How to Strategize Fiber Intake to Keep You on the Trail

The sky is a vibrant blue and your legs are fresher than an alpine breeze. Miles flow like a mountain stream, but a low abdominal churning warns of a storm brewing. Maybe you hold out for a few more miles, but eventually, you may drop your two-ply standards for hopes of a marginally soft trailside leaf. 

You’re not alone in your struggles. Studies suggest that up to 90% of endurance athletes report experiencing gastrointestinal distress during exercise. If you feel like you’ve dialed in every aspect of your routine, it’s possible the culprit is pre-run dietary fiber. There is hope.

The Role of Fiber in Your Diet

First thing’s first: fiber is great, and you should be eating it! 

Fiber is the non-digestible carbohydrates in fruits, vegetables, grains, and legumes. While our bodies can’t digest it easily, it’s still important. Fiber feeds critical bacteria that live in our intestines. These powerful microbes contribute to nutrient metabolism and immune function. Nourishing them supports health, performance, and recovery. 

Soluble fiber also slows digestion, helping our bodies maximize nutrient absorption from food, while insoluble fiber keeps digestive materials moving. If you like staying healthy and dislike constipation, aim for the USDA’s recommended intakes: 

Ages	Female	Male
19-30	28 g	34 g
31-50	25 g	31 g
51+	22 g	28 g

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Fiber During Exercise

Because one of fiber’s key roles is to motivate a healthy rate of digestion, some of its effects are not as desirable during endurance events. Breakdown of fiber generates gasses, and fiber can draw water into the intestines. Since exercising also changes the rate at which food passes through our systems, these factors often lead to mid-run discomfort. 

It can also take the intestinal microbiota time to adjust to changes in fiber intake. Making abrupt dietary changes can increase the likelihood of experiencing gastrointestinal discomfort during runs. The prevailing wisdom applies here: avoid too many changes to your usual diet (unless to decrease your fiber intake) in the time leading up to a run or race. It could save you from trailside misery. Individuals vary, but a three-hour window of low or no fiber before exercising is a good place to start.

Being adequately fueled improves performance, so simple food swaps can be made in the time leading up to a race or training run to ensure the tank is full of easily digested energy. 

Let go of any fear of refined carbohydrates; they’ll be your best friend for avoiding GI issues. Although they are less nutrient-dense than their unrefined counterparts, simpler carbohydrates are a great source of lower-fiber energy. There will be plenty of time for antioxidant-rich, fibrous foods such as kale, quinoa, red cabbage, raspberries, and peas to boost recovery post-run! 

It may take some time to find the low-fiber foods that work best for you, but here are some tips to get started. 

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In the week(s) leading up to a race:

• Avoid abrupt changes to your fiber intake. If you are increasing fiber, try increments of around 5g per day for a more comfortable approach. Five grams would be like incorporating 1 cup of brown rice  and 1 cup of raw chopped kale, or ¾ cup of blackberries, each day. 
• In your training journal or notes, indicate the last food you ate before a run and whether it sat well. Listen to your body and look for patterns that might be contributing to GI distress.

The  day before a race:

• Try white pastas and breads as carbohydrate sources (they contain less fiber than whole grain or whole wheat versions).
• Consume fewer legumes, like beans and lentils.
• Try cooking your vegetables rather than eating them raw (this kickstarts the breakdown of some of the fiber, making them easier to digest).

RELATED: There’s No Space For Guilt In Endurance Fueling

Immediately before race time:

• Eat a carbohydrate-rich meal in the three hours before you race. Save fiber-heavy foods like oats, apples, and seeds  for later, and try incorporating more easily digested options: 
  • white toast with fruit jam or an egg
  • fruit juice
  • rice-puff cereal with soy milk
  • white bread PB&J
  • melon slices 
  • white bagel with honey, cream cheese, butter, or smooth peanut butter (if fat doesn’t cause you gut issues while running ) 
  • a white bread sandwich with tofu or deli meat
  • white rice with soy sauce 
  • instant mashed potatoes
  • gnocchi with tomato sauce

During a run or race:

High-fiber foods will be more likely to cause gastrointestinal upset during exercise and will slow down absorption of the energy you need to perform. If you fuel longer events with food rather than sports gels or drinks, stick to simple and well-tolerated carbohydrate sources like those above. 

Disclaimer: This information is not intended as medical nutrition therapy. If you have a medical condition or concern, please consult your healthcare provider or a registered dietitian.

Kara Sampsell is a trail runner from Vermont with a BSc. in Dietetics. She completed her MSc. in Nutrition, Genetics, and Metabolism in Calgary, Canada while playing in the beautiful Canadian Rockies. When she isn’t running, Kara enjoys catching the sunrise, dreamy backcountry tours, morning bagels, evening climbs, and nerding out on science news.