“More marathons are won or lost in the porta-toilets than at the dinner table.” — Bill Rodgers, four-time Boston and New York Marathon winner

Research estimates that 30–90% of endurance runners experience gastrointestinal distress during training or racing, depending on conditions and distance. The good news: the majority of cases are preventable. The core prevention framework is straightforward: avoid high-fibre, high-fat, and high-protein food in the 3 hours before running; test every gel, drink, and supplement in training before race day; drink to thirst rather than to a fixed schedule; and build your gut’s tolerance to running-intensity digestion progressively. What follows is the mechanism, the evidence, and the practical protocol.

Use our Pace Calculator to identify your training paces. Higher-intensity sessions increase GI risk, so knowing when you’re pushing into threshold territory helps you plan fuelling more carefully.

Why Running Causes Stomach Problems: The Physiology

Understanding why running causes GI distress makes the prevention strategies obvious rather than arbitrary.

Blood Flow Redistribution

During hard running, your body prioritises oxygen delivery to working muscles and the cardiovascular system. At high intensities, up to 80% of the blood flow that normally supports the gastrointestinal system is redirected away from the gut. The result: the gut receives less oxygen, gut wall permeability increases (intestinal lining becomes more permeable sometimes called “leaky gut”), and normal digestive function is impaired.

The symptoms that follow nausea, cramping, urgency, and in severe cases bloody stools are downstream of this mechanical blood flow reduction, not primarily caused by food choices alone.

The training adaptation: Consistent endurance training improves the gut’s tolerance to this blood flow reduction. Well-trained runners experience less severe GI disruption at equivalent intensities than beginners — the gut adapts, just like muscles do. This is the basis of “training the gut,” covered below.

Mechanical Jostling

Running’s repetitive vertical impact physically stimulates the intestines in a way that cycling or swimming does not. This mechanical stimulation accelerates gut motility the speed at which contents move through the GI tract which is why runners’ diarrhea occurs far more frequently in runners than in equivalent-effort cyclists. The jostling effect is most pronounced at faster paces and on harder surfaces.

Hormonal and Nervous System Response

Exercise stress triggers the release of hormones, including adrenaline and cortisol, which directly stimulate gut motility. This is partly why pre-race anxiety is a genuine contributor to pre-race bathroom urgency — the stress response and the exercise response both activate the same pathway. Racing conditions reliably produce more GI symptoms than equivalent training runs for this reason.

Acid Reflux at High Intensity

High-intensity running can temporarily reduce lower oesophageal sphincter pressure, allowing stomach acid to travel upward. This produces the burning sensation or bitter taste some runners experience during a mid-tempo run. It’s more common in runners who eat close to a hard session and in those prone to reflux generally.

Foods to Avoid Before Running

Dietary choices in the 12–24 hours before a long run or race significantly influence GI risk. The categories to avoid are predictable from the physiology: anything that slows gastric emptying (fat, protein, fibre) or increases gut motility (caffeine in excess, some artificial sweeteners) creates problems when blood flow to the gut is already compromised.

In the 12–24 hours before a long run or race, reduce or avoid:

• High-fibre foods: Brassica vegetables (broccoli, cauliflower, cabbage), raw salads, legumes (beans, lentils, chickpeas), high-bran cereals, seeded bread. Fibre accelerates motility and increases gas production — exactly what you don’t want at pace.
• High-fat foods: Fried food, fatty meat, full-fat cheese, cream sauces, large portions of avocado. Fat slows gastric emptying significantly, meaning food remains in the stomach well into the run.
• Spicy food: Stimulates GI motility directly and can cause heartburn at running intensity.
• Large protein portions at the pre-run meal: Protein slows gastric emptying and adds little to immediate energy availability.
• Lactose (if sensitive): Dairy causes cramping and loose stools in lactose-sensitive runners, which is more common than most people realise. If you regularly experience bloating after milk or cheese, treat this as a pre-run avoidance item.
• Concentrated fruit juice: High fructose load without accompanying fibre can cause osmotic diarrhea.
• NSAIDs and aspirin: Ibuprofen, naproxen, and aspirin directly irritate the gut lining and compromise the intestinal barrier. They also have interactions with dehydration that increase acute kidney injury risk during endurance events. Avoid for 24 hours before any long run or race.

For a complete pre-run meal guide covering what to eat (not just what to avoid) and timing by window, see our what to eat before a long run guide.

In-Run Fuelling and GI Risk

Most race-day GI problems originate in fuelling decisions, not in underlying gut pathology. The two most common fuelling errors:

1. Gels Without Enough Water

Energy gels are hypertonic — their carbohydrate concentration is higher than that of blood plasma. Swallowing a gel without water means the gut must draw fluid from the body to dilute it before absorption can proceed, which slows gastric emptying and causes the cramping, sloshing, and nausea that many runners attribute to the gel brand itself. Always chase every gel with 150–200ml of water. Do not take a gel at the same time as a sports drink — the combined concentration is too high.

2. Exceeding Gut Absorption Capacity

The gut can absorb a maximum of approximately 60g of carbohydrate per hour from glucose-based sources alone. Above this ceiling, carbohydrate accumulates in the gut and ferments, causing cramping, bloating, and diarrhea.

The way to exceed 60g/hour without GI consequences is to use a combination of glucose and fructose, which use separate intestinal transport pathways (SGLT1 for glucose, GLUT5 for fructose). Research by Dr. Asker Jeukendrup established that this dual-transporter approach allows absorption of up to 90g of carbohydrate per hour — achieved through a combination of gels and isotonic drinks across the hour, not from a single hypertonic concentrated drink.

