Fibre for Athletes: How Much, and When to Cut Back
Nearly one in four endurance runners deliberately avoid high-fibre foods before a race, and among runners racing beyond 5km, women are several times more likely to do it than men (Parnell et al., JISSN, 2020). They're onto something. The bowl of rajma-chawal or bean chilli that serves you well on a Tuesday can send you looking for a portaloo at kilometre 14 on race morning.
Most athletes need 25–38g of fibre a day for gut health, but fibre is also a frequent trigger of race-day stomach trouble. Eat it freely through training weeks, then cut back in the 24–48 hours before a race or long hard session. Timing, not avoidance, is the answer.
Quick overview
How much fibre (fiber) do athletes need?
Adults need roughly 25g of fibre a day for women and 38g for men. That's the adequate intake set by the US Institute of Medicine and carried into the Academy of Nutrition and Dietetics position on fibre (Dahl & Stewart, J Acad Nutr Diet, 2015). Both figures come from a simpler rule: 14g of fibre for every 1,000 kcal you eat.
That rule matters because athletes eat more. A 60kg (132lb) runner eating 2,400 kcal on a training day lands at about 34g. A rower eating 3,500 kcal lands closer to 49g. There's no separate sports nutrition target for fibre, so treat 25–38g as a floor. If you're a big eater, let the energy-scaled figure guide you.
A WHO-commissioned series of meta-analyses found that people eating 25–29g a day had lower rates of heart disease, stroke, type 2 diabetes and colorectal cancer than people eating less, with further benefit above that (Reynolds et al., Lancet, 2019). Most people fall short, athletes included. The problem I see in practice is rarely too much fibre on an ordinary day.
What does fibre do for an athlete in training?
Fibre feeds the bacteria in your large intestine, which ferment it into short-chain fatty acids that help keep the gut lining healthy. It slows digestion, which smooths blood sugar after meals, and it keeps bowel habits regular through heavy training blocks and travel.
It also tends to arrive with carbohydrate attached. Oats, whole wheat roti, brown rice, dal, beans, fruit and potatoes in their skins are some of the most useful training-week carbohydrate foods there are. They help you reach the carbohydrate targets for athletes that your training calls for.
The catch runs the other way too. Fibre fills you up, and an athlete with high energy needs who builds every meal on salad, lentils and whole grains can end up full before they're fed. If you're training hard, tired all the time and still stuffed after meals, the bulk of your food may be the problem rather than the amount.
Why does fibre upset the stomach on race day?
Hard exercise diverts blood away from the gut toward working muscle, and a gut short of blood flow digests poorly and becomes more irritable (Costa et al., Aliment Pharmacol Ther, 2017). Heat, dehydration and high intensity all make that worse, and fibre adds to the load in three ways.
It leaves residue: insoluble fibre from bran, whole grains, skins and seeds adds bulk to your bowel, and running jostles it. Soluble fibre slows stomach emptying, which is useful at a desk and unhelpful when you need a gel absorbed at 25km. And many fibre-rich foods are high in fermentable carbohydrates (FODMAPs), which gut bacteria turn into gas.
That last one has the best evidence. In a crossover trial, recreational runners reported fewer gut symptoms, particularly pain and bloating, after a week on a low-FODMAP diet than after a high-FODMAP week (Wiffin et al., JISSN, 2019). A systematic review in ultra-endurance athletes reached a similar conclusion, while noting the studies are small (Ryan et al., Nutrients, 2023). Beans, lentils, onion, garlic, wheat and apples all sit high on FODMAP lists. That's why a dal-heavy or bean-heavy dinner the night before is such a common culprit.
Fibre isn't the only trigger. Racing harder than you trained, a new gel or a hot day can each wreck a gut on its own. Fibre is simply the one you control most easily.
When should you cut back on fibre before a race?
Cut back in the 24–48 hours before a race or a long hard session. The joint sports nutrition position stand recommends low-fibre, low-residue carbohydrate when fuelling up in the day before an event, and a pre-event meal low in fibre and fat (Thomas et al., J Acad Nutr Diet, 2016). Stretching that to 48 hours for longer races is how sports dietitians apply it in practice. It hasn't been tested head to head in trials, so treat it as a starting point.
A 5km parkrun, a gym session or an easy hour on the bike rarely justifies changing what you eat. The athletes who gain most are those racing 90 minutes or longer, anyone with a history of race-day stomach problems, and anyone carbohydrate loading. For a half marathon or longer, the low-fibre days overlap with the high-carbohydrate days. Lower-fibre carbohydrate is the easiest way to hit a loading target without feeling like you've eaten a sack of grain, and what carb loading actually involves covers the rest of that plan.
