50 Foods to Avoid Before a Marathon

You followed your training plan to the letter. Your fundamental endurance is solid, your long runs went well, and race day is approaching. There's one thing many runners neglect until the last moment: what they put on their plate.

Making poor food choices in the 48 to 72 hours before the race — or even on race morning — can turn a well-prepared marathon into a gastrointestinal nightmare. Stomach cramps, diarrhea, bloating, nausea, energy crashes: all these problems often have a dietary origin.

Here are the 50 foods and drinks to absolutely avoid before a marathon, organized by category, with the physiological explanations behind each one. Some will surprise you.

Warning window: this list applies primarily to the 72 hours before the race, with maximum caution in the 24 hours before the start and on race morning. The closer you get to the gun, the more careful you need to be.

Category 1 — Fermentable Vegetables (Nos. 1 to 10)

These vegetables are excellent for everyday health… but devastating before an endurance event. Rich in insoluble fiber and sulfur compounds, they ferment in the colon and generate gas, bloating, and sometimes abdominal pain during effort.

1. Broccoli

The champion of fermentable vegetables, broccoli contains glucosinolates and a large amount of fiber. During digestion, it produces intestinal gas in quantity. Absolutely avoid it in the 3 days before the race.

2. Cauliflower

Same family as broccoli (the crucifers), same problem. Cauliflower is particularly gas-producing. Even cooked, it remains a poor companion before a race.

3. Brussels Sprouts

The most digestively troublesome of the cruciferous vegetables. A single serving can be enough to trigger intestinal cramps during the race. Ban them at least 72 hours beforehand.

4. Green Cabbage and Red Cabbage

Whether in salad, braised, or in soup, cabbage stimulates bacterial fermentation in the colon. Runners with sensitive digestion may feel its effects up to 48 hours after eating it.

5. Leeks

Rich in fructo-oligosaccharides (FOS), fermentable prebiotic fibers, leeks frequently cause bloating and flatulence. Even cooked, they remain problematic.

6. Raw Onion

Cooked onion is more digestible, but raw onion contains fructans that escape intestinal digestion and ferment in the colon. Mid-marathon, the effects can be painful.

7. Raw Garlic

A powerful prebiotic, garlic stimulates the growth of intestinal bacteria — which is great for daily health, but a disaster a few hours before a race. Cooked garlic is slightly less problematic, but still worth limiting.

8. Lentils

A favorite legume among athletes for their protein and carbohydrate content, lentils contain oligosaccharides (FODMAPs) that the intestine cannot digest on its own. The result: fermentation, gas, and sometimes accelerated transit. The same applies to red lentils.

9. Red and White Beans

"Beans make you gassy" — this is no joke. The phytates and oligosaccharides they contain are digestive time bombs. Avoid them at least 4 to 5 days before a marathon if you're sensitive to them.

10. Chickpeas

Even if hummus and chickpea salads are trendy, eating chickpeas 24 to 48 hours before a race is a common mistake. The fermentative effects can linger for a long time in the digestive tract.

Category 2 — Fatty and Fried Foods (Nos. 11 to 18)

Fats considerably slow gastric emptying. A fatty meal stays in the stomach far longer than a carbohydrate-based meal, meaning that during the race, your digestive system is still processing your dinner from two nights ago.

11. French Fries and Chips

Double problem: fatty AND excessively salty. French fries absorb an astronomical amount of frying oil, slow digestion, and provide virtually no useful carbohydrate value (high GI, fiber destroyed by frying).

12. Pizza (the Evening Before or Race Morning)

The pasta dinner is a tradition among marathon runners — but pizza is not. The combination of cheese + processed meats + fatty dough takes a very long time to digest. Even a reasonably sized pizza can weigh on the stomach for 6 to 8 hours. Absolutely avoid it the night before and on race morning.

13. Burgers

Same observation as pizza: brioche bun + ground beef + sauce + melted cheese = a fat bomb. A burger can take 4 to 6 hours to clear the gastric barrier. Save it for the post-marathon celebration.

