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Dietary Fiber and Endurance Athletes: A Delicate Balance for the Gut

Updated: July 31, 2026

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Dietary fiber has long been linked to better cardiovascular and metabolic health in the general population. But in endurance athletes, its role is more ambiguous: beneficial in the long term for the gut microbiome, fiber can also become a trigger for digestive problems when consumed at the wrong time. A review published in April 2026 in the International Journal of Sport Nutrition and Exercise Metabolism (IJSNEM) addresses this tension and proposes a more nuanced approach than simply avoiding fiber.

An underestimated role in current sports nutrition guidance

According to the authors, current sports nutrition guidelines lack specific recommendations on fiber, even though existing data link an adequate intake to gut microbiome stability, modulation of immune function, and optimization of body composition. In athletes under heavy training loads, a moderate, well-distributed intake has been associated with:

Better immune function, especially valuable during high training loads when infection risk rises
Improved energy metabolism, via production of short-chain fatty acids by gut bacteria
Better preservation of skeletal muscle mass

The digestive risk during exercise: a well-documented reality

The flip side is equally well established. Between 30% and 70% of endurance athletes report digestive symptoms during exercise — bloating, cramps, diarrhea, even vomiting — particularly during long events such as the marathon, triathlon, or ultra-trail. Because fiber slows digestion and ferments in the colon, it ranks among the main foods identified by a 2025 systematic review in the Journal of the International Society of Sports Nutrition as associated with these problems, alongside fats, proteins, and fermentable carbohydrates (FODMAPs).

Fiber periodization: a preferred approach

Rather than pitting benefits against risks, recent literature converges on a periodization principle: adjusting fiber intake according to the training phase rather than eliminating it permanently.

During regular training, away from intense sessions, a fiber intake close to general adult recommendations (about 25 to 38 g per day, according to the Academy of Nutrition and Dietetics) remains compatible with good digestive tolerance and supports long-term gut health.
In the 24 to 48 hours before a competition, as well as in the meal directly preceding an intense session or race, a temporary reduction in high-fiber foods is commonly recommended to limit the risk of gastrointestinal problems.
"Gut training" — gradually accustoming the digestive system to certain intakes ahead of competitions — is presented as a promising complementary strategy to improve tolerance over time.

What this means in practice

For an endurance athlete, the challenge is therefore not to ban fiber, but to manage it intelligently over time: keeping it at a sufficient daily level for overall gut and immune health, while temporarily limiting it around intense efforts or competitions. Needs vary greatly from one individual to another depending on digestive sensitivity, sport type, and effort intensity.

This article is for informational purposes only and does not replace individualized medical or nutritional advice. Athletes with recurrent digestive problems or gastrointestinal conditions should consult a sports physician or registered dietitian before changing their fiber intake.