Runner’s stomach can derail your marathon. Discover 10 simple ways to prevent cramps, nausea and urgent toilet stops through smarter race-day fuelling.
There are few sights more worrying during a marathon than the queue for a portable toilet.
One minute, your pacing feels controlled and your fuelling plan is working. The next, your stomach cramps and finding the nearest toilet becomes considerably more important than hitting your target time.
Runner’s stomach – also known as runner’s trots or runner’s diarrhoea – is common among endurance athletes. Symptoms can range from mild bloating and an inconvenient urge to go to severe cramping, nausea, diarrhoea and vomiting.
Although you cannot control every digestive problem, many marathon toilet stops can be prevented by training your gut, identifying your personal triggers and rehearsing your nutrition before race day.
THE SHORT ANSWER
To reduce your risk of runner’s stomach during a marathon:
- Practise your race nutrition during long runs
- Use familiar carbohydrate products
- Avoid suddenly increasing your gel intake
- Reduce fibre before the race if you are particularly sensitive
- Keep race-morning meals relatively low in fat and fibre
- Test caffeine before marathon day
- Take standard gels with water
- Avoid starting too quickly
- Adjust your pace and fuelling in hot conditions
- Do not experiment with food, supplements or medication on race morning
The objective is not to empty your digestive system or remove entire food groups. It is to make race-day nutrition familiar and manageable.
WHAT IS RUNNER’S STOMACH?
Runner’s stomach is an informal term covering several gastrointestinal symptoms caused or aggravated by running.
These can include:
- An urgent need to have a bowel movement
- Loose stools or diarrhoea
- Abdominal cramps
- Bloating and excessive wind
- Nausea
- Vomiting
- Acid reflux
- Burping
- Stomach “sloshing”
- Loss of appetite
Lower gastrointestinal symptoms – particularly urgency, cramps and diarrhoea – are especially common in runners.
A 2025 systematic review concluded that gastrointestinal problems remain frequent in endurance sport and can negatively affect both performance and an athlete’s ability to consume enough carbohydrate and fluid. Read the systematic review.
WHY DOES RUNNING MAKE YOU NEED THE TOILET?
Runner’s stomach rarely has one single cause. It normally develops from a combination of physical movement, exercise intensity, nutrition, hydration and individual sensitivity.
Blood is redirected towards your muscles
During demanding exercise, your body prioritises the muscles, heart, lungs and skin.
Blood flow to the digestive system can decrease, making digestion and absorption more difficult. The effect becomes more pronounced as exercise intensity and body temperature increase.
This is one reason your normal breakfast might feel perfectly comfortable on a rest day but cause problems when followed by two hours of running.
Running physically moves the digestive system
The repetitive impact of running can stimulate the bowel and increase the sensation of urgency.
Cyclists and swimmers can experience gastrointestinal symptoms, but the continual movement associated with running creates an additional mechanical challenge.
Intensity increases digestive stress
Running harder places greater stress on the gastrointestinal system.
If your marathon pace is too ambitious – or you are carried away during the opening miles – your body may struggle to process the carbohydrate and fluid you planned to consume.
Your gel may receive the blame when pacing was the real problem.
Heat can make symptoms worse
Hot and humid conditions increase blood flow to the skin as your body attempts to regulate its temperature. This can place further strain on the digestive system.
Dehydration may compound the problem, but forcing excessive amounts of fluid can also leave your stomach feeling full and uncomfortable.
Food and fluid can become concentrated in the gut
Large quantities of carbohydrate, especially when consumed with insufficient water, can sit heavily in the stomach or draw additional water into the intestine.
Problems are more likely when runners combine several concentrated sources without calculating their total intake – for example, taking a gel with a carbohydrate drink and then immediately eating chews.
Pre-race nerves can stimulate the bowel
The nervous system and digestive system are closely connected.
Race-day anxiety can increase bowel activity, which explains why some runners visit the toilet repeatedly before the start despite following the same breakfast routine used in training.
10 WAYS TO PREVENT RUNNER’S STOMACH
1. TRAIN YOUR GUT DURING LONG RUNS
Your legs are not the only part of your body that needs marathon training.
The digestive system can become more accustomed to receiving carbohydrate and fluid during exercise when it is exposed to them regularly.
In a controlled study, two weeks of repeated carbohydrate intake during running reduced gastrointestinal symptoms and improved running performance compared with a placebo intervention. Read the gut-training study.
Begin with an amount you can tolerate and gradually work towards your intended marathon intake. Practise:
- The same breakfast
- The same gel or carbohydrate product
- The same amount of carbohydrate per hour
- Similar gel timings
- Drinking while running
- Fuelling at marathon pace
- Carrying and opening your products
Use several long runs as complete rehearsals. One successful gel during an easy 10K does not prove that six gels will feel comfortable during a marathon.
