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
This table shows a conservative order for experiments, not a compulsory weekly dose. 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.
| Example week | Practice | Decision before progressing |
|---|---|---|
| 1 | Record your usual intake and symptoms during a familiar session | Establish a baseline without changing several variables |
| 2 | Repeat one familiar carbohydrate source at planned intervals | Confirm ordinary tolerance during and after the session |
| 3 | If tolerated, make one small change toward the race plan | Hold or reverse the change if symptoms increase |
| 4 | Test the planned drink and food combination in representative conditions | Check concentration, thirst, access, and stomach response |
| 5 | Trial one backup texture or flavor; test caffeine separately if used | Keep only options that are practical and well tolerated |
| 6 | Rehearse the simple plan during a familiar, non-exhaustive session | Document the primary plan, backups, and reasons to stop |
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.
- Tiller, N.B. et al. (2019). ISSN position stand on ultramarathon nutrition. Journal of the International Society of Sports Nutrition.
- 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.