Practical implication: If you’re targeting 60–90g of carbohydrate per hour in a marathon (appropriate for 3+ hour efforts), that might be one gel (20–25g) plus 500ml of isotonic sports drink (15–20g) per 30 minutes — not a single concentrated product at maximum carbohydrate content.

For a full breakdown of carbohydrate sources and their GI implications, see our best carbohydrates for runners guide and our sports nutrition guide.

3. High-Fructose Single-Source Products

Some cheaper sports drinks use high-fructose corn syrup as the primary carbohydrate source. At high concentrations, fructose draws water osmotically into the gut, causing diarrhea. Look for products that specify a glucose: fructose ratio not pure fructose or high-fructose corn syrup.

Hydration and GI Distress

Dehydration worsens GI symptoms by further reducing blood flow to the gut. Overhydration with plain water can cause hyponatraemia (low blood sodium) and GI cramping. The balance: drink to thirst, not to a fixed schedule, replacing fluid losses with sodium-containing drinks on runs over 90 minutes.

For specific hydration guidance, including how to calculate your personal sweat rate, see our running hydration guide. For the electrolyte strategy, our sports drinks vs electrolyte tablets guide covers the comparison in full.

Pre-Run Routine: Managing Bowel Urgency

Runner’s diarrhea the urgent need to defecate during a run, is one of the most common and least discussed runner problems. It’s caused by the combination of mechanical jostling, exercise-hormone-stimulated GI motility, and often anxiety. For most runners, it’s manageable with a deliberate pre-run routine.

What works:

Time your coffee strategically. Caffeine stimulates bowel motility, which works in your favour if you’re a regular coffee drinker and time it 30–45 minutes before you need to be at the start line. Establish a consistent morning ritual and your body will adapt to it.

Allow adequate time before hard sessions. The most reliable way to avoid mid-run urgency is to give your GI system time to complete its morning cycle. This typically means waking 60–90 minutes before a long run, having coffee and a small breakfast, and visiting the bathroom before heading out.

Know your route. For long training runs in areas without public facilities, plan your route to pass accessible toilets at 8–10km intervals, or carry a small supply of tissue. This removes the anxiety component — which itself worsens GI symptoms — and provides genuine practical backup.

Pre-race logistics: At major races, the bathroom queues are long and the start corrals close early. Factor bathroom time into your pre-race timeline as deliberately as you would breakfast timing. Arriving at a race without having visited the bathroom is the most avoidable GI emergency in running.

Training the Gut: The Most Underused Strategy

The gut is trainable; its ability to absorb carbohydrates during running, tolerate high-intensity blood flow reduction, and manage fuelling volume all improve with systematic practice. This is one of the most important and underused concepts in recreational runner nutrition.

How gut training works:

Consistently consuming carbohydrates during long training runs at race pace — using the same products, at the same intervals, at the same intensity you’ll use on race day — produces two adaptations:

1. Increased intestinal carbohydrate transport capacity. The density of intestinal glucose and fructose transporters increases with repeated exposure, allowing higher absorption rates over time.
2. Reduced gut sensitivity to exercise-related blood flow changes. Trained guts experience less wall permeability disruption at equivalent exercise intensities.

Practical protocol: Start practising race-day fuelling from week 8–10 of a marathon build. Use your target gel brand at your target interval (every 30–45 minutes) on runs of 75+ minutes at or near marathon pace. Adjust products if you experience consistent symptoms — individual response varies significantly between gel brands and formulations.

If you’re following a structured marathon training plan, the long runs in weeks 10–16 are your gut training sessions. Don’t skip fuelling on these runs thinking you’ll “toughen yourself up” — you’ll arrive at race day with an untrained gut, not a toughened one.

What to Do When Your Stomach Goes Mid-Run

Prevention is the priority, but even well-prepared runners experience GI distress. Here’s how to manage it when it happens:

Slow down. Blood flow to the gut increases at lower intensities. Dropping pace by 30–60 seconds per kilometre for 5–10 minutes often allows symptoms to settle enough to resume goal pace. Attempting to push through sharp GI distress at race pace almost always makes it worse.

Switch to plain water only. If you’ve been taking gels and experience cramping or nausea, skip the next gel and drink plain water at the next aid station. Give your gut 10–15 minutes to clear before reintroducing carbohydrates.

Walk through aid stations. Walking while consuming fuel significantly improves gastric emptying versus running while eating or drinking. If you’re at an aid station and feeling symptomatic, walk the 50–100m through it rather than grabbing on the run.

If urgency becomes severe, leave the course and find a facility. Attempting to continue with severe cramps or urgency typically results in either a DNF later or a race-day experience that makes you less likely to return to the sport.

Avoid ibuprofen mid-race. Some runners take NSAIDs for pain management during long races. This compounds GI distress by directly irritating the gut lining at a point where blood flow is already reduced. It also increases kidney strain under dehydration. Don’t do it.

Post-Run Stomach Recovery

Acute GI symptoms typically resolve within 30–60 minutes of finishing, as blood flow returns to the gut and stress hormones clear.

For faster recovery:

• Don’t eat immediately after finishing if nauseous. Start with plain water or a light electrolyte drink. Wait 20–30 minutes before attempting solid food.
• Ginger. Ginger tea or ginger chews have a reasonable evidence base for reducing nausea specifically. Worth keeping in a race bag.
• Gradually reintroduce carbohydrates. A ripe banana, 20–40 minutes after finishing, is usually the most digestible option for the post-run window.
• Avoid high-fat or high-fibre foods for 1–2 hours post-run if your gut is still irritated — the digestive system is slow to normalise after a hard, long effort.

If symptoms (cramping, diarrhea, or nausea) persist for more than 2 hours post-run, or if you notice blood in your stools, seek medical attention.