Track3D Guide

When Your Stomach Stops Cooperating in an Ultra

Trail & ultramarathon guidance

You can have every gram of carbohydrate planned, every bottle labelled and every aid station timed. None of that matters if, ten or twelve hours into the race, the thought of eating anything is the last thing you want to think of.

Stomach problems can turn an otherwise good ultra into a very long day.

Nausea, Bloating, Reflux, Cramping, Diarrhoea, That horrible sloshy feeling where you know there is food and fluid sitting in your stomach but absolutely nothing seems interested in moving anywhere useful.

And once it starts, the instinct is often to blame the last thing you ate.

Sometimes that is the problem.

Often it is more complicated than that.

Your stomach does not race in isolation.

Pace, heat, hydration, how much you are eating, what you are eating, how concentrated it is, how long you have been moving and what you trained your gut to tolerate can all change what happens next.

This Guide is not about finding one magic food that never causes nausea.

It is about understanding why stomach problems happen, building a fueling strategy your gut has actually practised, and having a plan for what to do when the race stops cooperating.

GI problems are common in ultras

If you have had stomach problems during an ultra, you are not some strange exception.

They are extremely common in long endurance events.

A systematic review of single stage ultra trail races found gastrointestinal symptoms reported in 65 to 82% of runners across the included studies. In one 161 km ultramarathon study, 96% of runners reported at least one gastrointestinal symptom, with nausea, belching and flatulence among the most common.

That does not mean severe nausea and vomiting should simply be accepted as part of ultrarunning.

It means the problem deserves the same attention as pacing, shoes or training load.

Common does not mean unavoidable.

And it definitely does not mean there is only one cause.

What runners mean when they say their stomach shut down

“My stomach shut down” is useful runner language, but it is not a medical diagnosis and the stomach does not literally switch itself off.

During prolonged exercise the body is trying to do several things at once.

Working muscles need blood. If it is hot, the skin needs more blood for cooling. The digestive system still has to receive, move, digest and absorb whatever you keep putting into it.

As exercise becomes harder or longer, blood flow to the gastrointestinal system can be reduced and gut function can change. Heat can make this worse. Running also adds repeated mechanical jarring that cycling, for example, does not produce to the same degree.

The result can be slower gastric emptying, bloating, reflux, abdominal discomfort, nausea, urgency and other symptoms.

Then there is the simple volume problem.

If you keep putting food and fluid into a stomach that is already struggling to move its contents onward, adding more can make the sensation worse.

The answer to a backed up stomach is not automatically to put more into it.

It is not always the food

One of the easiest mistakes after a bad race is to identify the last gel, sandwich or drink you remember eating and ban it forever.

Sometimes a particular food genuinely does not agree with you.

But consider what else was happening at the time.

Pace

Were you working harder than planned? Higher exercise intensity can increase gastrointestinal disturbance and may reduce how well you tolerate feeding. One of the most useful pieces of advice I got was when I asked my coach how do I know if I am going to hard and his answer was simple - if you can't eat you're going to hard.

Heat

Was the temperature climbing? Heat stress is strongly associated with greater gastrointestinal disturbance and symptoms.

Hydration

Were you behind on fluid, or were you forcing fluid faster than you could tolerate? Both can create problems.

Concentration

Were you stacking gels, concentrated drink and food without enough fluid for the products and quantities involved?

Duration

Did the same strategy work perfectly for four hours in training but fall apart at hour nine in the race?

Training

Did you actually practise eating this amount while running, or did race day become the first serious test?

A good post race review should look at the whole situation, not just the wrapper in your pocket.

The carbohydrate problem is not simply more or less

Carbohydrate matters in a long endurance event.

Underfuel badly enough and eventually there will be a performance cost.

But there is also a point where the amount, concentration or type of carbohydrate you are trying to ingest can exceed what your gut can comfortably process at that moment.

That creates an awkward reality.

The fueling target that looks best on paper is useless if you cannot tolerate it in the race.

Current research does not support the idea that carbohydrate itself is simply bad for the gut. In fact, carbohydrate intake during exercise may help protect aspects of gastrointestinal function.

At the same time, excessive carbohydrate intake or poorly tolerated formulations can contribute to delayed gastric emptying, malabsorption and symptoms.

So the question is not:

“What is the biggest carbohydrate number I can find?”

It is:

“What intake can I repeatedly tolerate while running at the effort, duration and conditions I expect on race day?”

That number may change as the race changes.

The amount you tolerate on a cool flat section early in the day may not be the amount you tolerate climbing in the heat twelve hours later.

