Gut Training for Ultra Runners: How to Stop GI Distress on Race Day

Learn why GI distress hits ultra runners and the research-backed gut training protocol to build carb tolerance and finally stop stomach issues on race day.

Somewhere between hour four and hour eight of an ultramarathon, most runners hit the same wall. Not a glycogen wall. A gut wall. Nausea creeps in, the stomach stops emptying, and food that should be fuel just sits there. Gut training is the fix nobody talks about early enough, and the research on it is now specific enough to build a real plan around. Between 30% and 90% of endurance athletes report gastrointestinal symptoms during competition, with incidence climbing as events get longer (Costa et al., 2017). In ultramarathons specifically, GI distress is consistently cited as the leading cause of DNFs, ahead of musculoskeletal injury. This guide breaks down why the gut fails on long runs, what actually triggers it beyond pace, and the progressive protocol that lets you absorb more fuel without paying for it later.

Why Your Gut Shuts Down Mid-Race

The gut doesn't fail randomly. It fails because of a deliberate physiological trade-off.

During prolonged exercise, the sympathetic nervous system redirects blood away from the digestive tract toward working muscle and skin, where it's needed for power output and cooling. Researchers call the resulting drop in gut blood flow splanchnic hypoperfusion, and it can reduce intestinal perfusion by more than 50% during intense effort, with the sharpest drop happening in the first ten minutes of hard exercise (Van Wijck et al., 2011). Less blood flow means a slower-emptying stomach, weaker nutrient absorption, and a gut lining that becomes more permeable to bacterial byproducts, which is part of why long races leave some runners feeling flu-like even without an infection.

Costa and colleagues describe this as exercise-induced gastrointestinal syndrome, driven down two parallel pathways: a circulatory one (blood flow redirected away from the gut) and a neuroendocrine one (rising sympathetic activity that slows overall digestive function on its own) (Costa et al., 2017). The two compound each other. The harder and longer you run, the worse both pathways get, which explains why GI symptoms rarely show up on an easy 10k but show up reliably past the three-to-four hour mark.

The practical takeaway: your gut isn't broken during a race. It's doing exactly what evolution built it to do under sustained physical stress. Training it well means working with that response, not fighting it.

The Triggers Beyond Pace

Pace and duration set the baseline risk, but four other factors decide whether that risk turns into a bathroom emergency at mile 40.

High-fiber and high-fat meals close to race start. Both slow gastric emptying, leaving food sitting in a stomach that already has reduced blood flow. Cutting fiber and fat in the 24 to 48 hours before a long effort is one of the simplest, best-supported changes a runner can make.

FODMAPs. Fermentable carbohydrates found in wheat, certain fruits, dairy, and legumes draw water into the intestine and ferment rapidly in the colon, producing gas and cramping. A case study following a female ultra-endurance runner with IBS found that a short-term low-FODMAP diet in the days before and during a 186.7km multistage mountain race cut her symptoms from severe to mild (Gaskell & Costa, 2019). A follow-up controlled trial confirmed that a 24-hour high-FODMAP diet measurably worsened GI symptom markers compared to a low-FODMAP day under identical heat and exercise stress (Gaskell et al., 2020). You don't need to go low-FODMAP permanently, but a short trial run in training tells you fast whether it's worth doing race week.

NSAIDs. Ibuprofen and similar drugs taken "just in case" before a long run increase intestinal permeability and compound the gut damage already happening from exercise. If you're prone to GI issues, race-day NSAID use is one of the easiest things to simply stop.

Dehydration and heat. Fluid loss concentrates stomach contents and further restricts already-limited splanchnic blood flow. Heat acclimation over one to two weeks before a hot race, combined with a conservative early pace, measurably reduces the gut ischemia that drives symptoms later on.

None of these triggers are things you can gut-train your way past. They're separate levers, and pulling them is often more effective in the short term than any amount of carbohydrate adaptation.

The Gut-Training Protocol That Actually Works

Most runners assume GI tolerance is fixed, something you either have or don't. It isn't. The intestine adapts to repeated carbohydrate exposure much like a muscle adapts to repeated loading.

Glucose crosses the intestinal wall through a transporter called SGLT1, which caps out around 60g per hour regardless of how much you eat. Fructose uses a separate transporter, GLUT5, so combining the two carbohydrate types lets total absorption climb past 60g and into the 90g-plus range that ultrarunners increasingly rely on for long, hard efforts. Repeated exposure to carbohydrate during training increases the density and activity of these transporters (Jeukendrup, 2010), which is the actual mechanism behind "training your gut."

