You've trained hard, meticulously prepared for the competition – and then, of all things, your gastrointestinal tract rebels. Runner's stomach, also known as gastrointestinal discomfort in endurance sports, is a surprisingly common problem: depending on the study, 30 to 90 percent of endurance athletes report digestive problems during or after training. Runners are particularly affected – the vertical impact makes the intestines especially vulnerable – but triathletes, cyclists, and other endurance athletes are also familiar with this phenomenon.

The symptoms range from mildly unpleasant to career-ending. Side stitches and slight nausea on the one hand, uncontrollable diarrhea and severe cramps on the other. Marathon legends have abandoned races due to stomach problems, triathletes have to use portable toilets during the run, and cyclists struggle with bloating and belching. The problem is real, widespread – and largely preventable.

The causes are varied and differ from person to person. Mechanical factors play a role: the repetitive impact of running, the pressure on the abdomen when cycling in an aero position, and the changes in body position during a triathlon. Physiological factors are key: during intense exertion, blood is diverted from the digestive organs to the working muscles – resulting in up to 80 percent less blood flow to the splanchnic region. Nutritional factors are the biggest factor: what, when, and how much you eat and drink before and during exercise significantly determines whether your stomach can handle it.

The good news: With the right knowledge and consistent training of your gastrointestinal tract, you can minimize or even eliminate gastrointestinal discomfort. Your digestive system is adaptable – you can train it to function under stress, just like you train your muscles and cardiovascular system. This guide gives you the tools to do so.

It's important to understand that occasional mild discomfort is almost unavoidable, especially during very long or very intense exertion. The goal isn't perfection, but rather to reduce symptoms to the point where they don't impair your performance and well-being. And, for those instances where something does go wrong, to have strategies in place to manage the situation.

To effectively combat gastrointestinal discomfort during exercise, you need to understand what's happening in your body. The mechanisms are complex, but the basic principle is simple: under stress, your body prioritizes muscles – at the expense of other systems, including digestion.

Splanchnic hypoperfusion is the technical term for the central problem. At rest, approximately 20 to 25 percent of the heart's output flows to the digestive organs. During intense physical exertion, this percentage drops to below five percent – ​​a reduction of up to 80 percent. Your intestines are literally no longer adequately supplied with blood. This leads to impaired nutrient absorption, slowed motility, and increased intestinal permeability.

Intestinal permeability – colloquially known as 'leaky gut' – increases under stress. The tight junctions that connect the intestinal cells become more permeable. This allows bacterial toxins (lipopolysaccharides) to enter the bloodstream, which can trigger systemic inflammatory responses. Heat intensifies this effect – which is why gastrointestinal problems are particularly common during competitions in hot conditions.

Mechanical stress is particularly pronounced during running. Every step jolts the internal organs. The intestines are shaken slightly with each impact. This explains why runners experience more gastrointestinal problems than swimmers or cyclists – even though the metabolic load can be comparable. The so-called 'runner's trot' (diarrhea during or after running) is directly linked to this mechanical stimulation.

Hormonal stress also affects digestion. During intense exertion, stress hormones such as cortisol and adrenaline rise, which can alter intestinal motility. Prostaglandins are released, which can trigger cramps. The autonomic nervous system switches from 'rest and digest' to 'fight or flight' – digestion is not a priority in this mode.

Dehydration exacerbates all problems. Even moderate dehydration reduces gastric emptying and increases the risk of nausea and cramps. At the same time, excessive fluid intake or hypotonic drinks lead to other problems such as hyponatremia and gastric overload. Finding the right balance is an art.

Gastrointestinal complaints in athletes can be divided into upper and lower GI symptoms. This distinction is important because the causes and treatment approaches differ.

Upper gastrointestinal (GI) disorders affect the stomach and esophagus. Nausea is particularly common during intense exercise and in hot weather. It arises from reduced blood flow to the stomach, delayed gastric emptying, and the accumulation of fluid or food in the stomach. Heartburn and acid reflux are common among cyclists (due to abdominal pressure in the aero position) and swimmers (due to the horizontal position). These occur when the lower esophageal sphincter doesn't close properly, allowing stomach acid to reflux. Bloating and gas often result from too much food or fluid in the stomach that isn't being moved quickly enough. Carbonated beverages are also a frequent trigger.