Do not try to catch up with one huge hit

If you realise you are behind on fuel, it is tempting to throw a large amount into the system at once.

That can be exactly the wrong time to create another problem.

Steady, planned intake is normally easier to manage than repeatedly falling behind and then attempting to repay the entire debt at the next aid station.

The same applies when nausea has already started. Forcing a large amount of calories into a stomach that feels full and backed up is not automatically going to rescue the situation.

Real food versus gels is the wrong argument

There is a recurring debate in ultrarunning about whether real food is better than gels and sports nutrition products.

My personal experience is that there is a universal winner.

Gels and drink mixes can make carbohydrate intake simple, measurable and easy to carry. Real food can provide different flavours and textures and can be much more appealing after hours of sweetness.

Both can also cause problems.

Option	Potential advantage	Potential problem
Gels	Easy to measure, carry and consume	Sweetness fatigue, concentration and tolerance can become issues
Carbohydrate drink	Fuel and fluid can be delivered together	Mixing too strong or drinking beyond tolerance can create GI load
Chews	Small portions and easy pacing of intake	Can become difficult to chew or tolerate late in the race
Simple solid food	Different flavour and texture, often more satisfying	Fat, fibre, protein and larger portions can slow digestion or feel heavy
Aid station food	Variety and no need to carry everything	You may be trying food you did not practise with

The useful strategy is usually flexibility.

Have foods and products you know you tolerate. Have different flavour and texture options. Know which are easier for you when intensity is high and which work when you are moving more slowly.

Gels are not automatically the problem.

Real food is not automatically the cure.

You train the legs. Train the gut as well.

This is probably the most useful part of the research.

Gut training is exactly what it sounds like: repeatedly exposing the gastrointestinal system to the type and amount of food or drink you expect to use during exercise.

A systematic review of gut training studies found that repeated feeding during exercise can reduce gut discomfort and may improve carbohydrate absorption and gastrointestinal symptoms in some athletes.

The 2025 Sports Dietitians Australia and Ultra Sports Science Foundation position statement also identifies gut training and feeding tolerance work as evidence based strategies for managing exercise related GI problems.

That does not mean doing one long run with your race nutrition and ticking the box.

Treat it like part of the training block.

1Choose the likely race strategyProducts, foods, fluid and approximate intake.

2Start where you can tolerate itDo not jump from minimal training fuel to an aggressive race target overnight.

3Practise repeatedlyUse long runs and selected harder sessions to expose the gut to race fueling while moving.

4Practise the conditionsIf the race may be hot, hilly or long, test the strategy in relevant conditions where practical.

5Record what happensFuel, fluid, intensity, temperature and symptoms are more useful together than guessing later.

One important point: your normal training can hide a race day problem.

If most training runs are short enough that you barely eat, you may build excellent running fitness without ever testing whether your planned race nutrition still works after six, eight or ten hours.

You do not need to run 100 kilometres every weekend to train the gut.

You do need to give it repeated opportunities to practice the job you expect it to do.

Hydration can go wrong in both directions

When nausea begins, runners often decide they must be dehydrated and start drinking more.

Sometimes they are behind on fluid.

Sometimes they are not.

Both underhydration and overhydration can contribute to gastrointestinal symptoms.

Starting exercise significantly dehydrated can slow gastric emptying and increase nausea and upper abdominal discomfort. But drinking beyond individual tolerance can also distend the stomach and produce bloating, belching and regurgitation.

And there is a more serious reason not to respond to every problem by simply drinking more.

Excessive fluid intake during prolonged exercise can contribute to exercise associated hyponatraemia, a potentially dangerous fall in blood sodium concentration.

Important

Nausea does not tell you whether you are dehydrated, overhydrated or dealing with something else. Do not diagnose the cause from one symptom. If nausea is accompanied by confusion, severe headache, repeated vomiting, collapse, unusual swelling or a rapidly worsening condition, involve race medical staff.

The practical approach is to build a fluid plan around your own needs and conditions, then remain willing to adapt rather than forcing a fixed number regardless of what is happening.

Heat changes the equation

Heat is not just a hydration problem.

It can also become a gut problem.

As body temperature rises, more blood is required at the skin for cooling while exercise is already reducing blood flow available to the digestive system. Research consistently shows greater gastrointestinal disturbance when endurance exercise is performed in hot conditions.

This helps explain why a nutrition strategy that worked beautifully on a cool training morning can suddenly feel terrible in a hot race.

If the stomach starts failing in the heat, look at the whole heat problem.

Slowing down and cooling may be part of the fueling solution.

