Gut Check: Unlocking Ultra Performance 

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Ruby Wyles

By: Ruby Wyles

Runner, triathlete, and passionate coach, Ruby is most fulfilled by helping athletes overcome limiting beliefs with joy. She is also a proud science nerd, and advocate for athletes' mental and physical health.

Training the gut might be the best way to take your racing to the next level.

As discussed in detail in Unraveling The Carbohydrate Conundrum – carbohydrates are essential for optimizing endurance performance, and consuming them during exercise is recommended for events lasting longer than 60 minutes. If you’ve listened to any running podcast recently you’ve also likely heard the hype of runners packing in the grams of carbohydrates per hour! In fact, recent research has shown that there are performance enhancing effects of increasing in-workout fueling well beyond the older guidelines – with some athletes getting upwards of 100-120 grams of carbohydrates per hour. That being said, you can’t go from zero to hero! In order to maximize carbohydrate absorption and utilization, as well as to increase the amount of fuel you can tolerate during exercise without discomfort (you know what we’re talking about), training your gut is a must! 

Runner fueling at aid station. PC: James Holt
Runner fueling at an aid station during the Gorge Waterfalls 100km. PC: James Holt

What Is Gut Training?

As the name suggests, gut training involves challenging the gastrointestinal (GI) system to adapt to improve nutrient absorption and delivery – alleviating negative symptoms. As trail and ultra runners you’re likely familiar with the idea of “don’t try anything new on race day” and gut training follows that same principle. Gut training isn’t simply trialing different gels and chews in a couple runs ahead of race (which is still important), rather gut training demands a measured and systematic approach to in-run nutrition in the weeks and months prior to a goal race. As athletes, we train our heart and lungs to work harder so we can go faster and further, so why stop there and ignore our gut? 

Why Train The Gut?

Successful performance in endurance events requires athletes to consume exogenous fuel sources as we quickly burn through what we already have stored (endogenous) in our muscles and liver. In order to maintain or even increase your output during training or racing, your energy stores need to be continuously topped off so you don’t “overdraw your bank account.”

In theory, the more energy you have, the faster, further, and/or harder you can go. In practice, this is much easier said than done for several reasons:

  • Eating and drinking while running is hard and can leave you breathless, coughing, and spluttering.
  • Nutrition jostling in the stomach can feel unpleasant as you run.
  • Unpleasant GI symptoms like cramps, bloating, and diarrhea, as well as nausea and bonking, are common.
  • Extreme environments, such as the heat or altitude, can also impact gut symptoms and make taking on food and fluids more challenging.

Training the gut can help improve all of the factors listed above, making it easier and more comfortable to eat and drink more on the move. Not to mention, staying fueled during races with minimal GI distress massively increases your chances of a successful performance without bonking, as well as a speedy recovery afterwards. In fact, unpleasant gut symptoms are one of the most common reasons endurance athletes report for DNF-ing. And studies suggest up to 70% of elite endurance athletes experience exercise-induced GI symptoms.

Gut training allows runners to eat more on race day. PC: Somer Kreisman
Gut training allows runners to eat more on race day. PC: Somer Kreisman

For real-world proof that this actually works, just look at competitive eaters consuming huge amounts of food and fluids in record times. Regardless of your thoughts on competitive eating, to be a champion eater requires training!

Gut Training Science

How gut training works

  • Increasing gastric emptying.

 The rate at which substances move from the stomach into the small intestines

  • Improving gut tolerance and comfort.

Reducing athletes’ feeling of fullness.

  • Increasing digestive enzymes and transporters.

Quicker breakdown and absorption of carbs.

Increasing gastric emptying

Delayed gastric emptying, or a slowing of the rate at which substances leave the stomach, is a common side effect of prolonged and/or high intensity exercise, and potential trigger of GI distress. This is due to the secretion of certain hormones stimulated by exercise, diverting blood flow away from digestive organs. Repeated training with nutrients in the gut may have an inhibitory effect on the secretion of these hormones, resulting in more blood flow to the gut, thereby increasing the rate of gastric emptying.

Another way to increase gastric emptying is simply to increase your intake of dietary carbohydrates outside of training. Research demonstrates that just three to seven days of increasing carbohydrate intake by about 400g per day is enough to significantly increase carbohydrate gastric emptying. Similarly, consuming a higher fat diet for 14 days has also been shown to markedly improve the rate of gastric emptying of fats from the stomach, however, there appears to be no effect of increased protein intake on gastric emptying. Adaptations to the small intestines responsible for increased gastric emptying happen fast, so increasing dietary carbohydrate intake is one method of gut training that can be left to the week before race day. 