The clinical evidence for this is now solid. In a landmark study, trained runners who consumed 90g/hr of carbohydrate (a 2:1 glucose-fructose blend) during five sessions per week for two weeks showed reduced GI symptoms, less carbohydrate malabsorption, and better performance in a self-paced running test compared to a placebo group (Miall et al., 2018). A separate trial found that just 28 days of high daily carbohydrate intake increased how much ingested carbohydrate trained cyclists could actually oxidize during exercise, confirming the gut itself adapts independent of fitness (Cox et al., 2010). More recent work has tested feeding density and food form, and found that both carbohydrate gels and solid, carbohydrate-rich foods produce comparable symptom reductions when used consistently (Martinez et al., 2025).

A Practical Progression

Start at 30g of carbohydrate per hour during a long training run, using the exact products or foods you plan to race with. Hold that for one to two weeks, then add roughly 10g/hr every two to three weeks:

  • Weeks 1-2: 30g/hr, single carbohydrate source, easy-paced long runs
  • Weeks 3-4: 40-50g/hr, switch to a glucose-fructose blend
  • Weeks 5-6: 60-70g/hr, at goal race intensity for at least part of the run
  • Weeks 7-8+: 80-90g/hr, full race-pace and race-nutrition rehearsal

Measurable changes in carbohydrate malabsorption appear after roughly two weeks of consistent exposure (Miall et al., 2018), but a full 8 to 12 week block before an A-race gives you room to troubleshoot format issues along the way, since gut tolerance is specific to what and how you're eating, not just how much.

If symptoms show up at a given dose, change the delivery before you cut the amount. A concentrated gel with no water behind it behaves very differently in the stomach than the same carbohydrate spread across a diluted drink mix, and switching formats often solves a problem that looks like an intake problem.

This is exactly the kind of variable that benefits from structured tracking rather than guesswork. Trailia's Race Readiness tool logs your training fueling alongside HRV and workout data, so you can see whether a rough long run traces back to under-fueling, a new product, or something unrelated like sleep or heat.

Building Your Personal Gut-Training Plan

A gut-training block only works if it mirrors race conditions closely enough to mean something. Train at the intensity you'll actually race at, not an easy shuffle, because exercise above roughly 70% of VO2 max reduces gut blood flow far more than moderate efforts do, and a dose that's fine in zone 2 can fail at threshold. Use the identical gels, chews, or real food you'll carry on race day, since even minor formulation differences (added fiber, different sweeteners, higher fat content) change how a product behaves under stress. Rehearse once or twice a week rather than daily. The adaptation comes from repeated exposure over weeks, not from cramming every session with maximum carbohydrate.

For runners targeting a specific ultra, working backward from race day with a distance-appropriate fueling target makes the whole block easier to plan. Trailia's Race Predictor factors in your elevation profile and historical pacing to estimate finish time and effort distribution, which in turn tells you roughly how many hours you'll need to fuel through and at what intensity, so your gut-training sessions can target the right dose at the right effort level rather than a generic number pulled from a forum post.

Keep a short symptom log during these sessions: what you ate, how much, at what pace, and what happened in your gut over the following two hours. Patterns show up fast, usually within three or four long runs, and they're far more useful than any generic "60-90g/hr" guideline applied blindly.

When Symptoms Persist

Some GI distress responds to protocol changes and some doesn't. If nausea, cramping, or diarrhea persist even after fixing fueling timing, format, dose, and pre-race diet, it's worth ruling out an underlying issue like IBS, celiac disease, or a lactose or fructose intolerance with a doctor or sports dietitian, rather than continuing to cycle through gels hoping one will finally work. A short, structured low-FODMAP trial under professional guidance can clarify a lot here, since it isolates a specific mechanism rather than changing five variables at once.

Key Takeaways

  • GI distress during long efforts comes from reduced gut blood flow (splanchnic hypoperfusion) plus rising sympathetic activity, not a personal weakness or bad luck
  • Cut fiber and fat in the 24 to 48 hours before a long run or race, and skip NSAIDs on race day
  • Build carbohydrate tolerance gradually: start around 30g/hr and add roughly 10g/hr every two to three weeks toward your race target of 60-90g/hr
  • Combine glucose and fructose sources to push absorption past the 60g/hr single-transporter ceiling
  • If symptoms don't clear with a structured protocol, get a persistent low-FODMAP trial or a gut-focused checkup rather than guessing further

Conclusion

GI distress isn't a fixed trait some runners are cursed with and others aren't. It's a trainable system, governed by blood flow, sympathetic tone, and carbohydrate transporter density, all of which respond to consistent, specific practice. Start your next training block with 30g/hr of your race-day carbohydrate source on a long run, add roughly 10g/hr every couple of weeks, and treat gut training with the same seriousness you give your long runs and vert work. The runners who finish strong at hour ten aren't the ones with lucky stomachs. They're the ones who rehearsed this months earlier.

