Fiber is normally a great part of a healthy diet. But should you reduce fiber intake before an event to prevent stomach issues, minimize bloating, and even increasing your watts/kg?
You don’t want to have to do a ‘Dumoulin’
In the days leading up to an event or competition, athletes make various adjustments—tweaking training, refining equipment, optimizing sleep, and, of course, modifying nutrition. While the first dietary changes that come to mind might be increasing carbohydrates, avoiding alcohol, or steering clear of unfamiliar foods, a newer trend is emerging and that’s lowering dietary fiber intake.
What Is Dietary Fiber?
Dietary fiber is a type of non-digestible carbohydrate found in plant-based foods such as whole grains, beans, legumes, fruits, and vegetables. It passes through the digestive system relatively unchanged and offers the body numerous health benefits. Fiber supports the gut microbiome (helpful gut bacteria), aids in maintaining intestinal barrier function, prevents constipation, reduces hunger, aids in weight loss, improves insulin resistance, and promotes heart health by lowering LDL cholesterol. The recommended daily fiber intake is 25 g for women and 38 g for men, or about 14 g per 1000 kcal consumed. While those totals may not seem like much, many of us struggle to meet our fiber recommendations on a daily basis (1).
The two main types of fiber include:
- Soluble fiber – Dissolves in water, helps lower cholesterol, and stabilizes blood sugar.
Found in apples, bananas, oats, peas, and beans. - Insoluble fiber – Does not dissolve in water, promotes gut motility, and aids digestion.
Found in whole wheat flour, bran, nuts, seeds, and the skins of many fruits and vegetables.
What benefits overall health can sometimes be counterintuitive for performance, and to an extent that’s true with fiber. Considering fiber provides potent health benefits, lowering it in the name of performance should be done with caution.
Why Reduce Fiber Before Competition?
As mentioned earlier, fiber can slow digestion, lower blood sugar, and aid in gut motility. While this may be desirable during weight loss or when needing steady energy during the workday, during exercise this could cause issues like indigestion, bloating, gas, and an increased urge to defecate. None of these side effects are ideal mid-race, and thus modifying fiber intake before competition may help some athletes avoid GI distress (2). That said, not everyone benefits from fiber reduction. Some athletes with high fiber intake experience no stomach issues at all, but for others considering this lower fiber approach, the potential upsides include:
- Lower risk of GI distress or unexpected bathroom breaks
- Reduce gut residue to decrease weight and offset some gain from carb-loading
- Minimize bloating and discomfort leading up to competition
For athletes with a history of GI problems, this can be a game-changer. Many athletes can recall moments of doubling over in pain, making emergency stops, or worse—situations no one wants to repeat on race day or before a big event.
FODMAPs
While there are two main classifications of dietary fiber (soluble and insoluble), there is a subcategory of fiber that can be particularly troublesome for digestion, known as FODMAPs (fermentable oligo-, di-, monosaccharides, and polyols).
When consumed before or during exercise, FODMAPs may draw water into the intestines and produce gas, increasing the likelihood of bloating, cramping, and GI distress. Some athletes have been known to adopt a short-term low-FODMAP diet (1–3 days pre-race) as a preventive measure against GI distress. However, long-term low-FODMAP diets are not recommended. As mentioned, fiber fuels beneficial gut bacteria, and thus prolonged restriction may negatively impact gut health and/or lead to nutrient deficiencies (3,4).
A host of foods fall into the FODMAP category, and upon examination you may see why complete long-term elimination of these foods would leave your diet utterly stripped. For this reason, it’s best to work alongside a registered dietitian if aiming to adjust FODMAP intake beyond 1–3 days or if you frequently experience GI issues linked to some of these food items. Everyone responds differently to these foods, so eliminating 1–2 triggered foods in the days leading up to an event may be all you need rather than complete abstinence of all FODMAPs.
Temporary Weight Reduction
Beyond avoiding GI complaints, limiting fiber before competition may aid in achieving short-term weight loss by reducing undigested food and fecal weight. Studies show that reducing fiber to <10 g/day for several days can lead to a 0.5–1.0 kg body weight loss, or a modest 0.6–0.7% change in body mass. Researchers found that only 25% of this body mass loss was directly linked to fiber reduction, and that other factors such as differences in gut transit time and type of fiber could play an even bigger role in individuals’ response to this weight reduction approach. Less than 1% weight change may not be much, but depending on the competitive margins and your goals, it could be of notable effect for athletes in weight-sensitive sports (5).
Final Thoughts
For endurance and weight-sensitive athletes, a short-term low-fiber strategy may help reduce gut bulk, minimize bloating, improve race-day comfort, and maybe shave off a modest pound or two. The most beneficial component of this I’ve seen for athletes is mitigating excess fecal weight and the urge to defecate on race day. This, I think, becomes even more beneficial when paired with carbohydrate loading, which, if done with fiber-rich foods, may cause a host of issues.
It’s important to remember fiber is essential for long-term health, so any dietary fiber adjustments should be done selectively and strategically and not as a permanent diet unless working alongside a professional.
How to Implement a Low-Fiber Diet Before Competition
If you’re considering lowering fiber before an event, here’s how to do it effectively:
- Start 1–3 days before race day.
- Limit fiber intake to <10 g/day. If this is too aggressive, aim for 15–20 g.
- Focus on low-fiber, high-carb foods leading up to race day (e.g., white rice, white bread, low-fiber cereal, peeled fruits, well-cooked veg, sports drinks).
- Reduce foods high in lactose and fructose (e.g., cow’s milk, some fruits or juice) if prone to GI distress.
It’s crucial to test this strategy well before race week to ensure it works for you. Every athlete responds differently—what works for one may not work for another.
Nothing much worse than an upset stomach on the bike
References
- Mancin, L., Burke, L.M. & Rollo, I. Fibre: The Forgotten Carbohydrate in Sports Nutrition Recommendations. Sports Med (2025). https://doi.org/10.1007/s40279-024-02167-1
- de Oliveira, E. P., Burini, R. C., & Jeukendrup, A. (2014). Gastrointestinal complaints during exercise: prevalence, etiology, and nutritional recommendations. Sports medicine (Auckland, N.Z.), 44 Suppl 1(Suppl 1), S79–S85. https://doi.org/10.1007/s40279-014-0153-2
- Wiffin, M., Smith, L., Antonio, J., Johnstone, J., Beasley, L., & Roberts, J. (2019). Effect of a short-term low fermentable oligiosaccharide, disaccharide, monosaccharide and polyol (FODMAP) diet on exercise-related gastrointestinal symptoms Journal of the International Society of Sports Nutrition, 16(1), 1. https://doi.org/10.1186/s12970-019-0268-9
- Lis D. M. (2019). Exit Gluten-Free and Enter Low FODMAPs: A Novel Dietary Strategy to Reduce Gastrointestinal Symptoms in Athletes. Sports medicine (Auckland, N.Z.), 49(Suppl 1), 87–97. https://doi.org/10.1007/s40279-018-01034-0
- Foo, W. L., Harrison, J. D., Mhizha, F. T., Langan-Evans, C., Morton, J. P., Pugh, J. N., & Areta, J. L. (2022). A Short-Term Low-Fiber Diet Reduces Body Mass in Healthy Young Men: Implications for Weight-Sensitive Sports. International journal of sport nutrition and exercise metabolism, 32(4), 256–264.< https://doi.org/10.1123/ijsnem.2021-0324
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