Improving gut tolerance

Training over multiple sessions with a larger volume of food and fluid in the stomach, and/or a greater concentration of carbohydrates can increase the volume and pressure tolerated by the gut. This allows athletes on race day to consume greater quantities of carbohydrates and fluid without the same GI distress and discomfort triggered by not training the gut. At first, athletes may feel food and fluid jostling around in their stomach, as well as nausea, but with practice, nutrition will settle better, with reduced feelings of fullness and bloating too.

Increasing enzymes and transporters

Repeatedly ingesting carbohydrates during exercise stimulates the body to increase the number and activity of carbohydrate transporters and digestive enzymes in the small intestines, improving athletes’ abilities to digest and utilize carbohydrates while moving to fuel performance.

How To Gut Train

Training the gut is much like training any other part of the body, you have to challenge it to a greater load than it can currently comfortably handle in order to stimulate adaptations that allow it to handle more. Put simply, you have to tolerate some discomfort in training while your body is adapting to a greater quantity of food and fluid in the gut, so that you can optimize your performance on race day with minimal GI symptoms. 

Here are some research-backed and practical ways to train the gut: 

  • Training immediately after eating a meal.
  • Training with a relatively large volume of food/ fluid in the stomach.
  • Consuming more carbohydrates during exercise (i.e. if you currently consume 60g/hr, bump that up to 90g/hr and beyond).
  • Practicing consuming slightly more than your race nutrition plan in training.
  • Increasing daily intake of carbohydrates.

When should you consider doing this?

  • Incorporate at least two gut training sessions per week in the 2-3 months leading into race day: while research supports 12 to 15 sessions prior to race day, this is a situation where more is better at least initially. 
  • Practice some of these strategies across all types of training sessions: easy runs, long runs, interval and high intensity workouts, race simulations and training races. 

Other ways to optimize race fueling and avoid GI symptoms

Perhaps you’re reading this a week out from your goal race and you’re panicking that you don’t have time to gut train. Perhaps you are now worried about how you can fuel for optimal performance while minimizing GI symptoms on short notice?

Let me tell you, it’s not too late! Your last minute tip actually sounds a lot like carbohydrate loading! Again, increasing your daily intake of carbohydrates for just 3-5 days has been shown to increase carbohydrate processing by the gut. Other tips include limiting your intake of fiber, fat, and highly concentrated carbohydrate solutions in the days before and during race day.


Gut training is essential to maximize performance on race day, and you can do this by preparing your body to digest and utilize carbohydrates to power you through sustained exercise. Given that GI symptoms are one of the most common reasons athletes give for dropping out of a race, this area is well worth taking seriously! Much like running intervals challenge your aerobic system and burn the legs, training the gut necessitates some discomfort in training. However, the more you practice ingesting carbohydrates on the run, the more your body can tolerate without unpleasant GI symptoms. Adaptations to the GI systems can happen in just a few days, but for best results, starting gut training in the months leading into a goal race is ideal. Practicing your race nutrition plan (or even a little extra) in training, as well as exercising on a full stomach can get you to the finish line minus the nausea. While it’s easier said than done, over time you can improve your ability to fuel and your performances will skyrocket!


Cunningham, K. M., Daly, J., Horowitz, M., & Read, N. W. (1991). Gastrointestinal adaptation to diets of differing fat composition in human volunteers. Gut, 32(5), 483–486. https://doi.org/10.1136/gut.32.5.483 

Horowitz, M., Cunningham, K. M., Wishart, J. M., Jones, K. L., & Read, N. W. (1996). The effect of short-term dietary supplementation with glucose on gastric emptying of glucose and fructose and oral glucose tolerance in normal subjects. Diabetologia, 39(4), 481–486. https://doi.org/10.1007/BF00400681 

Jeukendrup A. E. (2017). Training the Gut for Athletes. Sports medicine (Auckland, N.Z.), 47(Suppl 1), 101–110. https://doi.org/10.1007/s40279-017-0690-6

de Oliveira, E. P., Burini, R. C., & Jeukendrup, A. (2014). Gastrointestinal complaints during exercise: prevalence, etiology, and nutritional recommendations. Sports Medicine (Auckland, N.Z.), 44 Suppl 1(Suppl 1), S79–S85. https://doi.org/10.1007/s40279-014-0153-2

Keep exploring


Rising to The Challenge: How to Prep For Hilly Races


 Leveling Up: Why Even Seasoned Runners Can Benefit from Coaching


Fueling the Distance: How Protein Can Propel Endurance Athletes Forward

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