Do not keep treating the gut as though it is independent from the effort you are asking the rest of the body to produce.

FODMAP, fibre, fat and caffeine are things to test, not universal bans

When runners start searching for solutions to GI distress, the list of foods they are told to remove can become enormous.

Gluten. Dairy. Fibre. Fat. Caffeine. FODMAPs. Artificial sweeteners. Real food. Gels.

Pretty soon the fueling strategy consists of water and fear.

The evidence is more nuanced.

FODMAP

FODMAPs are a group of carbohydrates found in foods such as onions, garlic, some fruits, wheat, milk and certain sweeteners. In some people they are poorly absorbed in the small intestine, which can lead to bloating, gas, cramping, diarrhoea or urgency. For ultrarunners, reducing high FODMAP foods for a short period may help if GI problems are a recurring issue, but it is not something everyone needs to do.

Short term low FODMAP strategies can reduce exercise related GI symptoms in some susceptible athletes, particularly lower GI symptoms such as bloating, gas and urgency.

But the 2025 position statement does not support turning this into a permanent blanket restriction. Longer restriction may have nutritional and microbiome consequences and should be individualised.

If recurrent symptoms are significant enough that you are considering a restrictive diet, that is a good time to involve an Accredited Sports Dietitian rather than deleting entire food groups from a Google search.

Fibre and fat

Large amounts of fibre and fat close to or during running can be harder for some athletes to tolerate because they can slow digestion and increase gastrointestinal load.

That does not mean fat and fibre are bad foods. It means timing and individual tolerance matter.

Caffeine and polyols

A 2024 study of 44 runners in a 56 km trail ultramarathon did not find total FODMAP intake associated with GI symptoms, but did find associations involving caffeine before the race and fat and sorbitol intake during the event.

That study was observational, so it cannot prove those factors caused the symptoms.

It does reinforce the point that the problem may be more specific than simply “carbs upset my stomach”.

Gluten free

For non coeliac athletes, current evidence does not support a gluten free diet as a general strategy for preventing exercise related GI problems.

If gluten is a genuine medical problem for you, that is different. If it is not, removing it because another runner did so is not evidence.

When nausea starts during the race

The worst time to invent a stomach strategy is while standing at an aid station feeling sick.

Have a simple troubleshooting process.

Question	What to consider
Am I going too hard?	Reduce effort. Walking a climb or easing pace can reduce the stress competing with digestion.
Am I overheating?	Use appropriate cooling, shade and race heat strategies rather than treating the stomach alone.
Does my stomach feel full or sloshy?	Do not keep stacking large amounts of food and fluid on top of a stomach that already feels overloaded.
Have I been drinking appropriately?	Review the pattern. Both too little and too much fluid can create problems.
Did I change products or foods?	Return to familiar options if practical.
Is this flavour fatigue or actual nausea?	A different taste or texture may solve food aversion. It will not solve a physiological GI problem.
Can I tolerate a smaller amount?	Once symptoms settle, restart with familiar intake in smaller portions rather than trying to catch up all at once.

Then give the change a chance to work.

If you slow down for five minutes and then immediately hammer the next climb, you have not really tested whether reducing intensity helped.

The same goes for cooling and intake.

Do not keep doing more of the thing that is clearly not working.

Change one useful variable, give it time, then reassess.

If vomiting starts

Vomiting is where this stops being just an annoying fueling problem.

One episode can happen in an ultra and some runners recover.

Repeated vomiting is different.

You are losing fluid and whatever nutrition you are trying to take in. Continuing to force food and drink while repeatedly vomiting can make the situation worse and makes it increasingly difficult to meet basic hydration and energy needs.

If vomiting persists, the cause is unclear, you cannot keep fluid down, or your overall condition is deteriorating, involve race medical staff.

Do not assume the answer is automatically more salt, more water, more calories or a random medication offered by another runner.

Medication is not a substitute for diagnosis

Current exercise gastroenterology guidance does not support treating pharmaceutical interventions as a general solution to exercise related GI problems.

NSAIDs such as ibuprofen have also been shown to aggravate exercise related intestinal injury. Race medical advice comes before somebody else's improvised trail pharmacy.

Build a fallback plan before race day

Your primary fueling plan should not be your only fueling plan.

Before the race, decide what you will do if the foods and products you expected to use become difficult to tolerate.

Different texture

If sweet gels become unappealing, what familiar liquid, soft or solid option have you already tested?

Different flavour

Have both sweet and savoury options rather than discovering flavour fatigue after ten hours.

Lower intensity

Know that the correct response may be to walk or slow down while the gut recovers rather than protecting a split.