Lower GI disorders affect the small and large intestines. Abdominal cramps can originate in the intestines, but also in the abdominal wall muscles or the diaphragm. Side stitches are probably not a true GI disorder, but rather related to the diaphragm and ligaments, although they feel similarly unpleasant. Urgent bowel movements and diarrhea are the most dreaded symptoms – they can ruin competitions and are difficult to ignore. The cause is multifactorial: mechanical stimulation, stress hormones, reduced water absorption in the large intestine, and increased motility. Bloating and flatulence result from the fermentation of carbohydrates in the large intestine and swallowed air.

Timing and intensity strongly influence the symptoms. Upper GI symptoms typically appear early in training or competition – they are often related to the pre-event meal. Lower GI symptoms usually develop later, as the cumulative workload increases and bowel function becomes progressively impaired.

Individual variability is enormous. Some athletes can seemingly eat and drink anything and never have any problems. Others react to the slightest triggers. These differences have genetic, microbial, and training-related components. What works for your training partner might not work for you—and vice versa. This is what makes individual testing and adjustments so important.

What and when you eat before training or competition is the most important modifiable factor for gastrointestinal tolerance. The hours before exertion often determine whether your stomach cooperates or rebels.

Timing is crucial. The last large meal should be finished three to four hours before intense exercise. This gives the stomach time to empty. For most people, this means: a very early breakfast before a morning race, or a good last meal the evening before, and only a small snack in the morning. A light snack (100-200 kcal) one to two hours beforehand is well tolerated by most people. The closer to the exercise, the smaller and easier to digest the snack should be.

Fat and fiber dramatically slow down gastric emptying. The recommendation to eat a balanced diet doesn't apply to the pre-event meal. Here, you want easily digestible carbohydrates with little fat and fiber. White bread instead of whole grain, honey instead of nut butter, banana instead of an apple with the peel. This often feels "wrong" to health-conscious people, but it's the smarter choice before exertion.

Protein in moderate amounts is fine – it slows down gastric emptying less than fat. An egg with toast, some yogurt with a banana. But avoid protein-rich meals – large amounts of protein can cause stomach upset.

Problem foods before exercise are varied and individual. Generally risky: legumes (gas), cabbage and cruciferous vegetables, spicy seasonings (heartburn), sugar alcohols in protein bars (diarrhea), high doses of caffeine (problematic for some people), dairy products for those with lactose intolerance, and large amounts of fructose.

Drinking fluids before the start should ensure you're well-hydrated, but not on a full stomach. A good rule of thumb is 5-7 ml per kg of body weight two to four hours beforehand, then more as needed. Avoid large amounts right before the start – a sloshing stomach is uncomfortable and can impair performance.

Test everything in training. The golden rule of sports nutrition is: 'Nothing new on race day.' Your pre-event routine should be tried and tested dozens of times before important competitions. Every meal, every snack, every drink – test it under race-like conditions in training.

For exertion lasting 60 to 90 minutes, you need external energy and fluids. But this is precisely what poses a challenge for the stomach: absorbing nutrients while the body needs all its resources for the muscles. The good news: the gastrointestinal tract can be trained.

Carbohydrates are the primary fuel for endurance performance. Current recommendations for intense exercise lasting over two hours are 60-90 grams of carbohydrates per hour. For very long efforts (over three hours), up to 120 grams are even recommended. However, these amounts are only tolerated by athletes with a well-trained stomach. For untrained athletes, 30-40 grams per hour is more realistic.

Multiple transportable carbohydrates are key to high absorption. Glucose and fructose are absorbed via different transporters. The combination (typically 2:1 glucose to fructose) allows for higher absorption rates than either sugar alone. Sports drinks and gels utilize this principle. Maltodextrin behaves like glucose and is often used because it tastes less sweet.

