Why Reduce Fiber Before a Marathon?
You’ve trained for weeks, dialed in your sleep, managed your training load. And then you blow it all with a lentil salad the night before? It happens more often than you’d think. Reducing dietary fiber in the 48 hours before a marathon isn’t a dietitian’s quirk — it’s a digestive strategy backed by research.
Fiber is excellent in everyday life — it feeds the microbiome, slows digestion, and regulates blood sugar. But that’s exactly the problem: it slows digestion and ferments in the colon. When you’re running at high intensity for 3 to 6 hours, blood flow concentrates on your muscles. The digestive tract is under serious stress — mechanical contractions, progressive dehydration, and transient intestinal ischemia all create conditions ripe for cramps, diarrhea, and nausea. Fermenting fiber amplifies these symptoms.
A landmark review — Costa RJS et al. (2017), Sports Medicine — documents that up to 93% of endurance runners report gastrointestinal symptoms during long races. Among the dietary factors identified: high fiber, fat, and protein intake in the hours before effort. This isn’t about eating “clean” — it’s about digestive mechanics.
Good news: this restriction is temporary. Post-race, you go straight back to vegetables, legumes, and wholegrains. These 48 low-fiber hours are part of your race preparation, just like your last long run or active recovery.
What Ograal Does for You
Planning your meals for the 48 hours before a race means juggling several variables at once: carb-loading, fiber reduction, start time, body weight, and your eating habits. That’s exactly what Ograal does.
The app generates a complete 7-8 day pre-marathon meal plan, calibrated gram by gram based on your profile (weight, time goal, start time). Carb-loading management is built in natively: you don’t have to manually calculate your carbohydrate intake to top off your glycogen stores — the app handles it, factoring in the progressive reduction of fiber.
On race day, Ograal adapts the timing of your last meal to your start time (6am, 8am, 10am, 1pm…), and plans your gels, drinks, and bars across the entire effort — up to 7 hours of racing if needed. No more guessing whether you should take a gel at km 35 or whether you waited too long: it’s already planned.
The result: you arrive at the start line with a calm gut, full stores, and a precise plan in your head. With Ograal, you can automatically plan your meals from D-2 through race day, including carb-loading management — tailored to your start time and profile. Try the app at ograal.app.
D-2 (48h Before): What to Eat, What to Avoid
This is where your nutritional preparation really begins. At D-2, you continue loading up on carbohydrates (pasta, rice, bread) while progressively removing high-fiber, high-fat, and fermentable foods. As Jeukendrup AE (2017), Sports Medicine notes, “training the gut” also means knowing what to spare it from at critical moments.
EAT:
- White rice, white pasta, semolina — your primary fuel sources
- White bread, plain baguette (no seeds or grains)
- Ripe banana — easy to digest, rich in potassium
- Hard cheese (emmental, comte, light mozzarella)
- Lean white meat: chicken, turkey, veal — simply cooked
- White fish (cod, sole, pollack) — low fat, easy on the gut
- Unsweetened applesauce, filtered fruit juices
- Eggs — scrambled or omelette, minimal added fat
AVOID:
- Raw vegetables (lettuce, raw carrots, peppers, raw tomatoes)
- Legumes: lentils, chickpeas, kidney beans, broad beans
- Wholegrains: wholemeal bread, brown rice, oats, muesli
- High-skin fruits: grapes, cherries, plums, unpeeled raw apples
- Cabbage, broccoli, leeks, raw onions — highly fermentable
- Nuts, seeds, nut butters
- Heavy sauces, fried food, large amounts of soft cheese
- Alcohol and sugary sodas in excess
Sample D-2 day: breakfast of white bread + jam + banana + coffee; lunch of white pasta + chicken + moderate olive oil; dinner of white rice + turkey + applesauce.
D-1 (24h Before): Simplify Further
At D-1, the principle is simple: go even further in simplification. Carbohydrate volumes stay high, but meals become less varied — and that’s intentional. You already know these foods, you know how your body handles them. This is not the moment to experiment.