Cooling plan

If the race can be hot, know where cooling, ice, water and shade are available.

Crew instructions

Tell crew what your normal fallback options are so they do not present twenty random foods when you are nauseous.

Medical boundary

Agree on the symptoms that mean problem solving stops and race medical becomes the priority.

This is also where the Race Plan helps.

If you know the next leg is a long exposed climb in the heat, the decision at the aid station is different from leaving for a cool runnable descent.

Food, fluid and effort should make sense for the section you are about to race, not only for the hour you just completed.

Know when this is no longer a fueling problem

Most exercise related GI symptoms are unpleasant rather than dangerous.

Some are not.

Seek race medical support for symptoms such as persistent or repeated vomiting, inability to keep fluids down, blood in vomit or stool, severe or worsening abdominal pain, confusion, collapse, serious weakness, altered behaviour, significant breathing or chest symptoms, or suspected heat illness.

Nausea, vomiting and headache can also occur with exercise associated hyponatraemia. Symptoms alone cannot reliably distinguish it from dehydration, heat illness or other race problems, which is exactly why guessing can be dangerous.

A stomach problem can be something to manage.

A deteriorating runner is a medical problem.

What the research says

The most useful recent development is that exercise gastroenterology is moving away from one size fits all advice.

The 2025 joint position statement from Sports Dietitians Australia and the Ultra Sports Science Foundation concludes that exercise related GI symptoms are influenced by multiple physiological, nutritional and environmental factors. Strategies with the most consistent support include appropriate carbohydrate and protein intake, maintaining appropriate hydration, short term FODMAP manipulation for selected athletes, and gut training or feeding tolerance work.

A separate 2025 systematic review of 29 studies reached a similar conclusion. Gut training looked promising, carbohydrate type and amount mattered, low FODMAP strategies may help some athletes, while hydrogel carbohydrate products did not show a clear GI advantage over standard products and probiotic results were mixed.

The gut training evidence is not perfect. A 2023 systematic review found only eight eligible studies and noted substantial variation in methods. But several studies showed reductions in gut discomfort and carbohydrate malabsorption after repeated feeding exposure during exercise.

Ultramarathon specific field studies add an important reality check.

GI symptoms are common, but different runners can reach the same symptom through different routes. A 2024 56 km trail study did not find total FODMAP intake or measured carbohydrate malabsorption clearly associated with symptoms. A 2026 Marathon des Sables qualitative study also found large differences between individual runners, with nausea and vomiting affecting race performance in some participants.

The evidence does not point to one perfect ultramarathon diet.

It points toward individual tolerance, repeated practice and understanding the conditions that make your own gut fail.

A practical stomach strategy checklist

Before the race	During the race	If problems start
Practice the intended carbohydrate intake	Fuel steadily rather than repeatedly falling behind	Reduce intensity and reassess
Test gels, drinks and real food	Use familiar products and foods	Check heat and cooling
Train in relevant heat where practical	Adjust to conditions rather than blindly following numbers	Review fluid intake without automatically drinking more
Test aid station foods you may rely on	Watch for early fullness, bloating or nausea	Avoid adding large boluses to a backed up stomach
Have sweet and savoury options	Keep the next section in mind	Restart familiar intake gradually if symptoms settle
Know your fallback plan	Tell crew when the plan changes	Use race medical support when symptoms are persistent or concerning

Common questions

Why do I get nauseous in races when the same nutrition works in training?

Race duration, intensity, heat, hydration, nerves and total feeding load may be different from training. A strategy that works for four hours may not have been tested deeply enough for a twelve or twenty hour race. Review the entire race context rather than assuming one food suddenly became the problem.

Should I stop eating if I feel nauseous?

Do not blindly force the planned intake into a stomach that already feels overloaded. Reduce effort, assess heat and hydration, and give the gut a chance to settle. If symptoms improve, restart familiar intake gradually. Prolonged complete underfueling also creates problems, so the goal is recovery of tolerance rather than simply deciding not to eat for the rest of the race.

Are gels worse for the stomach than real food?

Not universally. Some runners tolerate gels and carbohydrate drinks very well, while others prefer more solid food. Real food can also create problems when portions are large or high in fat or fibre. Test the exact foods and products you intend to use while running.

Does a hydrogel carbohydrate product prevent stomach problems?

Current systematic review evidence does not show a clear gastrointestinal advantage for hydrogel carbohydrate products over standard carbohydrate products. Individual runners may still prefer or tolerate one product better than another.

Should I use a low FODMAP diet before an ultra?