2. REHEARSE YOUR RACE-MORNING BREAKFAST
Your marathon breakfast should already have passed several tests in training.
Eat it before long runs and record:
- What you ate
- Portion size
- How long you waited before running
- Whether you needed the toilet
- How you felt during the run
- Whether you became hungry
- Any cramping, nausea or reflux
Most runners benefit from a carbohydrate-rich meal eaten approximately two to four hours before the start. However, the correct timing and quantity are individual.
Suitable options may include:
- White toast or a bagel with jam
- Low-fibre cereal with milk or a suitable alternative
- Porridge, if you already tolerate it
- Banana and toast
- Rice pudding
- Pancakes with syrup or honey
Race morning is not the time to copy another runner’s enormous bowl of porridge because it looked impressive on social media.
3. CONSIDER REDUCING FIBRE BEFORE THE RACE
Fibre is important for general health and normal bowel function. It should not be unnecessarily restricted throughout marathon training.
However, reducing fibre during the final 24–48 hours before a marathon may help runners who frequently experience urgency or diarrhoea.
This might involve temporarily swapping:
- Wholemeal bread for white bread
- Brown rice for white rice
- High-fibre cereal for a lower-fibre option
- Potatoes with their skins for peeled potatoes
- Large salads for smaller portions of cooked vegetables
- Beans and lentils for a familiar lower-fibre protein source
Do not make the change for the first time during race week. Test it before an important long run to see whether it actually helps you.
4. IDENTIFY YOUR PERSONAL TRIGGER FOODS
Common triggers reported by runners include:
- Large amounts of fibre
- High-fat or fried foods
- Spicy meals
- Excessive caffeine
- Alcohol
- Large portions of dairy for people with lactose intolerance
- Sugar alcohols such as sorbitol and xylitol
- Very large meals
- Unfamiliar supplements
This does not mean every runner needs to avoid every item on the list.
Keep a simple food and symptom diary during training. Look for patterns rather than blaming the last food you happened to eat.
5. DO NOT START A RESTRICTIVE LOW-FODMAP DIET WITHOUT GOOD REASON
FODMAPs are fermentable carbohydrates found in foods including certain fruits, vegetables, grains, dairy products and sweeteners.
Short-term reductions may help some athletes with recurring gastrointestinal symptoms. Small studies have reported improvements in symptoms such as bloating, loose stools and urgency when runners followed a lower-FODMAP diet. One endurance-running trial found greater symptom severity after a high-FODMAP day than a low-FODMAP day during exercise in the heat.
However, the evidence is not strong enough to recommend a restrictive diet to every runner. The 2025 systematic review found that low-FODMAP approaches appear promising but warned that unnecessary restriction may have nutritional disadvantages.
If symptoms persist, work with a registered dietitian – ideally one experienced in sports nutrition and gastrointestinal conditions – to identify specific triggers and reintroduce foods appropriately.
A low-FODMAP diet is a structured assessment tool, not automatically a permanent way of eating.
6. BUILD YOUR CARBOHYDRATE INTAKE GRADUALLY
Marathon runners are frequently told to consume more carbohydrate per hour. The advice can be appropriate, but suddenly attempting to absorb 90g per hour after training with 30g is likely to cause problems.
Work upwards gradually during long runs.
Count carbohydrate from every source:
- Gels
- Sports drinks
- Chews
- Bars
- Sweets
- Real food
For example, three 30g gels and a carbohydrate drink could push your intake considerably higher than expected.
Higher intakes are normally easier to tolerate when products combine different carbohydrate sources, such as glucose and fructose. But individual tolerance still matters.
The best carbohydrate target is not the largest number you can force down. It is the highest useful intake that you have repeatedly tolerated in training.
7. USE WATER AND SPORTS DRINKS CORRECTLY
Most standard energy gels are designed to be taken with water. This helps dilute the carbohydrate and may make it easier to tolerate.
Check the manufacturer’s instructions because isotonic gels are formulated differently and may not require additional water.
Be careful when taking a gel at an aid station:
- Choose water rather than carbohydrate drink if the gel is already concentrated
- Take several controlled sips rather than throwing a full cup towards your face
- Avoid swallowing large volumes at once
- Practise drinking while running before race day
Stomach sloshing does not always mean you need more water. It may mean fluid is not emptying from the stomach quickly enough because the pace, volume or carbohydrate concentration is too high.
