How to Use Gut Training to Improve Endurance Exercise Performance

 Note from Greg: Below is another terrific guest blog from our friend Adam McDonald. Adam has an impressive track record in natural bodybuilding, but he has a strong understanding of sports nutrition in general and is very interested in endurance exercise too. Be sure to follow him on Instagram and YouTube!


We've previously discussed why, if you’re aiming to maximise your endurance exercise performance, fuelling well during the event is paramount to giving your absolute best (see this guide for more). However, what most new endurance athletes don’t realise — and what experienced athletes are all too familiar with — is that consuming the wrong nutrients during long bouts of exercise can lead to some pretty nasty digestive issues — nausea, flatulence, stomach pain, bloating, and diarrhoea are common during endurance exercise. Sometimes digestive issues are merely mild, uncomfortable hindrances. However, in some instances they can be so severe that they result in a person having to stop a race or event. In fact, it’s not really a matter of if you will suffer from gastrointestinal (GI) issues during a race, but when! Research has shown that 30 to 50% of endurance athletes experience GI problems, and the longer the race, the higher the incidence of issues. The good news is that you can use gut training to improve endurance exercise performance and reduce the likelihood of digestive problems. This blog outlines how.


Key takeaways

  • By determining how well you take up nutrients, your gut plays a vital role in your endurance performance.
  • Because digestive problems are so common and can take you out of an event, planning your race day nutrition can be all-but as important as your physical training.
  • You can train your gut to take up nutrients at a faster rate. Without gut training, you're more likely to experience GI issues.
  • Since glucose and fructose are taken up by different transporters in the gut, combining them can let you comfortably digest carbohydrate at a higher rate than possible when consuming only one of them. A ratio of about 5 parts glucose to 4 parts fructose seems best.
  • It’s best to gradually increase your food intake from session to session. The goal in training should be to exceed your plan for race day.
  • If your event takes place during the night, you should practice your nutrition during the night too.


Factors affecting gut function during exercise

On one hand, we high-performance athletes want to maximise our performance, which requires adequate in-race fuelling. On the other, however, we need to mitigate the potential negative effects that fuelling can give rise to.

The reasons why some athletes experience GI issues and others don’t remain largely unclear, but this is surely influenced by both genetics and behaviours. Type of exercise (modality, intensity, etc), history of GI complaints, nutrition and hydration, weather, and psychological state (e.g., anxiety) can all influence GI issues.

With proper planning and gut training, you can speed the rate at which nutrients pass through your stomach and are absorbed by your intestines. Additionally, gut training can help you tolerate larger amounts of nutrients. All these changes help alleviate common GI problems.


Slowly increase your carbohydrate intake from session to session and consume a mix of glucose and fructose

Your body takes up carbohydrates (e.g., glucose and fructose) via your small intestine using certain transport proteins that allow the nutrients to pass through the gut lining. Following this, most glucose can be shuttled by the blood and then transported into various peripheral tissues (e.g., skeletal muscles) where it is stored or burned for energy. Most fructose is sent to the liver where it can be converted into different metabolites, including glucose.

When carbohydrates are consumed more quickly than they can be taken up in the gut, however, they’re likely to be fermented by some gut bacteria, which can lead to bloating and pain. Although studies of humans are lacking, research on other animals shows that increasing dietary carbohydrate intake raises the levels of certain glucose transporters in the gut. Assuming this translates to humans, this means your body can adapt to take up more carbohydrate during exercise, allowing you to avoid GI issues at high intakes.

Importantly, some types of carbohydrate are taken up in the gut using different transporters (e.g., glucose is taken up by SGLT1, but fructose is taken up by GLUT5). This means that by combining glucose with fructose, you can absorb carbohydrate at a higher maximal rate than if you only consumed either one alone. This is why many sports nutrition companies advocate a ratio of 2 parts glucose to 1 part fructose, consumed at up to 90 g total carbohydrate per hour. Interestingly, however, recent research suggests that this 2:1 ratio probably isn’t optimal — instead, a ratio closer to 5 parts glucose to 4 parts fructose seems best.


Practice makes perfect

The most important point in this blog is to practise your race-day nutrition. Although many events have people giving out complimentary gels and foods, avoid consuming these before or during events and stick to your plan. Once you have a plan for your in-race nutrition, your goal should be to work up to intakes during your training sessions that are above what you’re aiming for during the event itself. A good goal is for your final training sessions to exceed your race intake goal by about 20%.

You shouldn’t start at 90 g carbohydrate per hour and should instead start lower and work your way up in the weeks leading to the event. It's probably best to do at least 6 of these gut-training sessions, with each session lasting at least 1 hour. By doing this, you’ll likely find an upper limit of intake that you can tolerate.

Some people prefer to have carbohydrates during exercise while others, particularly in ultra-endurance events, prefer a higher fat approach. We’ve discussed the rationale behind why our Long Range Fuel ultra-endurance products are high in fat (see THE SCIENCE OF LONG RANGE FUEL section on this page), and you can learn more about the best way to use them by using this calculator. Scientists have demonstrated that consuming high amounts of fat for several days speeds the rate at which it passes through the stomach, which might reduce GI issues. So while less is known about training the gut to take up non-carbohydrate macronutrients, it seems reasonable to try gut training for these other nutrients too.

When training before an event, it’s best to mimic race conditions as closely as possible. This includes the terrain, climate, and so on. It also includes time of day. If, for example, you’ll be exercising through the night, some pre-event sessions should include training at night and practising your nutrition at this time too.


Is there something to learn from competitive eaters?

Although the sports nutrition science on this subject continues to evolve, competitive eaters have been using gut training techniques for years. The current hotdog eating World Record is an astonishing 76 hotdogs consumed in 10 minutes.


The techniques competitors use include consuming large volumes of liquid and food and work by 1) distending the stomach to tolerate more food, and 2) reducing feelings of fullness with a full stomach.

Doing training sessions directly after eating a large meal could help induce adaptations in the gut. Another strategy you could try is consuming large amounts of fluid during training — more than normal. Although neither of these strategies will be comfortable or lead to your best training session, they might pay off come race day.


Go get ‘em!

If you do experience any issues on race day and are worried about them getting worse, be flexible and not afraid to deviate from your plan — you might need to temporarily reduce your intake or stop consuming anything.

Since you’re reading this, you’re already ahead of many people who just show up on race day and consume whatever freebies they get on the day. Try giving one or two of the above strategies a go if you’re starting out, and don’t hesitate to let us know how you get on via social media.