How to Increase Your Fiber Without Wrecking Your Long Runs

Runners have a complicated relationship with fiber. On the one hand, fiber is key for digestion, the immune system, and healthy body composition. On the other, get too much before a long run and, well, you know—sudden necessary trips to the bathroom.To resolve this dilemma, you need to master “fiber periodization,” according to a recent review article in International Journal of Sport Nutrition and Exercise Metabolism.What you shouldn’t do is ignore overall fiber requirements. Fewer than 1 in 10 U.S. adults meet daily fiber targets, and near half of health-conscious runners, research shows, fall short—often because high-fiber foods are the first thing cut when GI symptoms strike.Instead, learn exactly how to increase your overall intake of fiber while using the slow taper method to decrease what you eat before a long run or race.Why Fiber Matters for Runners"Fiber supports a healthy gut microbiome, helps with digestion, promotes heart health, and can also help with blood sugar regulation and keeping you satisfied between meals,” explains Claire Shorenstein, MS, RD, CSSD, a board-certified specialist in sports dietetics in Santa Cruz, California. Too little fiber and you can become constipated; too much fiber, and you can experience bloating and diarrhea.In the right amounts, fiber may even have direct benefits for runners. A study in Nutrients found that beta-glucan, a soluble fiber found in oats, barley, some mushrooms, and yeast, helped lower markers of inflammation following a treadmill run in a hot, humid environment. Inflammation is a hallmark of exercise-induced muscle damage, so fighting it with the help of beta-glucan may promote faster recovery, helping shorten your downtime between training sessions. While the study was funded using a grant from yeast manufacturer Leiber GmbH, there is other evidence that beta-glucan can improve muscle strength and aerobic endurance in athletes.How Much Fiber Runners NeedWomen need about 25 grams of fiber per day and men 38 grams. “We just have to be more intentional with the timing around our training,” Courtney Smith, MS, RD, CDCES, an endurance runner and sports dietitian in Salida, Colorado, tells Runner’s World.What “intentional” looks like in practice depends on the runner.“There is no one approach that works best for every athlete, but I generally recommend distributing fiber intake across the day while considering the timing of training and competition,” Patrick B. Wilson, PhD, RDN, associate professor of exercise science at Old Dominion University in Norfolk, Virginia, says.In general, avoid eating more than 5 grams per feeding in the two hours before running and immediately afterward. “While the slowing of gastric emptying is responsible for some of the fiber's health benefits, that’s not ideal right before exercise,” Wilson says. Similarly, high-fiber foods aren’t ideal for replenishing glycogen stores rapidly after a workout.How to Add Fiber Without Causing GI IssuesShorenstein suggests tracking your food for at least two to three days to establish your baseline fiber intake. If you fall short of the recommended amounts, gradually ramp up. To do this, increase by 3 to 5 grams every two to three days and see how well you tolerate it before bumping it up again.For example, if you typically eat 1 cup of white rice, swap it for 1 cup of brown rice to add 3 grams of fiber to your diet, Shorenstein suggests. Try different foods to see which fiber sources sit well on the run. “It’s not so different from how we train the gut when we’re working on a fueling plan during exercise,” Shorenstein says.Preparation helps, too. Cooking your vegetables before you eat them and blending fruits, vegetables, oats, and seeds into a smoothie can make those foods more tolerable.Shorenstein also recommends choosing whole-food sources of fiber over processed ones whenever possible. Many nutrition bars, enriched sandwich wraps, and supplements have isolated fibers that may trigger bloating or gas. You want whole foods high in soluble or insoluble fiber.What 25 Grams of Fiber Looks Like in a DayBroken into meals and snacks, the daily fiber target is less daunting than it sounds. Here’s an example of how a morning runner might spread roughly 25 to 30 grams of fiber throughout the day, according to Smith:Prerun snack: 2 to 5 grams of fiber1 ripe banana for a 30-minute run, or1 ripe banana with a slice of whole-wheat toast drizzled with honey for a longer runPostrun breakfast: 4 to 6 grams of fiberBlueberry smoothie with Greek yogurt for runners who can’t eat solids after a workout, or ½-cup dry oatmeal with ½-cup blueberries, Greek yogurt, and honeyLunch: 10 grams of fiberWhole-wheat wrap with chicken, spinach, tomato, and hummus, and a side saladAfternoon snack: 5 grams of fiberMedium Pink Lady apple with 1 Tbsp of peanut butterDinner: 5 grams of fiberSalmon and roasted carrots with wild riceHow to “Fiber Taper” Before a RaceGo into your race taper with an accompanying fiber taper plan. “Reducing the amount of fiber that’s moving through the digestive tract means less of a chance of needing an emergency bathroom stop, dealing with bloating, and other issues during the race,” Shorenstein says.How much you taper your fiber intake depends on your race distance. For shorter races like 5Ks and 10Ks, you don’t need to worry too much about it. Simply cut fiber after lunch the day before, Smith suggests.Events lasting longer than 90 minutes are a different story. “I usually tell people to reduce fiber for two to three days before their race,” Shorenstein says. The goal isn’t to eliminate fiber entirely. Rather, it’s to make room for a carbohydrate load—consisting of roughly 4.5 to 5.4 grams of carbohydrates per pound of body weight—while minimizing GI distress.However, cutting fiber shouldn’t mean cutting carbs. The goal is to swap high-fiber sources for ones that deliver the same fuel without the roughage. “Juices can be a good replacement for fruit because they provide the same carbohydrates and polyphenols as whole fruit without the fiber,” Wilson says. White rice and pasta, as well as gummy bears, also work well, he adds.Taper your cutback gradually. For example, if you normally eat 30 grams per day, aim for 20 grams three days before your race, 15 grams two days prior, 10 grams the day before (front-loaded before lunch), and 5 grams on race day. Shorenstein stresses that this is a general guideline, so don’t worry too much about hitting a specific number.Finally, as you drop fiber prerace, be sure to bump up your fluids—your body needs more water to store the carbs from your carb load in your muscles, Shorenstein says.Lauren Bedosky is a freelance health and fitness writer who specializes in covering running and strength training topics. She writes for a variety of national publications, including Runner’s World, Prevention, Experience Life and Women’s Running.