As endurance enthusiasts we diligently plan our training, with great emphasis on mileage, pace and even recovery. But one variable we often overlook is the gut. The fact is up to 96% of us experience some kind of debilitating gastrointestinal issue such as bloating, cramping, nausea and even diarrhea during a run or a race. These symptoms can sabotage training, race day performance and make running far less enjoyable.
Gastrointestinal issues are more common during longer runs and races, where fuel and fluid intakes increase, placing greater stress on the gut. We rock up to a race with belts loaded with gels, chews and drinks, and a gut that is just not ready for it all. The root cause of these symptoms are varied, but include reduced blood flow to the gastrointestinal system, impaired gut motility, changes in gut permeability and microbial diversity, all of which can be exacerbated by poorly planned fuelling strategies.
On a more positive note, the gastrointestinal system is highly adaptable and responds to stimuli much like any other muscle. So regular exposure to specific fuelling and hydration strategies in training can result in physiological adaptations that reduce gastrointestinal distress and enhance enjoyment, comfort and performance.
Gut Training
In recent years research has explored the concept of gut training. A 2023 systematic review by Martinez et al. found that just two weeks of gut training with 30 – 90 g/hour of carbohydrates reduced gut discomfort by up to 47% and malabsorption of carbohydrates by 45–54%. This led to improved tolerance and performance in time-trial efforts. Ingesting multiple transportable carbohydrates - the Dual Fuels found in our energy gels - allows oxidation rates to exceed 90 grams per hour, improving performance and reducing GI symptoms. Simply increasing the amount of carbohydrate consumed within your habitual diet from 40% to 70% of total energy can also double the intestinal SGLT1 transporter content, enhancing glucose absorption.
Gut training involves systematically practicing race-day nutrition in training, periodising fuel and fluid intake to gradually increase the stress in the gut to induce positive physiological adaptations. The key outcomes include:
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Enhanced carbohydrate absorption via upregulation of SGLT1 and GLUT2 transporters.
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Improved gastric emptying due to better tolerance of fluid and volume.
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Reduced malabsorption and gastrointestinal distress through desensitization and improved motility.
Practical Strategies
The research clearly confirms that gut training works, so let’s now look at how to implement gut training into your program.
Start Early and Build Gradually
Avoid rocking up on race day without prior preparation. Begin at least 6 weeks before your race. Start with 30g of carbohydrates per hour during long runs, increasing gradually each week until you reach the race day target of 90 g/hour. This allows the gut to adapt to higher carbohydrate loads.
Choose Appropriate Carbohydrate Sources
Ideally trial a variety of different carbohydrate options in training to find which suits you best.
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Sports drinks
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Gels
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Chews
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Solid foods (bars)
Practice Timing and Volume
Select specific sessions each week to simulate your race day nutrition timing, where you aim to consume 10 - 30 grams of carbohydrate every 20 minutes with 200 – 300 ml of water. Gradually increase the volume of carbohydrate and fluid as the weeks progress to stress the stomach and improve tolerance.
Monitor Fiber and Fat
Reduce the amount of both fiber and fat consumed before and certainly during long runs. Stick to low-residue, easy-to-digest and familiar foods in the 4-hour window prior to runs to prevent gastrointestinal issues. Consider a short-term low FODMAP diet if you experience debilitating symptoms.
Include Electrolytes and Fluids
Calculate your individual sweat rate and assess sweat sodium losses to refine and practice your unique hydration strategy. That likely involves ~500–700 ml of fluid per hour and around 600 mg of sodium per hour on average, adjusted for running intensity and climate of course.
Record and Adjust
We are all very good at monitoring and recording our runs and training sessions, but ignore the other elements. To train the gut well, it’s highly recommended that you maintain a detailed food, fuel and fluid log, where you document everything that you have consumed before and during the run alongside any relevant symptoms so that you can reflect back and adjust the strategy accordingly.
Practical Application
The below table demonstrates how gut training might be practically applied during a 6 week pre-race training programme.
Context is everything in sports nutrition, so the below plan is based on an individual aiming for a sub 3-hour marathon time, with a sweat rate of 1.5 L/hour and a sweat sodium loss of 1000 mg/hour. In this circumstance it’s recommended that they aim for 90-grams of carbohydrate per hour and replace around 93% of the fluid lost through sweat to support performance.
|
Week |
Objective |
Carbohydrate (grams per hour) |
Fluid (ml per hour) |
Products |
|
1 |
Introduce carbohydrates and fluid |
20 - 30 |
500 |
1 energy gel every 45-mins 500 ml water + 1 Core sachet per hour |
|
2 |
Increase exposure |
40 - 60 |
750 |
1 energy gel every 30-mins 250 ml sports drink per hour 500 ml water per hour |
|
3 |
Mid level loading |
60 - 70 |
1000 |
2 energy gels per hour 250 ml sports drink per hour 500 ml water + 1 Core sachet per hour |
|
4 |
Race fuel introduction |
80 - 90 |
1200 |
1 Fuel Bar 60-minutes pre-run 1 energy gel every 20 minutes 600 ml sports drink per hour 600 ml water + 1 Core sachet per hour |
|
5 |
Race fuel testing |
80 - 90 |
1400 |
1 Fuel Bar 60-minutes pre-run 1 energy gel every 20 minutes 600 ml sports drink per hour 800 ml water + 1 Core sachet per hour |
|
6 |
Taper and simulation Reduce training volume but maintain carb fuelling plan |
90 - 100 |
1400 |
1 Fuel Bar 60-minutes pre-run 1 energy gel every 20 minutes 700 ml sports drink per hour 700 ml water + 1 Core sachet per hour |
Scientific Insight and Athlete Experience
The current research confirms that most of us will experience some kind of stomach issue during a run and that half of those symptoms can be avoided by training the gut in a specific way as we prepare for a race. Beneficial effects of gut training can be achieved in as little as 2-weeks with repeated ingestion of gels and drinks. Athletes who implemented these strategies displayed improvements in time-trial performance, likely a result of enhanced absorption of ingested carbohydrates, improving fuel availability and lower incidence of gastrointestinal symptoms. Well-trained runners also displayed improved tolerance of larger fluid volumes during a race after this strategic gut training, allowing them to keep up with sweat rates and maintain hydration status.
Conclusion
Gut training might not be as sexy as clocking miles on Strava, but it’s likely as, if not more important to your race day performance. Adopting this 6-week evidence-based strategy as part of your race day preparations will allow you to absorb more of the carbohydrate and fluid you ingest, keep pace with with the gold-standard fuel and fluid requirements, reduce discomfort and optimize performance and enjoyment, which is the reason we all run in the first place, right?