14. Fatty Cheeses (Raclette, Camembert, Brie)

Having raclette the night before a marathon is a classic beginner mistake. Melted cheese, very fatty, considerably weighs down digestion. Camembert or brie are no better — opt for plain fromage blanc or light mozzarella if needed.

15. Fatty Cured Meats (Salami, Rillettes, Pâté)

Rillettes, country pâté, dry salami: very rich in saturated fats, these foods overload the digestive system for hours. Moreover, their high salt content can disrupt the hydration balance, already under pressure during prolonged effort.

16. Sausages and Spicy Merguez

Often consumed at pre-race "social" meals (the night-before barbecue…), sausages and merguez are fatty, spicy, and hard to digest. A perfect recipe for ruining your marathon.

17. Cream-Based Sauces and Dishes

Blanquette de veau, gratin dauphinois, béchamel sauce… These preparations concentrate fats and dairy products. Even in small quantities, they can cause persistent digestive heaviness.

18. Foie Gras

Luxurious but merciless before a race: foie gras is composed of more than 50% fat. It has absolutely no place on your plate in the 72 hours before a marathon.

Category 3 — Spicy and Irritating Foods (Nos. 19 to 24)

Some foods directly irritate the intestinal lining or accelerate transit in an uncontrolled way. Under intense physical stress (a marathon certainly qualifies), this effect is amplified.

19. Hot Curry and Chili Peppers

The capsaicin found in chili peppers is a powerful intestinal transit stimulant. It accelerates colon contractions — exactly what you don't want at the 10 km mark from the finish. Even moderately spicy curries can cause loose stools in sensitive runners.

20. Hot Mustard

The allyl isothiocyanate that gives mustard its pungency can irritate the gastric lining. In small amounts in a sandwich, it's trivial. Before a marathon, avoid it.

21. Hot Sauce (Tabasco, Sriracha)

Same logic as chili peppers: accelerated transit guaranteed. Some runners report intense abdominal pain after consuming hot sauce the night before a race.

22. Horseradish and Whole-Grain Mustard

Popular in traditional cuisine, horseradish and whole-grain mustard can cause gastric reflux, especially combined with intense physical effort that compresses the abdomen.

23. Very Acidic Dressings

A salad with a drizzle of balsamic vinegar is fine. But very acidic dressings (pure lemon, white vinegar, large amounts of cider vinegar) can irritate the gastric lining, especially in runners who suffer from reflux.

24. Citrus Fruits in Excess

Large amounts of grapefruit or orange juice can increase gastric acidity. One glass in the morning is acceptable for some — a liter of freshly squeezed lemon juice, no.

Category 4 — Problematic Drinks (Nos. 25 to 33)

Hydration is critical before a marathon, but not all drinks are equal. Some can destabilize your blood sugar, disrupt your sleep, or accelerate dehydration.

25. Alcohol (Even Moderate)

This is the number-one mistake among amateur runners: a glass or two of wine the night before "to celebrate." Alcohol is a diuretic, it profoundly disrupts sleep (reduced REM), it interferes with muscle glycogen synthesis, and it increases inflammation. Even a 33 cl beer can negatively impact the next day's marathon. Zero alcohol in the 48 hours before the race.

26. Energy Drinks (Red Bull, Monster, Rockstar)

Their caffeine content is high and uncontrolled, they contain very high-GI sugar (glycemic spike followed by a crash), and the acids they contain irritate the stomach. Avoid them entirely.

27. Too Much Coffee (More Than 2 Cups)

Caffeine at moderate doses can improve performance (this is proven), but in excess it accelerates transit and can cause loose stools during the race. If you're not accustomed to coffee, don't start on race morning to "wake yourself up."

28. Industrial Sweetened Fruit Juices

Orange, apple, tropical… Industrial juices concentrate fructose without the fiber of whole fruit. The result: a brutal glycemic spike, insulin drop, and sometimes too-high osmolality in the intestine that draws in water and causes diarrhea. Prefer water or an isotonic drink.

29. Carbonated Drinks (Sodas, Lemonades)

Carbon dioxide expands the stomach and creates bloating. Running with a distended belly is uncomfortable and debilitating. Moreover, sugary sodas cause glycemic spikes unsuited to an effort lasting several hours.