Gut training – the systematic training of gastrointestinal tolerance – is crucial. Start with small amounts and increase gradually. If you currently tolerate 20 grams per hour, increase to 25, then 30, and so on. The training takes weeks to months, but the adaptation is real: absorption capacity, gastric emptying, and intestinal tolerance measurably improve.

Practical fueling tips: Consume carbohydrates regularly and in small portions – a few sips of sports drink or a gel with water every 15-20 minutes. Avoid large amounts at once. Choose liquids over solids during high-intensity exercise or in hot weather – gastric emptying is faster. Always combine gels with water, not with sports drinks (too concentrated). Try different brands and consistencies – not all stomachs tolerate all products.

Fluid intake should aim for 400-800 ml per hour, depending on sweat rate, intensity, and environmental conditions. Drinking more than one liter per hour is rarely beneficial and increases the risk of stomach problems and hyponatremia. Drinking according to thirst is a good approach in most situations.

Certain factors increase the risk of gastrointestinal problems during exercise. Some you can't change, but many you can. Understanding these risk factors will help you take targeted preventative measures.

Heat is one of the biggest risk factors. High temperatures exacerbate splanchnic hypoperfusion because additional blood flows to the skin (thermoregulation). Intestinal permeability increases more significantly in the heat. Dehydration sets in more quickly. Gastrointestinal problems are significantly more common during hot competitions. Countermeasures: Acclimatize, adjust pacing, use cooling, and prioritize fluid intake.

Intensity correlates with gastrointestinal (GI) issues. The higher the intensity, the greater the diversion of blood away from the intestines. During threshold training or above, digestive function is most impaired. This explains why many athletes experience fewer problems in training than in competition – the intensity is higher, the stress greater. Tactical pacing can help: Don't start too fast, avoid intensity spikes.

Nervousness and stress before a competition activate the sympathetic nervous system and can trigger gastrointestinal symptoms even before the start. Pre-race diarrhea is a classic phenomenon. Relaxation techniques, familiar routines, and mental preparation can help.

Many athletes take NSAIDs (non-steroidal anti-inflammatory drugs such as ibuprofen) before or during competitions. This is risky: NSAIDs increase intestinal permeability, impair the mucosal barrier, and increase the risk of gastrointestinal bleeding—especially during exercise. Avoid NSAIDs before and during sports.

Regular blood tests can help you identify underlying factors that contribute to GI problems. The DoctorBox Metabolic Check Plus measures inflammatory markers, liver and kidney function, and key metabolic parameters. Abnormal values ​​can indicate subclinical stress that impairs gastrointestinal function.

Microbial dysbiosis can increase susceptibility to gastrointestinal (GI) problems. A healthy, diverse microbiome appears to have a protective effect. In the long term, a fiber-rich, varied diet is beneficial – although fiber intake should be reduced before competitions.

Despite the best preparation, GI problems can occur. Knowing how to react in the moment can make the difference between giving up and making it through.

For nausea: Reduce the intensity until your stomach settles. Take small sips of water or cola. Temporarily avoid solid food. Keep your head up and breathe deeply. If it doesn't get better, take a break. Caution: Persistent nausea accompanied by confusion or coordination problems may indicate hyponatremia or heatstroke – seek medical help.

For side stitches: Reduce intensity, breathe rhythmically, step on the affected side while exhaling, place your hand on the spot and apply gentle pressure while exhaling. Side stitches are unpleasant but harmless – they will go away.

If you have an urgent need to defecate: significantly reduce the intensity of your bowel movements; this can lessen the urge. If a toilet is within reach, use it – wasting time is better than an accident. For competitions: scan the course for toilets and bring toilet paper if necessary. Some athletes train in the morning after their coffee to empty their bowels before the race.

If you experience bloating and cramps: reduce your pace and switch to flat terrain if possible. Avoid consuming highly concentrated carbohydrates. Peppermint oil capsules can have an antispasmodic effect (test beforehand!). Try to get rid of the gas – social conventions only apply to a limited extent in competition.

Mental strategies are important. GI symptoms during exercise are often cyclical – they come and go. The realization that 'this too shall pass' can help avoid panic. Distraction techniques, focusing on technique, self-talk – use what works for you.