8. TEST YOUR CAFFEINE STRATEGY
Caffeine can improve alertness and endurance performance, but it can also stimulate bowel activity in some people.
If your normal morning coffee reliably sends you to the toilet, this may be useful several hours before the start. Taking repeated caffeinated gels during the race could produce a very different result.
Calculate caffeine from:
- Coffee
- Tea
- Pre-workout supplements
- Energy drinks
- Caffeinated gels
- Chews or tablets
Test the complete quantity and timing during training. Do not add a highly caffeinated gel at mile 20 simply because another runner recommends it.
9. CONTROL YOUR PACE – PARTICULARLY IN THE HEAT
An aggressive start can make a well-practised nutrition plan suddenly feel intolerable.
Start at the pace you have trained for. If conditions are hotter than expected, accept that your planned pace and fluid requirements may need adjusting.
Taking more gels will not compensate for running beyond a sustainable intensity.
If nausea develops:
- Reduce the pace
- Stop taking additional concentrated carbohydrate temporarily
- Sip small amounts of water if appropriate
- Allow your stomach time to settle
- Seek medical assistance if symptoms become severe
Continuing to force food and fluid into a stomach that is no longer emptying effectively may make the problem worse.
10. AVOID UNTESTED MEDICATION AND SUPPLEMENTS
Some runners take ibuprofen or another non-steroidal anti-inflammatory drug before a marathon in an attempt to prevent pain.
This can increase gastrointestinal and kidney-related risks during prolonged exercise. A controlled study found that ibuprofen aggravated markers of exercise-induced intestinal injury in healthy athletes. Read the study.
Do not use painkillers preventatively around a marathon without appropriate medical or pharmaceutical advice.
The same principle applies to:
- Anti-diarrhoeal medication
- Antacids
- Magnesium supplements
- New probiotics
- Pre-workout products
- Concentrated electrolyte tablets
The NHS advises that loperamide should not be taken to prevent diarrhoea unless directed by a doctor because it has not been approved or adequately tested for preventative use. Read the NHS guidance.
Medication should not be used to disguise an unresolved digestive problem.
A SIMPLE PRE-MARATHON STOMACH PLAN
Three to seven days before
- Continue eating familiar meals
- Avoid experimenting with restaurants, supplements or new products
- Confirm your race breakfast and fuelling plan
- Check which products will be available on the course
- Pack your own fuel if you have not trained with the event’s products
During the final 24–48 hours
If you are prone to digestive symptoms:
- Reduce fibre moderately
- Choose familiar, easily digested carbohydrates
- Avoid unusually fatty, spicy or enormous meals
- Limit alcohol
- Continue drinking normally
- Do not force excessive quantities of water
- Spread carbohydrate across several meals and snacks
Carb loading should not mean overwhelming your stomach with one enormous pasta dinner.
On race morning
- Wake with enough time to eat and use the toilet
- Eat your tested breakfast approximately two to four hours before the start
- Follow your practised caffeine routine
- Sip fluid rather than drinking a large amount immediately before starting
- Allow time for a gentle walk or warm-up
- Use the toilet when you have the opportunity – even if the queue looks long
- Avoid untested snacks handed out in the start area
During the marathon
- Begin fuelling according to your rehearsed schedule
- Do not wait until you feel completely depleted
- Take standard gels with water unless instructed otherwise
- Count all sources of carbohydrate
- Adjust your pace for heat and humidity
- Avoid taking additional products merely because they are available
- Never try anything for the first time
FOODS THAT MAY BE EASIER BEFORE A MARATHON
| Often easier to tolerate | More likely to cause problems in sensitive runners |
|---|---|
| White rice | Large portions of brown rice |
| White bread or bagels | Dense wholegrain bread |
| Low-fibre cereal | Bran or very high-fibre cereal |
| Peeled potatoes | Potatoes with large amounts of skin |
| Banana | Large servings of dried fruit |
| Jam or honey | Products containing sugar alcohols |
| Plain pancakes | Fried breakfast foods |
| Small cooked vegetable portions | Large raw salads |
| Familiar low-fat meals | Rich, creamy or spicy meals |
These are examples, not universal rules. A food you tolerate repeatedly in training is normally a better choice than a supposedly perfect race food you have never tried.
WHAT IF YOU NEED THE TOILET DURING THE MARATHON?
Sometimes the best preparation still does not prevent an urgent stop.
If you need to go:
- Use an official event toilet where possible
- Do not ignore severe cramps or escalating urgency
- Restart gradually rather than immediately sprinting to regain lost time
- Reassess your pace
- Sip fluid if appropriate
- Return to your planned fuelling only once your stomach has settled
Losing several minutes is frustrating, but attempting to hold off significant diarrhoea for the remainder of a marathon is unlikely to improve your performance.
