How To Avoid GI Issues During A Marathon

Gastrointestinal (GI) issues such as bloating, cramping, nausea and diarrhoea are a common cause of slowed performance or DNF in endurance running. Research consistently shows that 30-90% of endurance athletes experience GI symptoms during events, particularly in marathons and longer races¹⁻³

These symptoms are largely multifactorial and preventable, and are strongly influenced by fuelling strategy, hydration status, intensity, and gut training status¹,²,⁴. A structured fuelling system that is trialled in training can significantly reduce risk and improve race-day energy availability³,⁵.

 

Quick Answer: How to Avoid GI Issues During a Marathon

To reduce GI issues, athletes should:

• Trial all fuelling in training

• Use carbohydrates the gut tolerates

• Reduce fibre before racing

• Maintain consistent carbohydrate + fluid intake during the marathon

• Follow a practiced race-day plan¹⁻³,⁵

 

The GI Stability Framework: 5 Steps to Prevent GI Issues

1. Train Your Gut Progressively
   Introduce race-day foods, gels and fluids in training first. Gradually increase volume and frequency to help your digestive system adapt⁶,⁷. Aim to consume around 60-90g of carbs per hour³,⁶.
   
   

2. Use the Right Type of Fuel
   Choose easily digestible carbohydrate sources that work for you such as  gels, chews, or electrolyte/carb drink mixes. Blends of glucose + fructose (e.g. 2:1 or 1:0.8) improve absorption and reduce GI distress compared to glucose-only intake 8,9. Stick with products you’ve tried in training¹,³.
   
   

3. Match Carbohydrate Intake to Tolerance
   Most marathon runners tolerate around 60-90g carbohydrate/hour, but GI symptoms increase when intake exceeds individual gut capacity without training adaptation³,⁶.
   
   Start at the lower end and increase gradually only if:

• • Energy is dropping

  • GI symptoms remain minimal
    High intakes require both gut training and appropriate carbohydrate type selection⁶,⁸.
    
    

4. Balance Fluids and Sodium
   
   Dehydration and high carbohydrate concentration both delay gastric emptying and increase GI distress risk⁴,¹⁰.
   
   Guidelines:
   150-300mL of electrolytes and water every 15-20 minutes
   300-800mg sodium/hour (higher in heat/ultra events)⁴,¹⁰.
   
   

5. Execute a Tested Race-Day Plan
   Follow the exact nutrition and hydration strategy practiced in training. Avoid last-minute changes and relying on on-course aid stations to prevent surprises in your gut¹,³.

 

What Are GI Issues In Running?

GI issues in endurance running include nausea, bloating, cramping, reflux and diarrhoea. They occur when the digestive system cannot cope with reduced blood flow, mechanical jostling, and high fuelling demands during exercise, leading to impaired comfort and performance¹,³.

 

Why GI Issues Happen & How They Affect Performance

During running, blood is redirected away from the gut to the muscles, slowing digestion²,⁴. Combined with dehydration, high-intensity effort, or poorly tolerated fuel, this can trigger nausea, cramping, or diarrhoea. These symptoms slow energy delivery to your muscles and can make you slow down, cause discomfort and lead to unwanted toilet stops, all hurting your performance²,⁴,¹⁰.

 

How Many Carbs Per Hour Without Causing GI Issues?

Most athletes tolerate 60-90g/hour during endurance exercise when appropriately trained³,⁶.

However, exceeding this requires: progressive gut training, multiple transportable carbohydrates (glucose + fructose), Individual tolerance testing. High intake without adaptation significantly increases GI symptom risk⁶,⁸,⁹.

 

Signs Your Gut Is Not Trained For Race Day

If your gut isn’t properly trained, even small amounts of fuel can cause problems. Common signs include bloating, cramping or nausea during or after consuming carbohydrates. You may also notice a drop in performance despite adequate fuelling, as your muscles aren’t getting the energy they need. These symptoms indicate your gut needs more practice with race-day nutrition before attempting higher carbohydrate intakes in competition⁶,⁷.

 

What Is Gut Training & How to Build It

Gut training improves gastric emptying, intestinal carbohydrate transport and symptom tolerance during exercise⁶,⁷. Progressive exposure to race fuelling in training reduces GI symptoms and improves carbohydrate delivery capacity⁶.

 

Choosing The Right Fuel To Reduce GI Risk

The type of fuel you choose plays a major role in reducing GI issues during exercise. Isotonic gels and drinks are lower in concentration and easier to absorb, making them a good starting point, while more concentrated gels can increase the risk of bloating or discomfort if not taken with enough fluid⁴,¹⁰.

Some athletes may also experience intolerance to certain carbohydrate types, such as fructose, which can lead to symptoms like bloating or unexpected toilet stops⁹. Low FODMAP carbohydrate products may be better for these athletes⁹. Testing different options from an energy gel collection in training allows you to identify what works best and build a reliable race-day strategy.

 

Gel vs Drink vs Chews: Which Causes Less GI Stress?