Short term low FODMAP strategies can reduce symptoms in some athletes who are susceptible to lower GI problems, but it is not a universal recommendation and long term restriction may have downsides. If symptoms are recurrent or severe, work with an Accredited Sports Dietitian rather than starting a restrictive diet without guidance.

Can I train my stomach to handle more carbohydrate?

There is evidence that repeated feeding during exercise can improve tolerance and may reduce carbohydrate malabsorption and GI symptoms in some athletes. The practical approach is to build toward the intended race intake progressively and repeatedly during training rather than testing the full strategy for the first time on race day.

Is vomiting just part of a 100 miler?

It can happen, but repeated vomiting should not be treated as harmless or inevitable. It prevents effective fueling and hydration and can also occur with more serious conditions. If vomiting persists, you cannot keep fluids down, or your condition is deteriorating, involve race medical staff.

Can too much water make me nauseous?

Yes. Drinking beyond your individual tolerance can cause stomach distension and symptoms, and excessive fluid intake during prolonged exercise can contribute to exercise associated hyponatraemia. Nausea alone does not reveal whether you have had too much or too little fluid, so do not diagnose hydration status from one symptom.

The final thought

A race nutrition plan should give you structure.

It should not become a contract you are afraid to change.

If your stomach starts objecting, do not immediately panic and do not keep forcing the exact same strategy because a spreadsheet says you are due another 30 grams of carbohydrate.

Slow the situation down.

Look at the effort.

Look at the heat.

Look at what you have eaten and drunk.

Work out whether the stomach is overloaded, underfuelled, overheating, poorly hydrated, reacting to a particular food, or simply being asked to do something you never trained it to do.

The goal is not to win an argument with your stomach.

The goal is to understand the problem early enough that you still have options.

Then solve the next problem.

Plan the course, the checkpoints and the problems you may need to solve.

Use Track3D Race Planner to build your course, checkpoint timing and race strategy. Then use your training to work out what fueling and hydration your gut can actually support along the way.

Open Race Planner

Research notes and sources

Research snapshot: 28 September 2026. Exercise related gastrointestinal symptoms are highly individual. This Guide summarises current research for practical race planning and is not medical or individual dietetic advice. Persistent or severe symptoms should be assessed by an appropriate health professional.

Source	Why it matters here
Costa et al., 2025 — Sports Dietitians Australia and Ultra Sports Science Foundation Joint Position Statement	Current practitioner guidance covering exercise induced gastrointestinal syndrome, carbohydrate, hydration, heat, FODMAP strategies, gut training and individual assessment.
Mlinaric & Mohorko, 2025 — Nutritional strategies for minimizing gastrointestinal symptoms during endurance exercise	Systematic review of 29 studies covering gut training, carbohydrate formulations, low FODMAP diets, hydrogel carbohydrate and probiotics.
Martinez et al., 2023 — The Effect of Gut Training and Feeding Challenge on Gastrointestinal Status	Systematic review of repetitive feeding and gut training studies showing potential improvements in discomfort, carbohydrate malabsorption and symptoms.
Convit et al., 2024 — FODMAP intake, carbohydrate malabsorption and gastrointestinal symptoms during a 56 km trail ultramarathon	Ultramarathon field study showing the complexity of symptom relationships and highlighting possible roles for caffeine, fat and sorbitol that require further study.
Hoffman & Fogard, 2015 — Gastrointestinal distress is common during a 161 km ultramarathon	Field evidence showing very high symptom prevalence and the importance of nausea among both finishers and nonfinishers.
Arribalzaga et al., 2021 — Carbohydrate intake and GI problems in single stage ultra trail races	Systematic review showing frequent GI symptoms and the importance of balancing carbohydrate intake with gut tolerance and training.
Ryan, Daly & Ryan, 2026 — Nutrition practices and race experiences at the Marathon des Sables	Recent ultra endurance qualitative research showing large individual differences and performance effects from nausea and vomiting.
Tiller et al., 2019 — ISSN Position Stand: nutritional considerations for single stage ultramarathon training and racing	Ultramarathon specific nutrition guidance including carbohydrate intake, gastrointestinal distress and practical race fueling considerations.
Armstrong et al., 2025 — Exercise Associated Hyponatraemia: symptoms, severity and sport specificity	Recent review showing that nausea and vomiting can overlap with potentially serious hyponatraemia and cannot safely be diagnosed from symptoms alone.
Ultra Running Magazine — Practice What You Eat: Prepare Your Gut for an Ultra	Practical ultrarunning perspective on rehearsing race nutrition during training rather than leaving the first full test until race day.

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