Gut training for runners could help you absorb more carbohydrate, experience fewer stomach problems and fuel long-distance races more effectively. But does every runner really need 90–120g of carbohydrate per hour?
Learn how gut training for runners works, how many carbs you need per hour and how to practise race-day fuelling without stomach problems.
For years, runners were commonly told to take one gel every 30–45 minutes and hope their stomach cooperated. Now, endurance fuelling is entering a very different era.
Elite athletes are consuming increasingly large amounts of carbohydrate during marathons and ultras, with some reportedly reaching 90–120g per hour. That can mean the equivalent of three or four gels every hour – far more than many recreational runners could comfortably tolerate without practice.
But the digestive system is not necessarily fixed. In the same way that your muscles adapt to long runs, research suggests your gut can adapt to processing fuel during exercise. This is known as gut training.
The goal is not to force down as much sugar as possible. It is to gradually teach your body to tolerate the amount and type of fuel you will genuinely need on race day.
WHAT IS GUT TRAINING FOR RUNNERS?
Gut training is the repeated practice of eating or drinking carbohydrate while running. Over time, this may improve your comfort with food and fluid in the stomach and increase your ability to absorb carbohydrate during exercise.
During a long run, blood flow is redirected towards your working muscles and away from the digestive system. The repeated impact of running, rising body temperature, dehydration and race intensity can make digestion even more difficult. This is why a fuelling strategy that feels fine while sitting at home may cause bloating, nausea or urgent toilet stops during a race.
Gut training exposes your digestive system to race-like conditions before race day. You practise the same products, quantities, timings and fluid intake that you intend to use in competition, then adjust the plan according to how your body responds.
A review published in Sports Medicine concluded that the gastrointestinal system is highly adaptable and that nutritional training may improve stomach comfort, carbohydrate absorption and the likelihood or severity of gut problems during endurance exercise. In one trial, two weeks of repeated carbohydrate intake during runs reduced gastrointestinal symptoms and carbohydrate malabsorption in endurance runners.
That does not mean two weeks will solve every stomach problem. Individual responses vary, and the most effective approach and timeline are still being studied. However, the evidence supports a simple principle: race-day fuelling should be trained, not improvised.
WHY DO RUNNERS NEED CARBOHYDRATE DURING LONG RUNS?
Carbohydrate is stored in the muscles and liver as glycogen. During running, your body breaks glycogen down into glucose that can be used to produce energy.
Those stores are limited. As a long run continues, glycogen availability falls and your body becomes increasingly reliant on carbohydrate consumed during the session. If your pace drops dramatically, your legs feel empty or every kilometre suddenly requires more effort, low carbohydrate availability may be part of the problem.
Taking carbohydrate during prolonged running can help maintain energy availability, delay fatigue and support your intended race pace. It may also reduce the size of the energy deficit created during a long training session, which can support recovery afterwards.
However, consuming more carbohydrate does not automatically create a better performance. The carbohydrate still needs to leave the stomach, be absorbed through the intestine and reach the bloodstream. If you consume more than your gut can currently handle, the result may be sloshing, bloating, cramping or nausea rather than extra energy.
HOW MANY CARBS PER HOUR DO RUNNERS NEED?
Your target should depend on the length and intensity of the run, your pace, your goals and your individual tolerance.
As a broad guide:
- Less than 60 minutes: carbohydrate during the run is usually unnecessary if you begin adequately fuelled.
- Around 60–150 minutes: approximately 30–60g of carbohydrate per hour may be useful.
- Longer than 2.5 hours: runners may benefit from building towards as much as 90g per hour, particularly when racing at a demanding intensity.
- 90–120g per hour: this is an advanced strategy being explored by elite marathon and ultra-distance athletes. It is not a standard requirement for recreational runners.
Established sports-nutrition guidance recommends up to 90g per hour for events lasting longer than approximately 2.5 hours. Research into 120g per hour is promising, but it remains limited and has often involved small groups of highly trained or elite athletes.
A recent study involving eight elite male marathon runners found that 120g per hour increased the amount of ingested carbohydrate being used for energy compared with 60g or 90g. However, moderate or severe gastrointestinal symptoms occurred across the trials and nausea, fullness and cramping were greatest at 120g per hour. The researchers also noted that the performance implications still need to be established.
In other words, 120g per hour is not automatically better – and it is certainly not the new minimum.
HOW CAN THE BODY ABSORB MORE CARBOHYDRATE?
Different carbohydrates use different transport systems to cross the intestinal wall.
Glucose and maltodextrin rely mainly on one pathway, while fructose uses another. At higher intake rates, relying entirely on glucose can overwhelm its available transport route. Combining glucose or maltodextrin with fructose allows carbohydrate to be absorbed through more than one pathway.
