.[1]
The terms ‘soluble’ and ‘insoluble’ describe some properties without summarising all behaviour. Fibres also differ in their ability to form a gel, retain water or undergo fermentation. It would be too simple to say that one entire group is always comfortable while another is always problematic during exercise.
A label usually provides a total fibre quantity. That number helps calculate intake, but it does not fully describe the fibres present. The ingredient list may add context, for example by naming inulin. It still cannot tell you the precise dose of every individual fibre in a product.
Why can it affect digestive comfort?
Depending on type and amount, fibre can change the volume of intestinal contents, water retention and fermentation. Some effects may contribute to bloating, cramps or the urge to pass stool. These are possible mechanisms, not a guarantee that a product containing fibre will produce those symptoms in everyone.
Comfort during exercise also depends on exercise itself, intake volume, heat and individual sensitivity. Tolerating a serving in an everyday meal does not establish tolerance during an intense race. Conversely, difficulty in demanding conditions does not mean you need to remove that food from everyday life.
Fibre is not synonymous with FODMAP. Some fibres are fermentable, but the categories do not entirely overlap. Polyols are a separate topic too. To understand a food, consider its fibre quantity, relevant ingredients and other factors affecting tolerance separately rather than treating them as the same nutritional issue.
What do we actually know in athletes?
The 2025 SDA–USSF position statement highlights limited direct experimental evidence concerning low-fibre intake and symptoms during exercise. Evidence discussed includes field observations and interventions combining several changes. These do not make it easy to isolate the effect of fibre itself.[2]
This distinction matters when reading advice. A plausible mechanism, personal experience and a controlled trial answer different questions. Athletes may report better comfort after changing their diet without knowing which change caused the effect. Serving sizes, timing and other ingredients may have changed at the same time.
Choosing relatively low-fibre foods during an event may be a practical option, particularly for a sensitive individual. It still needs to fit experience and context. Evidence does not justify a general exclusion for all athletes or a number of grams per hour above which symptoms become certain.
Small amounts, solid food and long events
On a very long outing, solid foods can contribute variety and help you keep eating. Some contain fibre. A modest amount may be tolerated, so its presence alone is not a reason to reject a product. Look at the intended serving instead of deciding purely from a food category.
Food habits are useful when preparing a trial. A familiar product is easier to evaluate than several new foods mixed together on race day. Familiarity does not guarantee exercise tolerance, but it gives you a point of comparison. Then test the food in conditions matching how you plan to use it.
A food’s usefulness also depends on what it lets you consume. A very low-fibre product that you can no longer eat does not solve the intake question on its own. A well-accepted option may fit your approach even when its label does not say zero fibre. Compare practical constraints instead of one abstract nutritional ideal.
Read the label and understand your tolerance
Check whether fibre is listed per 100 g, per serving or per whole packet. Use the serving actually eaten. If you repeat servings, add the quantities over the period considered. A small amount per serving can become a larger total after several hours without that total automatically becoming a problem.
Keep missing values unknown. A fruit-based bar’s name does not let you calculate its exact fibre content, and an absent label entry does not demonstrate zero intake. Ingredients can indicate presence but cannot replace a declared nutritional quantity. The same rule applies when comparing a detailed label with an incomplete one.
For a trial, record the food, serving, time period, drinks and sensations. Avoid changing every part of your fuelling at once. When comparing products, keep conditions as similar as practical and also consider the carbohydrate supplied. That makes it easier to distinguish a serving-size difference from a difference in ingredients.
Review several trials before concluding. A symptom-free outing does not guarantee every situation, and one isolated episode does not condemn a whole food group. Recurrent or significant difficulties deserve individual assessment. A broad, lasting dietary restriction should not be inferred from a single number on a product label.
Give observations useful names: fullness, bloating, an urge to pass stool or difficulty chewing are different experiences. Note when they occurred rather than recording only that a product was ‘bad’. This does not diagnose a cause, but it helps you discuss the outing more clearly and choose what to compare next.
Worked example: three servings
Fictional teaching example: one serving contains 2 g of fibre. Three servings provide 2 × 3 = 6 g. This total describes the three intakes, not a limit to follow or a prediction of symptoms.
If another product’s label does not specify fibre, its contribution remains unknown. You can compare available information and ingredients, but you cannot replace the missing value with 0 g.
Frequently asked questions
Should you avoid all fibre during exercise?
A systematic exclusion is not justified for everyone. Reducing amounts may help in some situations, particularly with digestive sensitivity. Serving size, the complete food and your trials matter more than a rule based solely on fibre being present.
Do all fibres have the same effects?
No. Their properties and fermentation differ. Total fibre on a label cannot summarise every effect, and the soluble and insoluble categories alone do not predict tolerance during a race.
Is there a g/h limit?
There is no scientifically established universal maximum during exercise. A calculated total helps you understand consumption. Interpret it alongside context, all the food eaten and observed tolerance rather than treating it as a symptom threshold.
How Glyk considers this topic
Glyk considers fibre among the factors in its estimated digestibility assessment during exercise. A declared quantity and presence identified in ingredients are different information; a missing value remains a limitation.
The score is an interpretation for comparing products. It is not a digestive diagnosis and does not challenge fibre’s place in everyday eating. Scientific references provide context without validating the application’s rubric.
Scientific sources
These references inform the guide. Published recommendations, study findings and their interpretation in Glyk remain distinct. They do not validate the application’s scoring rubric.