When to stop: Vomiting blood, bloody diarrhea, severe and persistent abdominal pain, confusion, or disorientation are signs that something serious is happening. In these cases: Stop and seek medical help. Your health is more important than any race result.

The gastrointestinal tract is trainable – a finding that has revolutionized sports nutrition. Systematic gut training can dramatically improve gastrointestinal tolerance and allow for higher carbohydrate absorption during exercise.

The principle is progressive overload – just like with muscle training. You gradually challenge your stomach, it adapts, and you increase the challenge. Start with an amount of carbohydrates per hour that you can safely tolerate. Increase by five to ten grams every one to two weeks. Train at intensities that correspond to your race pace – your stomach needs to learn under realistic conditions.

Practical training plan: Phase 1 (weeks 1-4): Establish baseline. Consume 30-40 grams of carbohydrates per hour during two to three training sessions per week. Note your tolerance. Phase 2 (weeks 5-8): Gradually increase. Increase to 50-60 grams per hour. Experiment with different products. Phase 3 (weeks 9-12): Race simulation. Train with your planned race-day glycogen. Increase to 60-90 grams per hour if well tolerated.

Consistency is more important than intensity. Four weeks of consistent training are better than sporadic, aggressive attempts. The body needs time to adapt its absorption capacity and bowel function. This adaptation manifests as improved gastric emptying, increased intestinal transport capacity, and reduced symptom perception.

Train with the products you plan to use in the race. Different gels, bars, and drinks have different compositions and consistencies. Your stomach needs to learn exactly what it will receive during the race. Ideally, before an important race, you'll know which products will be offered at the aid stations—and have tested precisely those in training.

Keep a fueling diary. Note: What you ate, how much, when, the intensity, and any symptoms. Over several weeks, you'll recognize patterns – what quantities you can tolerate, which products work, and at what intensity it becomes problematic.

Certain sports and conditions place special demands on the digestive system. Here are specific strategies for the most demanding situations.

Ultra-endurance (over four hours) places a cumulative strain on the digestive system. The longer the exertion, the more the negative effects accumulate: intestinal permeability increases, inflammation intensifies, and nausea becomes more frequent. Strategies: Take it easy in the first few hours – high intake early on is riskier than later. Opt for real food: sandwiches, potatoes, and soup provide variety and are often better tolerated than endless gels. Schedule breaks: short walks allow for better digestion than constant running. Prioritize sodium: electrolytes become critical during very long exertion.

Triathlon combines three disciplines with different GI challenges. Swimming: A horizontal position and swallowed water can trigger nausea. Don't drink too much before the start. Cycling: The aero position compresses the abdomen. This is the window for nutrition – take advantage of it. Running: The highest mechanical load. Reduce nutrition intake and rely on the energy from the bike. Transitions are critical – abrupt changes in position can trigger nausea.

Heat exacerbates all GI problems. Intestinal permeability increases by up to 250 percent in the heat. Strategies: Heat acclimatization over 10-14 days also improves GI tolerance. Adjust pacing – in the heat, the same pace represents a higher relative intensity. Pre-cooling before starting (cold drinks, ice packs) can help. Ice slurry during exercise cools from the inside and provides hydration. Beware of overhydration – hyponatremia is paradoxically more common in the heat because people consume too much hypotonic fluid.

Altitude also affects digestion. At high altitudes, appetite is often reduced and GI function is impaired. Easily digestible food, smaller portions, and increased attention to hydration are important.

Why do I often have to go to the toilet when I'm running?

This is one of the most common problems among runners and has several causes. The mechanical impact of each step directly stimulates the intestines. Stress hormones released during exertion accelerate intestinal peristalsis. The reduced blood supply to the intestines impairs water reabsorption, resulting in looser stools. And the upright position promotes the natural downward movement of intestinal contents. Countermeasures: Schedule time for a toilet break in the morning before your run (coffee helps), eat a low-fiber pre-run meal, and choose routes with toilet access.

Does coffee help or harm you before exercise?

