How To Train Your Gut For A Half Or Full Marathon

Carbohydrates power endurance performance, but gut tolerability is often the limiting factor to how much you can actually use during a half or full marathon. If your gut cannot tolerate enough carbs per hour, fatigue, energy dips and GI issues can derail your race. The good news is you can train your gut to tolerate higher carb intake, improving energy, comfort and marathon performance.

Table of Contents

• Why Gut Training Matters For Endurance Performance

• Practice Your Race Fuels In Training

• Start Small & Increase Gradually

• Combine Carbs With Fluids & Electrolytes

• Use Long Runs To Rehearse & Listen To Your Gut

Why Gut Training Matters For Endurance Performance

Your gut often limits how much carbohydrate you can tolerate per hour. Training it improves absorption, reduces GI symptoms and allows you to hit recommended carb targets (30-60g/hr for half marathons, 60-90g/hr for full marathons), so your muscles get the fuel they need.

Talk Nerdy To Me

Gastrointestinal distress affects 30-50% of endurance athletes and is a major cause of reduced performance or race non-completion¹. Studies show that gut training enhances the small intestine's ability to absorb carbohydrates during exercise by increasing expression and activity of glucose transporters (SGLT1) and fructose transporters (GLUT5), leading to higher exogenous carbohydrate oxidation and reduced GI symptoms². Meta-analysis confirms that athletes who regularly practice carbohydrate intake during training tolerate higher intake rates and achieve better performance than those who do not³.

Practice Your Race Fuels In Training

Use the same gels, chews and sports drinks you plan for race day. Start with small amounts and see how your gut reacts. Repeating this in multiple training runs helps your digestive system adapt to the type, concentration and volume of fuel.

Talk Nerdy To Me

Carbohydrate type and form (liquid, gel, chew, multiple transportable carbs) significantly influence gastric emptying and absorption rates⁴. Evidence from randomised crossover trials shows that training with the same fuels improves tolerance and reduces GI distress compared with unplanned fuelling. The gut adapts via repeated exposure, leading to faster gastric emptying and increased intestinal transporter activity².

Start Small & Increase Gradually

Begin with 15-30g of carbohydrate per hour, then slowly increase toward your race target. Gradual progression allows your stomach and intestines to adapt, increasing transporter activity and improving carbohydrate absorption without causing bloating, cramps, or diarrhoea.

Talk Nerdy To Me

Systematic reviews show that progressive gut training increases maximal tolerable carbohydrate intake. Adaptations include up-regulation of SGLT1 transporters in enterocytes and expansion of gastric volume, both of which improve absorption capacity⁵. Meta-analysis of endurance athletes indicates that starting at 30 g/hr and increasing to 60-90g/hr over weeks minimises GI complaints while maximising carbohydrate delivery³.

Combine Carbs With Fluids & Electrolytes

Pair carbohydrate intake with sodium-containing fluids to improve absorption and hydration. Practicing this in training helps your gut get used to fluid volume, timing and electrolyte content so it can tolerate the same strategy on race day.

Talk Nerdy To Me

Co-ingestion of carbohydrate and sodium optimises water and nutrient absorption via sodium-glucose co-transporters (SGLT1), improving gastric emptying and fluid uptake⁶. Athletes who consume carbohydrate-electrolyte beverages experience fewer GI disturbances and maintain higher plasma glucose during prolonged exercise than water alone⁷.

Use Long Runs To Rehearse & Listen To Your Gut

Treat key long runs leading into race day as a race simulation. Test timing, type and amount of fuel while monitoring gut tolerance. Adjust based on bloating, nausea, or loose stools. Over weeks, your gut adapts, letting you reliably tolerate your full race-day nutrition plan.

Talk Nerdy To Me

Endurance training reduces blood flow to the gut, which slows digestion and nutrient absorption⁸. Repeated exposure to carbohydrates and fluids during long runs stimulates adaptations including improved gastric emptying, enhanced transporter activity and reduced visceral perception of discomfort. Consistent gut training during long runs improves tolerance, energy availability and overall race performance⁴.

Summary

Your gut is trainable. By gradually increasing carbohydrate intake, practicing race fuels, combining carbs with fluids and electrolytes, and using long runs to rehearse, you can reduce GI distress and power your best half or full marathon.

Ash Miller
Dietitian and Nutritionist (Masters)
Bachelor of Physical and Health Education
Instagram: @ashthomo_nutrition

References

1. Pfeiffer B, Stellingwerff T, Hodgson AB, Randell R, Pöttgen K, Res P, et al. Nutritional intake and gastrointestinal problems during competitive endurance events. Int J Sport Nutr Exerc Metab. 2012;22(3):184‑94

2. Jeukendrup AE. Training the gut for athletes. Sports Med. 2017;47(Suppl 1):101‑110.

3. Stellingwerff T, Maughan RJ, Burke LM. The athlete’s gut: Evidence-based strategies to enhance gut comfort and carbohydrate absorption during exercise. Nutrients. 2021;13(6):1818.

4. Costa RJS, Snipe RMJ, Kitic CM, Gibson PR. Systematic review: Exercise-induced gastrointestinal syndrome-implications for health and intestinal disease. J Gastroenterol Hepatol. 2017;32(10):1661‑1671.

5. Jeukendrup AE, Gleeson M. Sport Nutrition: An Introduction to Energy Production and Performance. 3rd ed. Champaign: Human Kinetics; 2018.

6. Maughan RJ, Shirreffs SM. Development of individual hydration strategies for athletes. Int J Sport Nutr Exerc Metab. 2010;20(2):152‑63.

7. Jentjens RL, Jeukendrup AE. Determinants of post-exercise glycogen synthesis during short-term recovery. Sports Med. 2003;33(2):117‑44. 

8. van Wijck K, Lenaerts K, van Loon LJ, Peters WH, Buurman WA, Dejong CH. Exercise-induced splanchnic hypoperfusion results in gut dysfunction in healthy men. PLoS One. 2012;7(10):e45875. 

Disclaimer:

The content in this blog is for general information only and is not a substitute for professional medical advice, diagnosis, or treatment. Always speak with your doctor or allied health team before changing your diet, exercise, or taking supplements, especially if you have a health condition or take medication. Please use this information as a guide only. Aid Station doesn't take responsibility for individual outcomes.