This is why many high-carbohydrate gels and drink mixes contain a glucose-fructose blend. For runners aiming above approximately 60g per hour, the type of carbohydrate can matter as much as the total amount.
Repeated exposure may also help the gut become better prepared for carbohydrate intake. Possible adaptations include improved stomach comfort, a lower sensation of fullness and an increased ability to absorb the nutrients regularly consumed during training.
You do not need to memorise carbohydrate chemistry while running. Check product labels, look for a blend of carbohydrate sources when targeting higher intakes and practise with the exact products you plan to race with.
HOW TO TRAIN YOUR GUT FOR A MARATHON
Gut training should be gradual. Going from one gel an hour to 90g of carbohydrate on your next long run is more likely to overwhelm your digestive system than train it.
1. WORK OUT YOUR CURRENT INTAKE
Record what you currently consume during a typical long run. Include gels, drinks, chews and food rather than counting gels alone.
For example, two gels containing 25g each provide 50g of carbohydrate. If your sports drink supplies another 20g during the same hour, your total is 70g – not 50g.
2. CHOOSE A REALISTIC TARGET
A runner completing a half marathon in under 90 minutes will have very different needs from someone running a five-hour marathon or an ultramarathon.
Many marathon runners may perform well in the 50–75g-per-hour range. Faster or more competitive athletes may choose to work towards 75–90g. There is no prize for reaching 120g if 60g supports your performance and leaves your stomach settled.
3. INCREASE GRADUALLY
Add around 10–15g per hour at a time and repeat that intake across several suitable long runs before increasing it again. If you currently tolerate 40g per hour, try 50g rather than jumping straight to 90g.
Practise during one key long session each week, allowing four to eight weeks to refine the strategy. Some runners may need longer, particularly if they have a history of gastrointestinal problems.
4. START FUELLING EARLY
Do not wait until you feel depleted. Starting early allows you to spread carbohydrate across the run instead of trying to recover with a large amount later.
If your target is 60g per hour, smaller servings every 20–30 minutes may feel easier than consuming the full amount at once. The exact timing will depend on the product and your preferences.
5. PRACTISE YOUR FULL RACE-DAY PLAN
Test more than the gels. Practise your pre-run breakfast, start time, caffeine, fluids, electrolytes and intended race pace. Heat and harder running can both increase the likelihood of stomach problems, so an easy winter jog may not accurately recreate race conditions.
Follow the fluid instructions on your fuel products. Taking several concentrated gels without enough water can make them harder to tolerate, while drinking excessive fluid also carries risks.
6. KEEP A GUT-TRAINING LOG
After each long run, record:
- Carbohydrate consumed per hour
- Products and flavours used
- Fluid consumed
- Timing of each serving
- Temperature and running intensity
- Energy levels
- Any fullness, reflux, cramps, nausea or toilet urgency
Patterns are more useful than one difficult run. A product may only cause problems when combined with caffeine, taken without water or used during hot conditions.
COMMON GUT-TRAINING MISTAKES
CHASING 90–120G BEFORE MASTERING THE BASICS
High carbohydrate numbers make attention-grabbing social content, but they are not appropriate targets for every runner. Begin with the amount that matches your event and build only when there is a clear reason.
TRYING A NEW PRODUCT ON RACE DAY
Even when two gels contain the same grams of carbohydrate, their concentration, ingredients, texture and caffeine content can differ. Nothing new on race day remains one of the most useful rules in endurance sport.
IGNORING FLUID
Carbohydrate intake, hydration and temperature interact. A strategy that works in cool conditions may feel completely different on a hot day. Follow product guidance and avoid treating hydration as an afterthought.
EATING A HEAVY PRE-RUN MEAL
A large meal high in fat, fibre or protein takes longer to digest and may make an aggressive fuelling plan harder to tolerate. Before key long runs and races, choose familiar, carbohydrate-rich foods that sit comfortably for you.
ASSUMING EVERY STOMACH PROBLEM MEANS “TOO MANY GELS”
Nausea and cramping may also be associated with starting too fast, heat stress, dehydration, excessive fluid intake, caffeine or unfamiliar ingredients. Review the entire session before blaming carbohydrate alone.
WHEN SHOULD YOU GET PROFESSIONAL ADVICE?
Occasional mild fullness while testing a new strategy is different from recurring or severe symptoms.
Speak to a doctor or registered sports dietitian if you regularly experience severe abdominal pain, vomiting, diarrhoea, blood in your stool, faintness, unexplained weight loss or symptoms that continue away from exercise. Runners with diabetes, gastrointestinal conditions, a history of disordered eating or other medical concerns should seek personalised guidance before substantially changing their fuelling plan.
