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Big race coming up? Are you training your gut?

As covid restrictions start to ease, endurance events are back on the agenda. Over the past two years some of us may have had more time to run and ride but less opportunities to compete. As race day begins to near an often-overlooked area of preparation is race day nutrition and specifically the concept of training the gut.  For those who experience a bad race, nutrition is often cited as a major reason. Regret and wanting to do things differently next time is often discussed post-race. Gastro-intestinal (GI) problems are commonplace in athletes, its estimated that 30-50% experience GI issues, with warmer conditions and longer races such as an Ironman distance increasing the prevalence (1).

At a recreational level GI issues are also common. The challenge many people face is they often may use events to motivate them across a weight loss or lifestyle modification journey. This may result in training with a reduced energy intake, a lower carbohydrate diet and little attention to consuming carbohydrate during prolonged exercise.

When the demands of your race requires additional fuel and fluid consumption to optimize performance, this is when problems can arise. Without sufficient planning and familiarization to nutrition requirements prior to the race and on the day, a sub optimal performance or GI distress can occur.

Why does your gut matter?

It is well established that carbohydrate ingestion during endurance exercise (>2 hr.) of moderate to high intensity significantly improves performance. It does this by sparing muscle glycogen, preventing liver glycogen depletion, maintaining blood glucose and allowing high rates of carbohydrate oxidation (providing energy for the exercising muscle) (2). Limiting significant dehydration in warm conditions will also enhance performance and the ability to tolerate fluid is an important part of race preparation (3). In an often-overlooked process carbohydrate and fluid needs to clear the gut and get into circulation to do it’s job. Each step along the digestive process can potentially be enhanced by specific dietary strategies that can improve your performance and tolerance.

Can your gut be trained?

In simple terms yes, your gut is an organ that responds to stress and adapts like muscle.

Gastric emptying is the first step in the digestion process where contents move from the stomach to the small intestine, this is where a feeling of bloating may occur. A South African study of runners demonstrated tolerance of fluid consumption of a sports drink like solution (amounts similar to sweat losses over 90mins) improved by practicing the intake over several weeks (4). Interestingly there was no measurable changes in gastric emptying rate and the authors proposed the improved perceived tolerance was due extending the stomach walls to allow for greater fluid space.

The next and most important step in the digestion process getting carbohydrate out of the gut and into the blood, this is termed carbohydrate absorption. This process can be altered by your diet. Animal studies have shown 30% increases in carbohydrate absorption after 8 weeks on a higher carbohydrate diet (5). Absorption occurs by sugars being transported across the gut and into the blood, the transporters are specific to the type of sugar. There is an upper limit to how much each transporter can carry across per hour but this is individual and can be effected by diet. In general glucose has an upper limit of absorption of 60g per hour, while fructose is 30g. When higher rates of carbohydrate are required such as 90g per hour it’s therefore important to get the right combination of glucose and fructose, if not an excess of glucose or fructose may sit in the gut and can lead to GI distress. This higher level of intake is recommended more for long duration exercise >2.5 hours where intensity is significant and gut training is highly recommended (6).

Can athletes increase absorption from the gut?

Yes, in a study at the Australian Institute of sport, athletes who consumed carbohydrate during trainings over a 4-week period, increased how much carbohydrate could be used for energy in the muscle by 9% compared to those who only consumed water only. The authors suggested this was primarily due to increased absorption from the gut (7).

What about those with sensitive guts?

For those with GI concerns a full assessment with an experienced Sports Dietitian is recommended. The good news is that training the gut can also help. Researchers at Monash university demonstrated that 2 weeks of a structured gut training program, where participants consumed a carbohydrate solution every 20mins during exercise resulted in a 60% reduction in GI symptoms (nausea, pain, vomiting and diarrhoea) and a 5% improvement in performance during a prolonged exercise test of 2-3 hours (7).

How can you incorporate gut training into your race preparation?

  1. In preparation for your race, target certain sessions as your race nears. Look to replicate required fuel and fluid intakes to that of your race demands. This might look like focusing on the 6 weeks leading up to your race where you may target your longer weekend training sessions.
  2. Try to break down your intakes of fuel to every 20mins across the hour, this is better tolerated. If you struggle initially you may want to build up over consecutive weeks, start with 50% of your required intake every 20mins and look to build up each week as the race nears.
  3. Find what texture and fuel options work best for you. In most cases with running, liquids from carbohydrate in the form of drinks or gels will be more easily tolerated. In cycling you may want to consume more solid foods. For longer rides taste fatigue is something to consider for your fuelling strategy.
  4. If you are going to use the aid stations at events research the products that will be provided, incorporate these in your training sessions to ensure these are tolerated.
  5. Consider the timing of your event, most races start early. Try to replicate your pre-event meal, ideally giving yourself 90mins – 2 hours to digest. Familiarize yourself with this routine in training and ensure the composition of your meal is tolerated during exercise. Experiment with lower forms of fibre if digestion is an issue.
  6. For longer events increasing your carbohydrate stores the day before your event maybe required. Similar to your pre-event meal, trial this in your preparation. Lower fibre foods use here can reduce feelings of being bloated on race day.

Consulting with an experienced Sports Dietitian can be a vital tool in your training preparation. At Axis we will provide you with a periodised nutrition program across your training week that is specific to your energy demands and body composition goals. Across each day of training, you will have a clear plan of meal timing, portion requirements as well as a detailed fuel plan for training sessions that require additional fuelling to optimize gut function.


1. Jeukendrup, A.E. (2017) Training the Gut for Athletes. Sports Med, 47 (Suppl 1): S101-S110

2. Cermak, N.M., van Loon, L.J.C. (2013) The Use of Carbohydrates During Exercise as an Ergogenic Aid. Sports Med, 43(11):1139-55.

3. James, L.J., Funnell, M.P., James, R.M., Mears, S.A. (2019) Does Hypohydration Really Impair Endurance Performance? Methodological Considerations for Interpreting Hydration Research. Sports Med, 49 (Supple 2):103-114.  

4. Lambert, G.P., Bull, J.L.A., Eckerson, J., Lanspa, S, O’Brien, J.O. (2008) Fluid tolerance while running: effect of repeated trials. Int J Sports Med, 29(11):878–82.

5. Ginsburg, J.M., Heggeness, F.W. (1968) Adaptation in monosaccharide absorption in infant and adult rats. J Nutr, 96:494–8.

6. Jentjens, R.L., Jeukendrup, A.E. (2005) High rates of exogenous carbohydrate oxidation from a mixture of glucose and fructose ingested during prolonged cycling exercise. Br J Nutr, 93:485–92.

7. Cox, G.R., Clark, S.A., Cox, A.J, et al. (2010) Daily training with high carbohydrate availability increases exogenous carbohydrate oxidation during endurance cycling. J Appl Physiol, 109:126–34.

8. Costa, R.J.S., Miall, A., Khoo, A., Rauch, C., et al. (2017) Gut-training: the impact of two weeks repetitive gut-challenge during exercise on gastrointestinal status, glucose availability, fuel kinetics, and running performance. Appl Physiol Nutr Metab, 42 (5): 547-557.