Why Does Exercise Upset My Stomach? Gut Symptoms in Athletes

You’re halfway through a run and your stomach starts cramping.

You feel nauseous during a hard training session. You get reflux when you exercise. You urgently need the bathroom every time you run. Or perhaps bloating makes training uncomfortable enough that you’ve started changing what you eat beforehand.

Gut symptoms during exercise can be incredibly frustrating, particularly when you’ve put time and effort into your training.

And when they keep happening, it’s easy to start experimenting.

Maybe you stop eating before training. Cut out dairy. Reduce gluten. Avoid fibre. Try different sports drinks. Change your breakfast. Or accept that having an upset stomach is simply part of your sport.

There can be many reasons why gastrointestinal symptoms occur during exercise, and the solution will depend on what is actually contributing to yours.

Working with a sports dietitian can help you identify likely triggers and develop a nutrition and fuelling strategy that supports both your gut comfort and your performance.

Why do I get stomach problems when I exercise?

Gastrointestinal symptoms are particularly common in endurance sports such as running, cycling and triathlon, although athletes participating in many different sports can experience them.

Exercise itself can affect the gastrointestinal system.

During harder or prolonged exercise, blood flow is redirected towards the working muscles. Movement and mechanical impact can also contribute, particularly with running. Hydration, exercise intensity, environmental conditions, stress and what you’ve eaten or drunk can all influence how your gut responds.

Common symptoms include:

• bloating
• abdominal cramping
• nausea
• reflux or heartburn
• stomach discomfort
• excessive gas
• an urgent need to use the bathroom
• loose bowel motions or diarrhoea
• vomiting in more severe cases.

For some athletes, symptoms occur occasionally. For others, they happen so consistently that they start influencing how, when and even whether they train.

That’s when it’s particularly useful to investigate what is happening.

What is “runner’s gut”?

“Runner’s gut” or “runner’s stomach” are common terms used to describe gastrointestinal symptoms associated with running.

You might experience abdominal cramping, nausea, urgency or diarrhoea during or after a run.

Running can be particularly challenging for the gastrointestinal system because of the combination of exercise intensity, changes in blood flow and the repetitive mechanical movement involved.

But having gut symptoms doesn’t mean you simply have to accept them as part of being a runner.

A sports dietitian can help you look for patterns around when symptoms occur, what you’ve eaten and drunk, the type of training you’re doing and other factors that may be contributing.

Could what I eat before exercise be causing my symptoms?

Potentially.

What you eat before training can influence how your gastrointestinal system feels during exercise.

But the answer usually isn’t as simple as identifying one “bad” food.

The amount you eat, how close you eat to training, the composition of the meal, your usual diet and your individual tolerance can all matter.

A meal that works perfectly well three hours before training might feel very different when eaten 30 minutes before a hard run.

A sports dietitian can help you explore what you’re eating, how much you’re eating and when you’re eating it, rather than unnecessarily removing foods from your diet.

This can be especially valuable when you’ve already tried several different approaches and still aren’t sure what’s causing your symptoms.

Should I avoid fibre before exercise?

Fibre is important for overall health and gut function.

However, higher-fibre foods can take longer to digest and may contribute to gastrointestinal discomfort for some athletes when eaten close to demanding exercise.

This doesn’t mean athletes need to follow a low-fibre diet.

Instead, timing and individual tolerance matter.

Depending on your symptoms and sport, a sports dietitian may explore whether adjusting the amount or type of fibre in meals before particular sessions or competitions could help.

For many athletes, the goal is to make targeted adjustments around exercise while maintaining a varied and nutritionally adequate diet overall.

Can dehydration cause stomach problems during exercise?

Hydration can play a role in gastrointestinal comfort.

Starting exercise inadequately hydrated or losing substantial amounts of fluid during prolonged activity can contribute to both reduced performance and gastrointestinal symptoms.

At the same time, simply drinking as much water as possible isn’t the answer.

Fluid needs vary according to:

• your sweat rate
• exercise duration and intensity
• temperature and humidity
• your sport
• individual physiology
• opportunities to drink during training or competition.

Drinking large amounts very quickly can also feel uncomfortable.

A sports dietitian can help you develop a hydration strategy based on your individual needs and sporting environment, rather than relying on a generic number of litres per day.

Why do I feel nauseous during exercise?

Nausea during exercise can have several potential contributors.

Exercise intensity, hydration, heat, food timing, the amount you’ve eaten, what you’re consuming during exercise and individual gastrointestinal tolerance may all play a role.

Competition nerves can influence the gut too.

Your gastrointestinal system and nervous system are closely connected, which means stress and anxiety before an important event may change appetite, digestion and gut symptoms.

If nausea regularly affects your ability to train or compete, looking at the whole picture can be more useful than simply trying different foods one at a time.

A sports dietitian can help you map your symptoms against your training, nutrition and hydration to identify patterns that may otherwise be difficult to spot.

Why do I get diarrhoea when I run?

Urgency and diarrhoea during or after running can be particularly distressing.

For some runners, it starts influencing everything from training routes to competition choices.