THE BOTTOM LINE
Yes, runners can train their gut.
Repeatedly practising carbohydrate intake during long runs may improve stomach comfort, reduce gastrointestinal symptoms and help the body absorb fuel more effectively. But gut training is not a competition to consume the highest possible number.
For most runners, the best strategy is the one that supplies enough carbohydrate to support performance, remains comfortable at race pace and can be repeated consistently. Build gradually, use a mixture of carbohydrate sources at higher intakes and rehearse the complete plan well before race morning.
Your legs should not be the only part of your body prepared for race day. Train the engine – and train the fuel system that keeps it running.
GUT TRAINING FOR RUNNERS: FREQUENTLY ASKED QUESTIONS
HOW LONG DOES IT TAKE TO TRAIN YOUR GUT FOR RUNNING?
Some research has found improvements after two weeks of repeated carbohydrate feeding during exercise. In practice, allowing four to eight weeks gives runners more opportunities to increase gradually, test different products and practise in race-like conditions.
HOW MANY GELS EQUAL 60G OF CARBOHYDRATE?
It depends on the product. If one gel contains 25g of carbohydrate, two gels provide 50g and three provide 75g. Sports drinks and chews must also be included in your hourly total. Always check the label rather than counting gels alone.
SHOULD EVERY MARATHON RUNNER AIM FOR 90G PER HOUR?
No. Up to 90g per hour may benefit some runners during events lasting longer than 2.5 hours, but the right target depends on intensity, duration and tolerance. Many marathon runners may find that a lower intake adequately supports their performance.
IS 120G OF CARBOHYDRATE PER HOUR SAFE FOR RUNNERS?
Research has tested 120g per hour in small groups of highly trained athletes, but it can produce gastrointestinal symptoms and its performance benefits are not yet clear for the wider running population. It should be considered an advanced strategy rather than a general recommendation.
WHAT SHOULD I DO IF GELS UPSET MY STOMACH?
Try a smaller amount taken more frequently, check whether the product needs water and consider a different format such as a carbohydrate drink or chews. Also review your pace, pre-run meal, caffeine and hydration. Persistent symptoms should be discussed with a healthcare professional or registered sports dietitian.
Related Reading
- Carbs for Runners: The Ultimate Guide
- Energy Gels Explained
- Runner’s Stomach: 10 Simple Ways to Save Your Marathon
- The Ultimate Half Marathon Nutrition Plan
- 8 Best Supplements for Runners
REFERENCES
- Jeukendrup, A. E. (2017). Training the gut for athletes. Sports Medicine, 47(Suppl 1), 101–110. https://doi.org/10.1007/s40279-017-0690-6
- Miall, A., Khoo, A., Rauch, C., Snipe, R. M. J., Camões-Costa, V. L., Gibson, P. R., & Costa, R. J. S. (2018). Two weeks of repetitive gut-challenge reduce exercise-associated gastrointestinal symptoms and malabsorption. Scandinavian Journal of Medicine & Science in Sports, 28(2), 630–640. https://doi.org/10.1111/sms.12912
- Burke, L. M., Hawley, J. A., Wong, S. H. S., & Jeukendrup, A. E. (2011). Carbohydrates for training and competition. Journal of Sports Sciences, 29(Suppl 1), S17–S27. https://doi.org/10.1080/02640414.2011.585473
- Jeukendrup, A. E. (2010). Carbohydrate and exercise performance: The role of multiple transportable carbohydrates. Current Opinion in Clinical Nutrition and Metabolic Care, 13(4), 452–457. https://doi.org/10.1097/MCO.0b013e328339de9f
- Urdampilleta, A., Arribalzaga, S., Viribay, A., Castañeda-Babarro, A., Seco-Calvo, J., & Mielgo-Ayuso, J. (2020). Effects of 120 vs. 60 and 90g/h carbohydrate intake during a trail marathon on neuromuscular function and high-intensity run-capacity recovery. Nutrients, 12(7), 2094. https://doi.org/10.3390/nu12072094
- Ravikanti, S., Silang, K. G., Martyn, H. J., Johnson, K. O., Louis, J. B., Bampouras, T. M., et al. (2025). 13C-labelled glucose-fructose show greater exogenous and whole-body carbohydrate oxidation and lower oxygen cost of running at 120 versus 60 and 90g/h in elite male marathoners. Journal of Applied Physiology, 139, 1581–1595. https://doi.org/10.1152/japplphysiol.00665.2025
This article is for general information and does not replace personalised medical or sports-nutrition advice.