WHEN RUNNER’S STOMACH NEEDS MEDICAL ATTENTION
Occasional urgency limited to long or intense runs is often manageable through training and nutrition changes.
Speak to a GP or appropriately qualified healthcare professional if symptoms:
- Also occur when you are not running
- Are becoming more frequent or severe
- Regularly interrupt training
- Include unexplained weight loss
- Wake you during the night
- Are accompanied by persistent fatigue
- Continue despite sensible dietary changes
- Involve recurrent bowel leakage
Black or dark-red stools, bloody diarrhoea, severe abdominal pain, repeated vomiting or an inability to keep fluids down require urgent medical advice. The NHS advises contacting NHS 111 for bloody diarrhoea or black or dark-red stools.
Do not automatically assume blood in your stool is “just running”.
THE RUNNER’S JOURNAL VERDICT
Runner’s stomach is common, but it should not be accepted as an unavoidable part of marathon running.
The digestive system responds to training. Practising your breakfast, carbohydrate intake, hydration and caffeine strategy gives your gut repeated opportunities to adapt.
Most importantly, avoid copying somebody else’s fuelling plan without testing it.
The runner beside you may tolerate 90g of carbohydrate, three coffees and a caffeinated gel without a second thought. Your ideal strategy may look completely different.
Successful marathon nutrition is not about consuming the most fuel possible. It is about taking enough to support performance without exceeding what your body has been trained to tolerate.
Train your legs. Train your pacing. Train your gut.
RUNNERS STOMACH: FREQUENTLY ASKED QUESTIONS
Why do I need the toilet as soon as I start running?
Running can stimulate bowel movement through physical impact, changes in blood flow and nervous-system activity. Pre-run caffeine, breakfast timing and anxiety may also contribute.
Should I run fasted to prevent runner’s stomach?
Running without food may reduce symptoms for some short, easy sessions, but it does not prepare your digestive system to tolerate the carbohydrate required during a marathon. Regularly avoiding fuel can leave the gut unprepared for race day.
How long before a marathon should I eat breakfast?
Most runners eat approximately two to four hours beforehand. The ideal timing depends on portion size, start time and personal digestion. Rehearse the exact schedule during training.
Can energy gels cause diarrhoea?
They can contribute when too much carbohydrate is consumed at once, they are taken without sufficient water or the runner has not practised using them. Ingredients such as caffeine or certain sweeteners may also trigger symptoms in sensitive runners.
Should I avoid fibre before a marathon?
Not permanently. A temporary reduction during the final 24–48 hours may help runners who frequently experience urgency or loose stools. Test this before a long run rather than trying it during race week.
Can a low-FODMAP diet prevent runner’s stomach?
It may reduce symptoms in some runners, but it is restrictive and not necessary for everyone. Persistent symptoms should be discussed with a registered dietitian or healthcare professional before removing multiple food groups.
Can I take Imodium before a marathon?
The NHS advises against taking loperamide preventatively unless a doctor tells you to. Speak to a doctor or pharmacist rather than experimenting with it on race morning.
Why does runner’s stomach get worse in hot weather?
Heat increases the demand for blood flow to the skin and can intensify strain on the digestive system. Faster dehydration, greater fluid intake and running at an unsustainable effort can add to the problem.
Does needing a toilet stop mean my gel plan is wrong?
Not necessarily. Symptoms can also result from pacing, heat, anxiety, breakfast choices, caffeine, hydration or a combination of factors. Review the complete race-day strategy rather than blaming one gel.
REFERENCES AND FURTHER READING
- Mlinaric J, Mohorko N. (2025). Nutritional Strategies for Minimizing Gastrointestinal Symptoms During Endurance Exercise.
- Costa RJS, et al. (2017). Gut Training: The Impact of Two Weeks’ Repetitive Gut Challenge.
- Jeukendrup AE. (2017). Training the Gut for Athletes.
- de Oliveira EP. (2017). Runner’s Diarrhea: What Is It, What Causes It and How Can It Be Prevented?.
- Gaskell SK, et al. (2020). Impact of a High- and Low-FODMAP Diet on Gastrointestinal Symptoms During Endurance Exercise.
- Van Wijck K, et al. (2012). Aggravation of Exercise-Induced Intestinal Injury by Ibuprofen in Athletes.
- NHS. Common Questions About Loperamide.
- NHS. Bleeding From the Bottom.
This article provides general educational information and is not a substitute for individual medical or dietary advice. Persistent, severe or unexplained gastrointestinal symptoms should be assessed by an appropriately qualified healthcare professional.
