Fiber – Friend And Foe To The Ultrarunner


When fiber is mentioned in conversation, there is usually a pause – then everyone has an opinion. We know that fiber is healthy; after all, fiber decreases transit time through the gut. And we know it can cause ultrarunners problems during a race; after all, fiber decreases transit time through the gut! So which is it? Do we eat it for health or avoid it for unscheduled nature stops? Obviously, the answer lies in the timing.

Before we talk about timing, it helps to understand exactly what fiber is, the different kinds and sources, and the role of fiber in digestion. Fiber is made up of a group of indigestible carbohydrates found in plant foods, but with no calories, as it is not absorbed by the body. There are two main categories of fiber: natural dietary fiber, and manufactured functional fiber. The sum of these two fibers is known as Total Fiber and is the amount listed on food labels.

Dietary fiber is further broken down into types: insoluble fiber and water-soluble fiber. Insoluble fiber speeds up gut transit time, increases stool bulk, moves potentially cancerous substances through the body and keeps the colon clean and functioning. Good sources are wheat bran, whole-grain cereals, vegetables such as green beans, beets, cauliflower, cabbage, potato skins, apple skins, nuts and seeds.


Water-soluble fiber forms a gel in the gut and slows down transit time, delays absorption of glucose (sugar) in the intestines (helping to control blood sugar) and can act like a sponge to help prevent cholesterol absorption. Good sources are oats, barley, legumes (beans, peas, lentils), some fruits (the insides of apples, blueberries), vegetables such as broccoli and okra, and psyllium.

Functional fiber may be extracted from food or may be manufactured synthetically. Some examples are gums, pectin, and resistant starch, and these may be added to foods for various purposes. To be classified as a functional fiber, the specific fiber has to have a physiological effect in the body, usually associated with a specific health benefit.

Despite all the choices we have, Americans, on average, consume only about half of the recommended daily intake. One way to solve this dilemma is to aim for 14 grams of fiber for every 1,000 calories you eat; more if you are diabetic. Choose a variety of plant foods to get a mix of the different types of fiber. Choose whole fruits and vegetables over juices and whole grains over refined grains. If you have not been getting enough fiber in your diet, add it in slowly, especially water-soluble fiber. It can take a few weeks, up to months, for your body to adjust to more fiber.

Increased consumption of fiber can help athletes who experience constipation – just make sure you increase your water intake at the same time. Fiber helps with feelings of fullness and assists in weight management as well as increasing insulin sensitivity. It helps reduce the risk of intestinal diseases and stimulates the production of healthy bacteria in the gut. All good reasons to include plenty of fiber in your diet, as long as you pay attention to timing.

Remember those phrases, “speeds up gut transit time” and “slows down transit time” and “delays absorption of glucose (sugar)”? Let’s think about how those things might affect an ultrarunner out on the trail…

Okay, let’s not.

Now you can understand how fiber might not be that great just before, and during, your long run or race. It still IS healthy and we want to include it in our diet, just not in our raceday nutrition plan. Also note, the faster you run (intensity), and the longer you run (duration), coupled with race anxiety (jitters), the more you need to pay special attention to the timing. Here’s the timing part.


Now we know why fiber is so good for us and important to our digestive system, but what about around a long or intense run or, especially, an ultra-distance race? The problem for ultrarunners is that fiber can give us a feeling of heaviness and/or fullness before a run and can increase GI distress. It takes about two hours for fiber to empty from the stomach and work its way into the small intestine. When fiber gets lower in the digestive tract, bacteria feast on it, producing gas and sometimes cramping. Here are some tips to avoid those unwanted frantic searches for bushes and trees.

  • Try a “Fiber Taper.” Decrease your fiber intake by 25 percent each day for two or three days out before your next big race. Focus on substituting white, refined starch for whole grains and try drinking more juices for your fruits and vegetables. Zucchini, tomatoes, olives, grapes and cantaloupe are all low in fiber. Avoid cruciferous vegetables (broccoli, cauliflower, Brussels sprouts) and bran cereals.
  • Be sure and reverse this Fiber Taper, gradually reintroducing fiber after competition so that you do not experience a fiber overload.
  • Substitute some liquid meal supplements if you have a particularly sensitive gut.
  • For early morning ultrarunners, eat your fiber after you run, during recovery or later in the day.
  • If you run after work or later in the day, eat your fiber in the morning or earlier in the day.
  • A recent study showed that runners tolerated five grams of fiber or less before a run, as long as they were accustomed to including fiber in their regular diet.
  • For some runners, harder-to-digest foods such as proteins and fats slow everything down. Eat less of these and more simple carbohydrates the night before the race.

About Author

Sunny Blende, M.S. is a Sports Nutritionist who writes and counsels individuals and teams on fueling for enhanced performance and making healthy food choices. Currently she writes the nutrition column for UltraRunning magazine and runs ultras herself. She has presented at the National RRCA Convention, the National Rowing Convention, Nike San Francisco Marathon Expo, and the Runners World San Francisco Marathon and worked as an assistant with the Los Angeles Marathon Association. An avid master competitor herself, she trains and competes in the San Francisco Bay Area and beyond. Sunny received her Bachelor of Science degree from University of Southern California and her Masters in Human Nutrition degree from University of New Haven. (Photo at left, by Luis Escobar).

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