Fueling is a race skill, but the gut is not something to overpower. Practice should make a tested plan more familiar while preserving the quality and recovery purpose of the training session.
Why Practice Fueling?
In small randomized studies, endurance runners who repeatedly consumed carbohydrate during exercise reported fewer gastrointestinal symptoms and showed less carbohydrate malabsorption during a later challenge. Those protocols were short and controlled; they do not prove that one intake rate, product, or six-week schedule works for every ultrarunner.
Start with the full guide to eating during a 24-hour race if you need the broader planning framework. If you have inflammatory bowel disease, celiac disease, diabetes, previous heat illness, recurrent exercise-related GI symptoms, or medication that affects digestion or fluid balance, get individualized advice before experimenting.
Set Individual Targets
Begin by recording the session duration and intensity, weather, foods or products, estimated carbohydrate, fluid, caffeine, and symptoms. Use that baseline to change one variable at a time. A target suitable for a hard, short event may not suit an easy multiday effort, and product concentration can matter as much as a headline hourly number.
Caffeine is optional. Individual performance response, anxiety, heart-rate effects, stomach tolerance, and sleep disruption vary. If you intend to use it, test a known amount separately from other changes and discuss it with a clinician or pharmacist when a health condition or medication may interact. Caffeine is not a remedy for sleepiness that makes continued movement unsafe.
An Optional 6-Week Practice Sequence
A gut-training sequence that does not raise the amount of carbohydrate is not gut training; it is repetition. The table below progresses the quantity explicitly, in grams of carbohydrate per hour, because that is the variable the practice is meant to change.
Stated baseline and target. The example starts at 20 g per hour — roughly one standard gel — and adds about 10 g per hour each week toward 50 g per hour, the top of the 30–50 g/h practical range that the ISSN single-stage ultramarathon position stand describes for ultra intensities. That is four steps: 20 → 30 → 40 → 50, reached in week 4, with weeks 5 and 6 spent confirming rather than adding. If your own recorded baseline is higher or lower, shift the whole ladder; the +10 g/h per week step is the illustrative part, not the starting number.
| Week | Carbohydrate | Session | What to practise | Decision before progressing |
|---|---|---|---|---|
| 1 | 20 g/h (baseline; 40 g total) | 2 h easy | Record what you already take and every symptom, without changing anything | You have a written baseline, not a guess |
| 2 | 30 g/h (+10; 60 g total) | 2 h easy | One familiar carbohydrate source, taken on a fixed interval — for example 15 g every 30 minutes | Ordinary tolerance during and after the session |
| 3 | 40 g/h (+10; 100 g total) | 2.5 h easy | Same source, higher rate. Change the rate only — not the product, the concentration, or the weather | Hold at 30 g/h for another week if symptoms increase |
| 4 | 50 g/h (+10; 150 g total) | 3 h, long-run effort | Target rate reached. Add the planned drink alongside the solid or gel and check the combined concentration | Thirst, access and stomach response all acceptable |
| 5 | 50 g/h (hold; 150 g total) | 3 h in race-like conditions | Same rate, harder context: race start time, race terrain, one backup texture or flavour. Test caffeine separately if you use it | Keep only options that are practical and well tolerated |
| 6 | 50 g/h (rehearse; 150–200 g total) | 3–4 h, familiar and non-exhaustive | Run the whole plan end to end, including the fallback you would use if the primary product fails | Document the primary plan, the backups, and the reasons to stop |
The arithmetic worth checking is per session, not per hour. Week 4 asks for 150 g of carbohydrate across three hours — at 25 g per sachet that is six gels, or two per hour, and about 600 kcal. Seeing the session total is usually what reveals whether a plan is portable and affordable before it reveals whether it is tolerable.
If your target is higher than 50 g/h. The 60–90 g/h figures quoted from broader endurance-nutrition guidance assume multiple transportable carbohydrates — glucose–fructose mixes and products built for high intake rates — and a deliberately trained gut. Continuing the same +10 g/h ladder past week 6 is the mechanism, but the ceiling is a ceiling and not a default; the two carbohydrate ranges are explained here. Jeukendrup's review of gut training concludes that repeated exposure can change gastric emptying and absorption, not that any published schedule produces tolerance in a given athlete.
