If your long runs end with urgent bathroom stops, cramping, or nausea, you are not broken. GI issues are common in endurance events.
You still need a fueling plan. The fix is usually not "push through" or "eat nothing." It is systematic trial-and-adjust with simpler inputs.
Why GI issues happen during long runs
Common contributors include:
- Too much carbohydrate per hour for your current gut tolerance.1
- Concentrated products without enough fluid.
- High fructose or caffeine load introduced too fast.
- Race anxiety, heat, dehydration, and pacing too hard too early.2, 3
Do not assume all nausea means too little carbohydrate. Excessive fluid intake is another possible concern; confusion, collapse, or severe symptoms need medical help.4
The minimum effective fueling target
For most marathon trainees:
- Start around 30-45 g carbohydrate/hour on long runs.
- Progress toward 45-60 g/hour if tolerated.
- Use small, regular doses every 20-30 minutes.
Higher intakes require appropriate product choice and individual practice; adaptation is possible but not guaranteed, and persistent symptoms need assessment.1, 5
A four-week fueling practice example
This is an editorial example, not a validated treatment protocol. Duration, targets, and progression depend on health, session length, existing tolerance, and professional advice. Do not progress while symptoms persist.
Week 1: establish a baseline
Checklist:
- Pick one simple fuel source.
- Target 30 g/hour.
- Pair each intake with water.
- Record timing, symptoms, and pace.
Week 2: increase only one variable
Checklist:
- Increase to 40-45 g/hour OR shorten interval between doses.
- Keep everything else constant (pre-run meal, caffeine, route).
- If symptoms spike, roll back one step.
Week 3: race-pace rehearsal
Checklist:
- Test fueling at planned marathon effort.
- Practice exact race-morning breakfast timing.6
- Include aid-station style drinking (small frequent sips).
Week 4: confirm race plan, no experiments
Checklist:
- Lock product, dose schedule, and backup option.
- Avoid new supplements, high-fiber meals, or unfamiliar gels.
- Prioritize sleep and stress management.
Sensitive stomach swaps that often help
- Use isotonic gels or diluted carb drink if concentrated gels trigger symptoms.
- Split one gel into two smaller intakes 10-15 minutes apart.
- Reduce caffeine dose or delay first caffeine serving.
- Lower pre-run fiber/fat in final 24 hours before key sessions.7
Race-week checklist
- Carbohydrate plan is written by time (not mood).
- Hydration plan is written by aid stations/intervals.
- Bathroom timing is rehearsed from wake-up to start corral.
- Emergency backup fuel is packed.
- Goal pace starts controlled to protect gut perfusion.
Build your personal fueling card (fill in your tested amounts)
Write this on your phone lock screen or race band:
- Write the carbohydrate grams per serving and your rehearsed grams per hour.
- Calculate the timing from those amounts, including carbohydrate in drinks.
- Record your practiced fluid strategy and aid-station availability; mouthfuls are not a measured or universal fluid dose.
- Include caffeine only if already tolerated and appropriate for you.
- Stop and seek help for severe symptoms; mild discomfort is a reason to ease off and reassess, not force the original intake.
A short card reduces race-day decision fatigue when stress is high.
Pre-run meal troubleshooting for sensitive guts
Most GI issues are not solved by removing all carbs. They are solved by finding the amount and timing you tolerate.
Practical framework:
- Eat the same breakfast before key long runs and race simulations.
- Keep pre-run fiber and fat lower than usual if symptoms are common.
- Finish main pre-run meal with enough time to digest.
- If nerves suppress appetite, split into smaller portions earlier.
Avoid changing everything at once. Keep a simple log and adjust one variable per long run.
Warning signs you are underfueling
- Unexpected fatigue or worsening effort during training.
- Repeated difficulty recovering after sessions.
- A pattern of inadequate intake alongside declining performance.8
These symptoms are not specific to under-fueling. Dizziness, confusion, collapse, or severe illness during a race require stopping and seeking medical help, rather than assuming another gel will fix the problem.
Underfueling can look like "mental weakness" but is often a solvable physiology and planning problem.
Red flags for overhydration and sodium mismatch
Some runners with sensitive stomachs overcorrect by drinking too much plain water.