30. Kombucha

Very trendy, kombucha is a fermented drink containing live bacteria and a small amount of residual alcohol. On an intestine already stressed by effort, the additional fermentation it generates can cause significant bloating. Consume it outside competition periods.

31. Whole Milk in Large Quantities

Lactose requires lactase to be digested. Many adults partially lack it, especially under stress. A large glass of whole milk on race morning can trigger cramps and diarrhea in slightly intolerant runners who don't yet know it.

32. Highly Concentrated Green Smoothies

Spinach, kale, chia seeds, plant milk, banana, protein powder… A "detox" green smoothie seems healthy, but it concentrates fiber, protein, and fat in an apparently easy-to-digest volume. The intestine must process all of this rapidly. Not ideal before a race.

33. Overly Concentrated Homemade Isotonic Drinks

Some runners prepare their own sports drinks with too much sugar. Too-high osmolality (>320 mOsm/kg) slows intestinal absorption and can cause nausea. Follow the recommended dosages.

Category 5 — Sugars, Sweets, and Desserts (Nos. 34 to 39)

Fast sugars seem like a good idea before a race… until they trigger reactive hypoglycemia or nausea mid-effort.

34. Butter-Rich Cakes (Brownies, Pound Cake, Madeleines)

Flour + butter + sugar = slow digestion. Homemade cakes may be tempting, but they combine fat and refined carbohydrates with no performance benefit. Enjoy them after the race as a well-earned reward.

35. Ice Cream and Frozen Desserts

Fatty, sugary, cold, and often made with whole milk. Ice cream causes a thermal shock in the stomach, slows digestion, and can irritate an intestine already sensitized by pre-race anxiety.

36. Gummy Candies (Haribo, etc.)

Animal-derived gelatin takes a long time to digest. Artificial colorings and flavors can irritate the intestine. And gummy candies contain high-GI glucose-fructose syrup that triggers an uncontrolled glycemic spike.

37. Donuts and Fried Pastries

The worst of both worlds: refined sugar + frying. Hard to digest, with no useful nutritional value for the marathon, and a GI that will give you an energy crash 45 minutes after eating them.

38. Dark Chocolate (85%+) in Large Quantities

Paradoxically, very concentrated dark chocolate can cause problems before a race: it contains caffeine and theobromine (stimulants), as well as a non-negligible amount of fiber and fat. One square is fine. A whole bar, no.

39. Compotes and Jams with Added Fructose

Excess fructose (more than 25 g in a single serving) is poorly absorbed by the small intestine and ends up fermented in the colon. Osmotic diarrhea is guaranteed for runners who are sensitive to it.

Category 6 — Hard-to-Digest Proteins (Nos. 40 to 45)

The classic mistake of a runner afraid of running out of energy: loading up on protein the night before. Proteins are not glycogen. They won't be available as fuel during the race, and they slow digestion.

40. Fatty Red Meat (Ribeye, Côte de Bœuf, Lamb)

Very slow to digest (4 to 6 hours for a thick steak), fatty red meat puts heavy demands on the stomach and digestive enzymes. Avoid it in the 48 hours before the race. Opt for grilled chicken or lean fish instead.

41. Fried Eggs in Large Quantities

One or two hard-boiled or poached eggs on race morning may work for some accustomed runners. Eggs fried in oil or butter, no. The cooking fat slows gastric emptying.

42. Fatty Fish (Mackerel, Sardine, Herring) on Race Morning

Excellent for general health, fatty fish (omega-3) are however rich in fat. On a marathon morning, they risk staying in the stomach too long. Choose lean white fish the evening before, or nothing at all on race day.

43. Seafood and Shellfish

Mussels, oysters, shrimp: triple risk before a marathon. First, they are hard to digest. Second, the bacteriological risk (food poisoning) is not negligible, especially if the cold chain was not perfectly maintained. Finally, cross-allergies can appear under physical stress. Not the moment to experiment.