You might plan every run around public toilets or feel anxious about whether you’ll make it through an event without needing to stop.

Food timing, fibre intake, caffeine, hydration, concentrated carbohydrate intake, exercise intensity and individual gastrointestinal sensitivity may all be relevant.

Sometimes there may also be an underlying gastrointestinal or medical issue that needs further investigation.

A sports dietitian can help determine which nutrition factors are worth exploring and, where appropriate, recommend that you speak with your GP or another health professional.

Should I cut out foods to stop exercise-related gut symptoms?

Be cautious about unnecessarily removing multiple foods or food groups.

When symptoms are uncomfortable, eliminating foods can feel like a logical place to start.

You remove one food. Symptoms continue. So you remove another.

Before long, you can end up with a fairly restrictive diet without actually knowing whether those foods were causing the problem.

For an athlete, this can create another issue.

The more foods you remove, the harder it can become to meet your energy and nutrition requirements for training and recovery.

A sports dietitian can help you take a more systematic approach.

Rather than guessing, we can look at your symptoms, food intake, training schedule and other potential contributors and determine which changes are genuinely worth testing.

What about FODMAPs and sports nutrition?

Some athletes hear about the low-FODMAP diet and wonder whether they should try it for bloating, diarrhoea or abdominal discomfort.

A low-FODMAP approach can be useful in particular clinical situations, especially for some people with diagnosed irritable bowel syndrome. It is also a structured dietary intervention involving restriction followed by reintroduction and personalisation.

For athletes, unnecessary or prolonged restriction can make meeting increased energy and carbohydrate requirements more difficult.

If FODMAPs appear relevant to your symptoms, working with a dietitian can help determine whether dietary modification is appropriate and how to do it while continuing to adequately fuel your training.

Can you train your gut for sport?

For some athletes, particularly those involved in endurance events, the gastrointestinal system can adapt to practising nutrition during training.

This is sometimes referred to as gut training.

If you need to consume carbohydrate or fluid during longer events, practising your competition nutrition during training gives your gut an opportunity to become familiar with the strategy.

A sports dietitian can help determine:

• whether you need fuel during exercise
• how much you may need
• what form of carbohydrate may work for you
• how frequently to take it
• how much fluid to consume
• how to practise the strategy during training.

This is another reason competition nutrition is best developed well before race day.

Don’t let gut symptoms compromise your fuelling

One of the biggest concerns we see when athletes experience gastrointestinal symptoms is that avoiding food becomes the solution.

If eating before training seems to trigger symptoms, training on an empty stomach can feel easier.

If sports nutrition products cause discomfort, you may stop fuelling during long sessions.

If certain foods seem problematic, you may gradually restrict more and more of your diet.

You might feel better in your stomach temporarily, while your body receives less of the energy it needs to train and recover.

Persistent under-fuelling can affect health, recovery and performance.

The goal is to find a way to fuel your body while managing your gastrointestinal symptoms, rather than choosing between the two.

Sports Nutrition: Are You Fuelling Enough for Your Training?

How can a sports dietitian help with gut symptoms?

There is rarely one universal solution for exercise-related gastrointestinal symptoms.

That’s why individual assessment matters.

A sports dietitian may explore:

• when your symptoms occur
• the type and intensity of training involved
• what you’re eating before exercise
• meal and snack timing
• fibre intake
• carbohydrate intake during exercise
• hydration
• caffeine
• sports foods and drinks
• competition nerves
• your usual bowel patterns
• your broader gastrointestinal history
• dietary restrictions you’ve already introduced.

From there, we can identify likely contributors and develop a structured plan to test changes.

This helps replace random trial and error with a more purposeful approach.

And when symptoms suggest something that needs further investigation, your sports dietitian can work alongside your GP or other healthcare professionals.

Practise your gut strategy before competition day

If gut symptoms tend to appear during races or competitions, planning ahead can make a significant difference.

Competition day has enough variables already.

Your nutrition doesn’t need to be another unknown.

A sports dietitian can help you develop a plan for:

• meals leading into your event
• your pre-event meal and snack
• fibre timing where appropriate
• hydration
• fuelling during competition
• caffeine if relevant
• recovery between events
• travel and food availability.

Most importantly, you can practise that strategy during training.

That gives you time to learn what your gut tolerates and refine the plan before performance matters most.

Stop planning your training around your stomach

If you’re choosing running routes based on bathroom access, avoiding food before training or worrying about whether your stomach will behave on competition day, it’s worth getting some support.

You’ve already invested time and effort into your training.

A sports dietitian can help you understand what’s happening, identify the nutrition factors that may be contributing and develop a strategy tailored to your body and your sport.

At Optimum Intake Dietitians, we work with athletes and active people across the Central Coast and Newcastle to develop individual sports nutrition strategies that support both gastrointestinal comfort and performance.

Ready to spend less time worrying about your stomach and more time focusing on your training?

Book an appointment with an Optimum Intake sports dietitian for individualised support with your fuelling, gut symptoms, training and performance.

Frequently Asked Questions