Watch for:
- bloating that worsens while running,
- nausea despite low carbohydrate intake,
- confusion or headache with significant fluid intake.
Fluid needs vary with sweat loss, weather, duration, and individual context. Do not force fluids or aim to finish heavier than you started; a sweat-rate estimate is a planning aid, not a compulsory replacement target.12
When to see a professional
This article is educational and not medical advice.
Clear "when to see a professional" guidance
- Persistent GI pain, bleeding, vomiting, or post-run symptoms lasting more than a day.
- Recurrent diarrhea despite reducing dose/concentration and improving hydration.
- Signs of low energy availability (fatigue, recurrent illness/injury, menstrual disruption).
- Ongoing anxiety around eating/fueling that affects training or daily life.
A sports dietitian and qualified clinician can help identify causes and build a safer plan.9, 10
Test pre-race loading and in-race fuel as separate variables
Gastrointestinal symptoms can come from meal timing, food composition, running intensity, heat, fluid, carbohydrate concentration, product choice, or several factors together. Use separate rehearsals: first stabilise a familiar pre-run meal, then test one race-fuel change, then compare conditions. Record symptoms during the run and for several hours afterward.
The three-day carb-loading plan includes a tolerance-first worksheet for the days before the race. It is not a treatment for recurrent symptoms. Persistent pain, vomiting, blood, unexplained weight change, restrictive eating, or symptoms that prevent adequate fueling needs qualified assessment.
26weeks.ai fit: fewer choices, better execution
Writing down a rehearsed fueling plan can make race-day decisions simpler.
A practical coaching system should turn fueling into repeatable defaults:
- what to take,
- when to take it,
- and what to change when symptoms appear.
That is exactly where 26weeks.ai can reduce friction and keep your training executable.
FAQs
Should I stop fueling if gels upset my stomach?
Do not force a gel through nausea, vomiting, or pain. For mild recurring discomfort, review product, concentration, timing, pace, and fluid with a dietitian, testing one change in training. Severe or persistent symptoms need medical assessment.
Are "natural" fuels always easier on the stomach?
Not always. Tolerance is individual. Test one product at a time.
How much caffeine should I use?
Do not add caffeine automatically if you have gastrointestinal sensitivity. Consider whether a non-caffeinated product is more suitable and discuss individual use with a qualified professional when needed.11
Can anxiety alone cause GI distress on race day?
Anxiety can contribute, but do not assume it is the only cause. Rehearsed routines may help you manage uncertainty; recurrent or severe symptoms deserve assessment.
Next step
Want an adaptive plan that adjusts your fueling rules when long-run feedback changes? Start training now.
References
Sources and associated claims reviewed October 17, 2026. Practical examples are editorial guidance, not independently validated protocols.
1 Contemporary Nutrition Strategies to Optimize Performance in Distance Runners and Race Walkers. (2019). Read the source.
2 Use of Carbohydrate (CHO), Gluten-Free, and FODMAP-Free Diets to Prevent Gastrointestinal Symptoms in Endurance Athletes: A Systematic Review. (2024). Read the source.
3 Gastrointestinal complaints during exercise: prevalence, etiology, and nutritional recommendations. (2014). Read the source.
4 Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. (2015). Read the source.
5 Training the Gut for Athletes. (2017). Read the source.
6 American College of Sports Medicine Joint Position Statement. Nutrition and Athletic Performance. (2016). Read the source.
7 Contemporary Nutrition Strategies to Optimize Performance in Distance Runners and Race Walkers. (2019). Read the source.
8 Overtraining Syndrome (OTS) and Relative Energy Deficiency in Sport (RED-S): Shared Pathways, Symptoms and Complexities. (2021). Read the source.
9 2023 International Olympic Committee's (IOC) consensus statement on Relative Energy Deficiency in Sport (REDs). (2023). Read the source.
10 Academy of Nutrition and Dietetics. Scope of Practice for the Registered Dietitian Nutritionist. Read the source.
11 International society of sports nutrition position stand: caffeine and exercise performance. (2021). Read the source.
12 American College of Sports Medicine position stand. Exercise and fluid replacement. (2007). Read the source.