Your Guide to Fiber Intake for Runners

Fiber intake for runners can feel like a tricky balance. You need fiber for health reasons, but too much fiber too close to a run can lead to discomfort and an urgent bathroom trip. Likewise, fiber intake around races will look different than normal dietary fiber consumption. So how do you find the balance as a runner? This article will delve into what you need to know about fiber intake for runners, including how to adjust fiber around a race. 

How fiber can benefit runners

Fiber is a type of carbohydrate – structurally a polysaccharide called cellulose – that is not broken down by the digestive process. There are two types of fiber: insoluble and soluble. Insoluble fiber does not dissolve in water and instead functions to increase bulk in the stool and speed up the digestive process. Soluble fiber dissolves in water and thus ferments during the digestive process. As a result, soluble fiber maintains healthy gut bacteria, which supports the function of both the digestive system (including nutrient absorption) and the immune system.

High-fiber diets are linked with positive health outcomes, whether you are a runner or not. Research demonstrates a positive association between fiber intake and reduced risks of cardiovascular disease, cancer, type-2 diabetes, and gastrointestinal diseases. Fiber promotes regular bowel movements, thus reducing the occurrence of constipation and other GI issues. High-fiber carbohydrates are slower to digest, thus helping you stay full for longer periods of time. 

High carbohydrate diets are typically recommended for runners. High fiber carbs are higher in micronutrients and lower in sugar compared to simple, low-fiber carbohydrates. Choosing complex carbohydrates with high fiber for most of your meals (except pre-run) provides you with the carbohydrates your body needs for running while also ensuring you get enough micronutrients to support yoru training. 

Recommended fiber intake for runners

The recommended fiber intake for runners is approximately 20 to 35 grams of fiber per day, depending on your overall calorie intake. You can achieve your recommended fiber intake by including vegetables, fruits, whole grains, legumes, seeds, and nuts in your daily diet. 

Higher fiber intakes are not necessarily better, unless you are medically directed to increase your fiber. If you exceed 25-35 grams of fiber per day, you may be at increased risk of gastrointestinal upset on runs. Additionally, fiber is very filling. While increased satiety is generally considered a benefit, it could be detrimental if a runner does not consume enough calories due to eating so much bulk from fiber. 

How to adjust your fiber intake for race day

While fiber is incredibly beneficial for overall health, it can trigger gastrointestinal upset during runs. For the exact reasons that it is beneficial for health, fiber can lead to GI issues on race day. The best time to eat fiber as a runner is after your runs—not before. 

Fiber’s slower digestion time means it may sit in your gut for longer than simple carbohydrates and draw more water into your intestines. As a result, you are more likely to experience GI discomfort or gassiness during running. 

Too much fiber intake for runners can also lead to the urgency to use the bathroom during running. While fiber slows digestion relative to simple carbs, it does expedite transit time of food in the gut. Generally speaking, this helps promote regularity – but too much fiber timed too closely to runs could cause unwanted bathroom breaks.

Because it can lead to gastrointestinal issues, most runners will want to adjust their fiber intake for race day. If you have experienced GI issues previously in races, you want to pay special attention to your fiber intake before a race. A 2018 randomized controlled trial in Medicine and Science in Sports and Exercise demonstrated the potential benefits. Runners who reduced their fiber intake (by reducing FODMAPs, or high-fiber foods that can ferment in the gut) for six days experienced fewer GI symptoms. 

Recommendations for fiber around race day include:

• Choosing low fiber carbohydrates for your pre-race breakfast
• Reducing fiber the day before the race, especially from gas-producing foods like beans/legumes and cruciferous vegetables
• If you have a history of GI symptoms, reduce fiber in the two days before race
• Being mindful not to increase fiber during a carb load

In general, it is recommended that if you are eating within two hours of a run, you choose low-fiber foods. It takes approximately two hours for most people to digest fiber. Beans and vegetables are beneficial for health, but they may leave you feeling uncomfortable on even a normal easy run if you eat them too soon before running. 

Low fiber foods for runners

For any pre-run or pre-race meal, you want to opt for low-fiber, carbohydrate-based foods. Too much fat and protein can also slow down digestion. Low fiber foods for runners include: 

• White bread/bagel
• Graham crackers
• Fruit juice 
• Pretzels
• White rice
• Fig bars/breakfast bars
• Applesauce
• Low-fiber cereal

For the day before a race, you want to make low fiber changes including:

• Choosing white bread, rice, and pasta instead of whole grain
• Choosing white potatoes over sweet potatoes
• Reducing vegetable intake; if you do eat vegetables, opt for cooked and skip cruciferous (broccoli, cauliflower, Brussels sprouts, kale)
• Reducing fruit intake
• Avoiding legumes, nuts, and seeds

Final thoughts on fiber for runners

Fiber offers significant and numerous health benefits, and therefore, runners should not completely avoid fiber in their regular diet. Most of the time, you want to opt for fiber-rich carbohydrates. However, in your pre-run meal, you want to choose lower-fiber foods to reduce the risk of GI upset. Likewise, in the day or two before a race, you want to consciously reduce the amount of fiber you eat to avoid GI upset during the event. 

References
Jeunkendrup & Gleeson (2016). Sport nutrition. 3rd edition. Human kinetics. 

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