Different carbohydrate sources place different demands on your gut, so choosing the right format can help minimise GI stress during exercise. Fluids are typically easiest to tolerate, while more concentrated or solid forms may increase the risk if not used correctly or without enough fluid.

 

 

How To Fuel Properly During Long Runs

Early and consistent intake improves carbohydrate availability and reduces GI overload risk³,⁶.

• Pre-race: 2-3 hours before, low fibre high carb meal

• 20-30 min before: small carbohydrate dose + fluids such as energy gels, chews, bars, or carbohydrate drinks.

• During: 60-90g carbohydrate/hour split into frequent doses

• Always pair with fluids⁴,⁶.

 

Hydration, Sodium & GI Comfort

Staying on top of hydration and sodium helps support energy absorption and reduce GI stress during training and racing⁴,¹⁰. Aim for 150-250mL of fluid every 15-20 minutes and take fluids alongside your fuel. As a guide, target 300-800mg of sodium per hour, increasing to 1000-1500mg per hour for ultra-endurance events¹⁰.

 

Step-by-Step Guide To Avoid GI Issues On Race Day

Day before

• Keep food familiar, low fibre

• Sip of water with electrolytes across the day

Race morning (2-3 hrs before)

• Low-fibre, high-carb breakfast e.g. Bagel with maple syrup

• Sip water with electrolytes

20-30 mins before race start

• Take an energy gel, chew, bar or waffle + water with electrolytes

During the marathon

• Start fuelling early ( Take an energy gel, chew, bar or waffle every 30-45 mins)

• Aim 60-90g carbs/hr³,⁶

• Take small, regular sips of water with a carb+ electrolyte drink mix

Managing Symptoms Mid-Race

• Switch to fluids and sip water¹,³

• Slow pace and skip gels if needed

• Small, frequent sips of water and electrolytes (try avoid further carb drink mixes)

• Reassess in 10-15 minutes

Key rule

• Nothing new on race day¹,³.

 

Why More Fuel Can Make GI Issues Worse

Higher intake does not always improve performance. Overconsumption can exceed intestinal transport capacity, leading to malabsorption and fluid shifts into the gut, increasing bloating and cramping⁶,⁸.

Poor fuelling strategies (high concentration, stacking gels, or mismatched carbohydrate types) can overwhelm absorption pathways and worsen symptoms rather than improve energy availability⁶,⁸,¹⁰.

 

Common Fuelling Mistakes That Cause GI Issues

Most GI issues are predictable and preventable. Common errors include:

• Rapid carbohydrate overloading

• Underhydration with gels

• Lack of gut training

Studies show that mismatched fuelling strategy and race-day intake exceeding training exposure are key predictors of GI symptoms¹,³,⁶.

The goal is not to reduce fuelling, but to match intake to trained gut capacity and hydration strategy³,⁶.

 

Key Takeaways

• Train your gut in training by gradually building carbohydrate intake toward 60-90g per hour rather than increasing on race day

• Use familiar, tested fuelling products and avoid introducing anything new during the marathon

• Match carbohydrates with fluids and sodium to support digestion and reduce GI stress

• Start fuelling early and consistently (don’t wait until you feel tired or depleted)

• If GI symptoms occur mid-race, reduce intake, switch to fluids, slow pace, and simplify your strategy temporarily

 

Ash Miller
Dietitian and Nutritionist (Masters)
Bachelor of Physical and Health Education
Instagram: @ashthomo_nutrition

References

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2. Costa RJS, Snipe RMJ, Kitic CM, Gibson PR. Systematic review: exercise-induced gastrointestinal syndrome. Aliment Pharmacol Ther. 2017;46(3):246-65.

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4. Jeukendrup AE. Nutrition for endurance sports: marathon, triathlon, and road cycling. J Sports Sci. 2011;29(Suppl 1):S91-9.

5. Thomas DT, Erdman KA, Burke LM. Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and ACSM: Nutrition and athletic performance. J Acad Nutr Diet. 2016;116(3):501-28.

6. Cox GR, Clark SA, Cox AJ, et al. Daily training with high carbohydrate availability increases exogenous carbohydrate oxidation. J Appl Physiol. 2010;109(1):151-60.

7. Snipe RMJ, Khoo A, Kitic CM, Gibson PR, Costa RJS. The impact of carbohydrate ingestion on gastrointestinal integrity during running. Am J Physiol Gastrointest Liver Physiol. 2018;314(1):G47-G58.

8. Rowlands DS, Houltham S. Multiple-transportable carbohydrates and endurance performance. Sports Med. 2017;47(Suppl 1):5-20.

9. Maughan RJ, Shirreffs SM. Gut tolerance of carbohydrate-electrolyte solutions. Sports Med. 2010;40(12):1013-23.

10. Costa RJS, Miall A, et al. Hydration and gastrointestinal function during exercise. Nutrients. 2019;11(11):E2567.

 

Disclaimer:

The content in this blog is for general information only and is not a substitute for professional medical advice, diagnosis, or treatment. Always speak with your doctor or allied health team before changing your diet, exercise, or taking supplements, especially if you have a health condition or take medication. Please use this information as a guide only. Aid Station doesn't take responsibility for individual outcomes.