44. Sushi and Sashimi (Raw Fish)

Same bacteriological risk as seafood. A poorly preserved fish can trigger rapid food poisoning. The day before a marathon is truly not the time to try the sushi place around the corner.

45. Thick Steaks "to Stock Up on Iron"

Iron stores over weeks, not overnight. Eating a 300 g steak the night before a marathon won't raise your VO2max or your endurance. On the other hand, it will weigh on your night and your race morning.

Category 7 — "Healthy" Traps and Untested Foods (Nos. 46 to 50)

These last foods are particularly tricky because they give the impression of being good choices. Be careful.

46. Protein Bars Rich in Polyols and Fiber

Many commercial protein bars contain polyol-type sweeteners (sorbitol, maltitol, xylitol) to reduce sugar content. These polyols are laxative beyond a certain dose, and their effect varies between individuals. Read the labels carefully.

47. Sugar-Free Gum and Candies

Same problem: sorbitol, xylitol, mannitol. These sugar alcohols cause osmotic diarrhea in many people, especially under stress. Avoid chewing gum the night before and on race morning.

48. Supplements and Products Never Tested Before

You've heard about the benefits of beta-alanine, sodium bicarbonate, or a new algae-based energy gel? The night before or morning of a marathon is absolutely not the time to test anything new. Any supplementation can cause unpredictable digestive side effects.

49. Even Healthy Foods in Very Large Quantities

Eating 500 g of pasta the night before as part of carb-loading is fine. Eating 1 kg of whole-grain rice because "carbs are good" overloads your digestive system. Quantity matters as much as quality. A stomach that's too full at the start line is a source of mid-race nausea.

50. Any Food Never Tested in Training

The golden rule of marathon running, summarized in one sentence: never try anything new on race day. Not a new gel brand, not an unknown grain bread, not a type of cheese discovered in the race region. If you haven't eaten it before your long training runs, don't eat it on marathon morning.

Summary — The Table of 50 Foods to Avoid

What You SHOULD Eat Before a Marathon

Now that you know what to avoid, here is the short version of what actually works. For the complete guide, see our dedicated article on marathon nutrition.

D-3 to D-1: White pasta, white rice, white bread, boiled potatoes (not fried), grilled chicken, lean white fish, bananas, plain yogurt, fromage blanc. Regular hydration.

The night before: White pasta al dente with light tomato sauce + a little grilled chicken. A familiar, known, tested meal. Nothing new.

Race morning (2.5 to 3 hours before the start): Oatmeal with banana and honey, or white bread + peanut butter + banana, or white rice with a drizzle of olive oil. One to two glasses of water. Light coffee if you're accustomed to it.

FAQ — Frequently Asked Questions

Can you eat whole-wheat pasta before a marathon? No, not in the final days. Whole-wheat pasta contains far more fiber than white pasta, which can cause digestive issues. Go for regular white pasta during carb-loading.

Is even a tiny bit of alcohol really a problem? Yes. Even a single glass of wine fragments deep sleep and interferes with muscle glycogen resynthesis. This is not a moral judgment — it's physiology. Wait until after the race.

What about coffee in the morning? If you habitually drink it, a coffee on race morning may even slightly improve your performance (caffeine is ergogenic at doses of 3–6 mg/kg). But don't start if you're not in the habit, and don't exceed two small cups.

I'm lactose intolerant — should I avoid everything containing dairy? Yes, doubly so. Lactose-intolerant people should avoid all unfermented dairy products (milk, cream, fresh cheeses) in the 48 hours before the race. Yogurts and hard cheeses are generally better tolerated because fermentation reduces their lactose content.

What should I do if I accidentally ate something on this list? Don't panic. Hydrate well, take activated charcoal if you have it (it absorbs some toxins), and plan a very simple, very digestible next meal. And remember the lesson for next time.

Further Reading

• Marathon nutrition: complete guide from race week to race day
• Magnesium for runners: benefits, risks, and how to take it
• Recovery after a marathon: nutrition, sleep, and return to running
• Running in hot weather: adaptation and hydration

This article is provided for informational purposes only. In case of chronic digestive disorders or medical conditions, consult a doctor or sports dietitian before your marathon.