Both can be true – it's very individual. Caffeine is a proven ergogenic aid with a proven performance-enhancing effect. However, caffeine stimulates intestinal peristalsis and can trigger or worsen diarrhea in some people. Others use precisely this effect to empty their bowels before a competition. Test how you react during training. If coffee sends you to the toilet – great, use that strategically before the start. If it exacerbates GI problems during exercise, reduce the amount or abstain.

Are gels easier to digest than solid food?

At high intensity, yes; at lower intensity, not necessarily. Gels are transported from the stomach more quickly, which is important during intense exertion because gastric emptying is slowed down anyway. At moderate intensity (longer ultra events), solid foods can even be better tolerated and offer more satiety and psychological satisfaction. The optimal strategy depends on duration and intensity: the shorter and more intense the event, the more liquid/gel fuel is recommended. The longer and more moderate the event, the more solid food is needed.

What is the best fluid strategy for sensitive stomachs?

Small, regular amounts are better than large amounts all at once. A full, sloshing stomach causes nausea and delays gastric emptying. Drinking 150-200 ml every 10-15 minutes is better than half a liter every 30 minutes. The concentration should be isotonic or slightly hypotonic (6-8% carbohydrates). Hypertonic drinks or gels without water can draw water into the intestines through osmosis and cause diarrhea. Temperature: Cool drinks are often better tolerated and empty from the stomach more quickly than warm ones.

Should I change my diet before a competition?

Yes, but not drastically. One to three days before important competitions, a low-residue diet is advisable: less fiber, fewer legumes, and fewer foods that are difficult to digest. This reduces the volume in the intestines and minimizes fermentation. At the same time, you shouldn't introduce completely new foods. Carbohydrate intake can be increased (carbo-loading), but with easily digestible sources like white bread, pasta, rice, and potatoes—not whole grains and vegetables.

Can probiotics prevent runner's stomach?

The evidence is promising, but not conclusive. Some studies show that certain probiotic strains can reduce the frequency and severity of gastrointestinal (GI) symptoms in endurance athletes. Lactobacillus and Bifidobacterium strains, in particular, have been studied. The mechanisms could include improved intestinal barrier function, reduced inflammation, and a more favorable microbiome. If you want to try probiotics, start at least two to four weeks before the competition, not immediately before. And remember: not all products are created equal – the effect is strain-specific.

How do I deal with GI problems during an important competition?

Mental preparation is key. Accept that problems can arise and have a plan. Know where the restrooms are along the course. Carry emergency options (Imodium, although controversial). Remember: GI symptoms often come and go – perseverance can pay off. If symptoms occur: reduce intensity, avoid solid food, take small sips of water or cola. And: know your limits. Sometimes quitting is the wiser decision – there will be other races.

Do NSAIDs like ibuprofen affect bowel function during exercise?

Yes, very negative, and that's an important point. NSAIDs taken before or during exercise significantly increase intestinal permeability—further increasing the already elevated permeability caused by exertion. This raises the risk of GI bleeding. Kidney function, which is already stressed under exertion, is further burdened. Despite this, a shockingly large number of athletes take NSAIDs 'preventively' before competitions. Don't do this. If you need painkillers to be able to compete, you should reconsider participating.

Why can I tolerate more during training than in competition?

This is a common phenomenon with several explanations. The intensity in competition is typically higher – and the higher the intensity, the more digestion suffers. Mental strain increases stress hormones, which exacerbate GI symptoms. You often consume more carbohydrates in competition than in training. Environmental factors can be different in competition – heat, humidity, unfamiliar food. The solution: Train regularly under race-like conditions – pace, nutrition, mental stress. Race simulation runs are invaluable.

Are there any medications for runner's stomach?

There are options, but none should be the standard. Loperamide (Imodium) can stop diarrhea, but it carries risks and shouldn't be used routinely. Antacids can help with heartburn. Antispasmodics like butylscopolamine can relieve cramps. Some athletes swear by peppermint oil capsules as a natural alternative. Important: Medications should be tested in training, not for the first time in competition. And: Medications treat symptoms, not causes. The better strategy is to address the causes through nutrition, training, and pacing.