Why Fibre Timing Matters Around Training

Plant-based athletes frequently report digestive trouble in sessions, and the usual cause is not intolerance. It is the ordinary consequence of a high-fibre diet meeting the demands of exercise.

What fibre does to transit

Soluble fibre absorbs water and forms viscous gels, which slows the rate at which the stomach empties into the small intestine.

Insoluble fibre adds bulk and stimulates movement through the large intestine, increasing the volume of material present at any moment.

Both effects are beneficial for satiety, blood glucose response and gut health. Both are inconvenient in the two hours before a hard effort.

Why exercise makes it worse

During intense exercise blood is redistributed away from the digestive tract towards working muscle, reducing digestive capacity substantially.

Mechanical jostling adds to it, which is why running produces more digestive complaints than cycling or swimming at equivalent effort.

The combination means food that would digest comfortably at rest can sit undigested during a session and cause discomfort or urgency.

Structuring intake around sessions

The workable approach is to keep the high-fibre meals away from the hours immediately preceding hard training, not to reduce fibre overall.

Lower-fibre carbohydrate in the pre-session window serves the purpose: white rice, white bread, ripe banana, fruit juice, or a simple sports drink.

After the session, when digestive blood flow has returned, the normal high-fibre pattern resumes and does the work it is there to do.

Individual variation and gut training

Tolerance varies widely, and it also adapts. Regularly consuming carbohydrate during training increases the gut's capacity to absorb it over weeks.

This adaptation involves changes in transporter expression in the intestinal lining, and it is one reason experienced endurance athletes tolerate fuelling that would trouble a novice.

Adaptation is specific to what is practised, which is why the standing advice is to rehearse race-day fuelling in training rather than trying it for the first time on the day.

The line between adjustment and problem

Occasional discomfort during hard sessions is common and usually manageable by shifting meal timing and composition.

Persistent symptoms, blood, unexplained weight change or symptoms that occur at rest are a different matter entirely and warrant medical assessment.

Anyone repeatedly restricting food groups to control training symptoms should be working with a sports dietitian, since the restriction itself can create the shortfalls it was meant to avoid.