The classic combination that works for the vast majority of runners: white rice + grilled chicken + applesauce. Simple, predictable, low fiber, rich in easily digestible carbohydrates. Stick with it for both main meals and you drastically reduce the risk of digestive surprises.
Progressively drop any cooked vegetables you might have still had at D-2 (cooked zucchini, cooked carrots — already better tolerated). At D-1, they disappear. On portions: listen to your hunger, don’t force yourself to “tank up” to the point of feeling heavy. Glycogen is stored over 24 to 48 hours — Jimenez-Alfageme R et al. (2025), Sports Medicine Open confirms that pre-marathon nutritional optimization depends as much on timing as on raw quantities.
Hydrate regularly throughout the day — without overdoing it. Pale yellow urine is a good indicator. Avoid compensating in the evening by drinking 1.5L all at once.
For detailed guidance on structuring your pre-race dinner, see this dedicated guide on endurance race eve nutrition.
Race Morning (3h Before): Your Last Strategic Meal
Race morning is when many runners either panic — or worse, try something new because they read it was a good idea. Both are mistakes.
Golden rule: never test a new food on race morning. This meal must have been rehearsed in training. Ideally 2 to 3 times on your long runs.
Recommended composition (2h30 to 3h before start):
- White bread or sandwich bread + jam or honey
- Ripe banana (1/2 to 1 depending on your tolerance)
- Plain low-fat yogurt or quark (if habitually well-tolerated)
- Coffee or tea if you normally drink it — not the time to try something new
- A large glass of water + optionally a diluted sports drink
This meal should represent around 1 to 1.5 g of carbohydrates per kg of body weight. Absolutely avoid “detox” orange juices, green smoothies, whole-oat bowls, eggs and bacon, or flaky croissants. Also avoid skipping the meal out of anxiety — liver glycogen is partially depleted overnight.
In the hour before the start, if you tolerate it: a banana or a warm-up gel 15 minutes before the gun. Drink 3 to 5 ml of water per kg of body weight 15 to 20 minutes before the start.
The 3 Most Common Mistakes
These mistakes repeat themselves race after race. Here they are, with the logic behind each one.
Mistake 1 — Eating “healthy” the night before with a green salad. This is the classic mistake of the health-conscious runner. A big bowl of rocket, avocado, chickpeas, and lemon dressing is delicious and nutritious… outside of the 48h pre-race window. The night before a marathon, it’s a digestive time bomb. The insoluble fiber in rocket, the FODMAPs in chickpeas, the fat in avocado — all of it lands in your gut at the worst possible moment.
Mistake 2 — Trying a new “special marathon” recipe. You saw a recipe for “energy balls with dates, psyllium, and hemp protein” on Instagram. You figured it was perfect for carb-loading. It’s not the time. Psyllium is a powerful soluble fiber, large quantities of dates are laxative, and new plant proteins can disrupt your digestion. Stick to what you know.
Mistake 3 — Cutting all carbs out of fear of gaining weight. Some runners, especially those who follow low-carb diets day-to-day, panic at the thought of eating white pasta. Peake JM et al. (2023), Int J Sport Nutr Exerc Metab is clear on this: without pre-marathon carb-loading, glycogen stores are incomplete. Yes, you’ll gain 1 to 2 kg of water (glycogen is stored with water). Yes, that’s normal. No, it’s not fat. And yes, you’ll need it at km 35.
Recommended Gear
Sources
Costa RJS et al. (2017). Gastrointestinal complaints during exercise: prevalence, etiology, and nutritional recommendations. Sports Medicine, 47(S1), 73-89. DOI
Jeukendrup AE (2017). Training the gut for athletes. Sports Medicine, 47(S1), 101-110. DOI
Peake JM et al. (2023). Pre-race nutrition for marathon runners: evidence and practice. Int J Sport Nutr Exerc Metab, 33(2). DOI
Jimenez-Alfageme R et al. (2025). Nutritional Intake and Timing of Marathon Runners. Sports Medicine Open, 11, 14. DOI