You don't need to reach zero. Most athletes I work with do well roughly halving their usual intake: refined grains in place of whole ones, vegetables peeled and cooked, and legumes, bran, nuts and seeds paused.
What a low-fibre day before a race looks like?
Breakfast can be white toast with honey, a plain bagel, rice porridge, idli with a thin chutney, or poha without the peanuts. Lunch is white rice or plain pasta with a small portion of well-cooked, peeled vegetables and a lean protein such as chicken, fish, eggs, tofu or paneer. Snacks can be ripe bananas, rice cakes or fruit juice. Dinner looks like lunch. None of it should be new to your gut in race week.
Pause rajma, chana, lentils and baked beans, bran cereals and muesli, raw salads, dried fruit, and generous amounts of onion and garlic.
Who shouldn't just cut fibre and hope?
Anyone whose stomach misbehaves in training and on rest days, not only on race day, needs to see a doctor before changing their diet. Ongoing bloating, a change in bowel habit, blood in the stool or unexplained weight loss need medical assessment. A low-FODMAP diet is a short-term elimination tool. It's meant to be run with a dietitian and followed by reintroduction.
Cutting fibre all year trades long-term health for comfort you need on a handful of days. The better fix for a sensitive gut is rehearsal: eat your pre-race low-fibre day before long runs, and train your gut with the carbohydrate you'll race on. Gut training for race day covers how to build that tolerance over several weeks.
The Fueletics Perspective
I'd rather an athlete eat 40g of fibre on most days and around 20g the day before a race than 20g every day of the year. Fibre is a training-week food, and it earns a short break when your gut has to perform under stress.
The right numbers are individual. Your usual intake, your race's carbohydrate needs, how early to start tapering fibre and how your gut copes with heat all move the plan. Get the timing wrong and a well-trained race can come apart in a portaloo queue. If you want your race-week numbers worked out rather than guessed, start with a Fueletics assessment.
Then use the first long run of your next block as the test. Eat low-fibre the day before, eat your planned breakfast, and note what your gut did at 90 minutes. That record will tell you more than any food list.
FAQs
Should I eat a low-fibre diet before a marathon?
For most runners, yes. Reduce fibre over the 24–48 hours before while keeping carbohydrate high, and practise it before a long run first.
Is fibre good for athletes?
Yes. It supports gut health, regular digestion and long-term heart and metabolic health, and fibre-rich foods supply much of a training diet's carbohydrate.
Can fibre cause stomach cramps while running?
It can contribute, especially FODMAP-rich foods such as beans, lentils, onion and wheat eaten the day before. Intensity, heat and race foods you haven't tried in training are common triggers too.
What are good low-fibre foods to eat before a race?
White rice, plain pasta, white bread, bagels, idli, ripe bananas, peeled potatoes, eggs, chicken, fish, tofu and paneer.
Research References
Parnell JA, Wagner-Jones K, Madden RF, Erdman KA. Dietary restrictions in endurance runners to mitigate exercise-induced gastrointestinal symptoms. J Int Soc Sports Nutr. 2020. doi:10.1186/s12970-020-00361-w
Dahl WJ, Stewart ML. Position of the Academy of Nutrition and Dietetics: Health Implications of Dietary Fiber. J Acad Nutr Diet. 2015;115(11):1861–1870. doi:10.1016/j.jand.2015.09.003
Reynolds A, Mann J, Cummings J, Winter N, Mete E, Te Morenga L. Carbohydrate quality and human health: a series of systematic reviews and meta-analyses. Lancet. 2019;393:434–445. doi:10.1016/S0140-6736(18)31809-9
Costa RJS, Snipe RMJ, Kitic CM, Gibson PR. Systematic review: exercise-induced gastrointestinal syndrome, implications for health and intestinal disease. Aliment Pharmacol Ther. 2017;46(3):246–265. doi:10.1111/apt.14157
Wiffin M, Smith L, Antonio J, Johnstone J, Beasley L, Roberts J. Effect of a short-term low FODMAP diet on exercise-related gastrointestinal symptoms. J Int Soc Sports Nutr. 2019;16(1). doi:10.1186/s12970-019-0268-9
Ryan T, Daly E, Ryan L. Exploring the nutrition strategies employed by ultra-endurance athletes to alleviate exercise-induced gastrointestinal symptoms: a systematic review. Nutrients. 2023;15(20):4330. doi:10.3390/nu15204330
Thomas DT, Erdman KA, Burke LM. Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance. J Acad Nutr Diet. 2016;116(3):501–528. doi:10.1016/j.jand.2015.12.006