Use suitable easy or long sessions already present in your plan. Repeat a step until it is tolerable, take a lighter week, or stop the sequence when symptoms or recovery worsen. Going backwards a step is a normal outcome, not a failed block.
Practice Food Options
Some runners prefer a mix of sports products and familiar foods as event duration increases. Test preparation, portability, food safety, chewing, and tolerance before race day. Variety is an option, not a requirement, and no item is universally easy to digest.
- Choose one simple familiar food or sports product per experiment.
- Keep portion size and timing consistent enough to interpret symptoms.
- Test hot or cold items only when your race setup can provide them safely.
- Record ingredients so a dietitian or clinician can spot repeat triggers.
Controlled Rehearsal
Rehearse the plan during the later part of a familiar session or a normal training weekend. Do not extend a workout solely to make eating harder. The useful outcome is a repeatable plan with known backups, not proof that you can force intake while exhausted.
Backyard runners can rehearse their between-loop organization with the backyard ultra nutrition guide. Keep the exercise load appropriate to the training plan and allow normal recovery before the next experiment.
Red Flags
Stop the session and seek urgent medical help for confusion, fainting, chest pain, severe breathing difficulty, blood in vomit or stool, or severe or rapidly worsening abdominal pain. Repeated vomiting, inability to keep fluid down, marked dizziness, or loss of coordination also requires prompt assessment rather than another fueling experiment.
Recurrent milder symptoms deserve review too, especially when they occur outside exercise, lead to food restriction, or interfere with hydration and recovery. A sports dietitian can help adjust the plan, while a clinician can assess causes that are not solved by gut training.
Sources
- Costa, R.J.S. et al. (2017). Two weeks of repetitive gut challenge during exercise. Applied Physiology, Nutrition, and Metabolism.
- Miall, A. et al. (2017). Repetitive gut challenge, gastrointestinal symptoms, and malabsorption. Scandinavian Journal of Medicine & Science in Sports.
- Jeukendrup, A.E. (2017). Training the Gut for Athletes. Sports Medicine, 47(Suppl 1), 101–110. PMID 28332114.
- Tiller, N.B. et al. (2019). International Society of Sports Nutrition Position Stand: nutritional considerations for single-stage ultra-marathon training and racing. Journal of the International Society of Sports Nutrition, 16(1), 50. PMID 31699159. Source of the 150–400 kcal and 30–50 g carbohydrate per hour ranges used above.
- Thomas, D.T., Erdman, K.A. & Burke, L.M. (2016). American College of Sports Medicine Joint Position Statement: Nutrition and Athletic Performance. Medicine & Science in Sports & Exercise, 48(3), 543–568. Source of the 60 g/h and up-to-90 g/h figures referenced above.
- Hew-Butler, T. et al. (2015). Third international exercise-associated hyponatremia consensus statement. British Journal of Sports Medicine.
- Guest, N.S. et al. (2021). ISSN position stand on caffeine and exercise performance. Journal of the International Society of Sports Nutrition.
Frequently Asked Questions
Can you train your stomach for ultramarathons?
Small controlled trials in endurance runners suggest that repeated carbohydrate practice during exercise can reduce gastrointestinal symptoms and malabsorption. That does not guarantee tolerance in every runner or justify forcing intake through significant symptoms.
How much should you eat during gut-training runs?
There is no universal calorie or carbohydrate target for every runner and session. Record what you already tolerate, then change one variable at a time toward an individualized race plan based on duration, intensity, conditions, body size, health, and product concentration. A sports dietitian can help set the target.
What if gels make you sick during ultras?
Do not force a product that repeatedly causes symptoms. Test one alternative at a time—such as a different carbohydrate form, concentration, texture, or familiar food—and seek a clinician or sports dietitian if symptoms are severe, recurrent, or present outside exercise.