Citations

Costa, R.J.S., Snipe, R.M.J., Kitic, C.M., & Gibson, P.R. (2017). Systematic review: exercise-induced gastrointestinal syndrome-implications for health and intestinal disease. Alimentary Pharmacology & Therapeutics, 46(3), 246-265. Van Wijck, K., Lenaerts, K., Grootjans, J., Wijnands, K.A., Poeze, M., van Loon, L.J., Dejong, C.H., & Buurman, W.A. (2011). Exercise-induced splanchnic hypoperfusion results in gut dysfunction in healthy men. PLoS ONE, 6(7), e22366. Miall, A., Khoo, A., Rauch, C., Snipe, R.M.J., Camões-Costa, V.L., Gibson, P.R., & Costa, R.J.S. (2018). Two weeks of repetitive gut-challenge reduce exercise-associated gastrointestinal symptoms and malabsorption. Scandinavian Journal of Medicine & Science in Sports, 28(2), 630-640. Cox, G.R., Clark, S.A., Cox, A.J., Halson, S.L., Hargreaves, M., Hawley, J.A., Jeacocke, N., Snow, R.J., Yeo, W.K., & Burke, L.M. (2010). Daily training with high carbohydrate availability increases exogenous carbohydrate oxidation during endurance cycling. Journal of Applied Physiology, 109(1), 126-134. Jeukendrup, A.E. (2010). Carbohydrate and exercise performance: the role of multiple transportable carbohydrates. Current Opinion in Clinical Nutrition and Metabolic Care, 13(4), 452-457. Gaskell, S.K., & Costa, R.J.S. (2019). Applying a low-FODMAP dietary intervention to a female ultra-endurance runner with irritable bowel syndrome during a multistage ultramarathon. International Journal of Sport Nutrition and Exercise Metabolism, 29(1), 61-67. Gaskell, S.K., Taylor, B., Muir, J., & Costa, R.J.S. (2020). Impact of 24-h high and low fermentable oligo-, di-, monosaccharide, and polyol diets on markers of exercise-induced gastrointestinal syndrome in response to exertional heat stress. Applied Physiology, Nutrition, and Metabolism, 45(6), 569-580. Martinez, I.G., Biesiekierski, J.R., Rauch, C.E., & Costa, R.J.S. (2025). Repetitive feeding-challenge with different nutritional densities on markers of gastrointestinal function, substrate oxidation, and endurance exercise performance. International Journal of Sport Nutrition and Exercise Metabolism, 35(3), 173-191.

Frequently Asked Questions

What is gut training for ultra runners?

Gut training is the deliberate, progressive practice of consuming carbohydrate during exercise at increasing rates to teach your digestive system to absorb more fuel without triggering nausea, cramping, or diarrhea. It works by increasing the density and activity of intestinal carbohydrate transporters through repeated exposure. Most protocols run 6 to 12 weeks, starting around 30g of carbohydrate per hour and building toward a race-day target of 60-90g/hr.

Why do I get stomach issues during long ultra races but not shorter runs?

Exercise redirects blood flow away from the gut toward working muscle and skin, a process called splanchnic hypoperfusion, and this effect compounds the longer and harder you run. Past roughly three to four hours, reduced gut blood flow combines with rising sympathetic nervous system activity to slow digestion and increase intestinal permeability. Shorter, easier runs rarely push either factor far enough to cause noticeable symptoms.

How long does gut training take to work?

Measurable improvements in carbohydrate absorption and reduced GI symptoms have been shown after as little as two weeks of consistent daily exposure in research settings. For a full race preparation, an 8 to 12 week block gives you enough time to also troubleshoot food formats, timing, and personal tolerance rather than just raising the carbohydrate dose.

How much carbohydrate per hour should I train my gut to handle?

Start at 30g/hr and add roughly 10g/hr every two to three weeks. A single carbohydrate source like glucose typically caps around 60g/hr because it relies on one intestinal transporter. Combining glucose and fructose uses two separate transporters, which lets a well-trained gut absorb 90g/hr or more, the range most ultrarunners target for long, hard efforts.

Should I avoid NSAIDs like ibuprofen before an ultra?

Yes. Taking ibuprofen or similar anti-inflammatories as a preventive measure before or during a race increases intestinal permeability and compounds the gut stress already caused by exercise. If you're prone to GI distress, skipping race-day NSAID use is one of the simplest changes you can make.

Does a low-FODMAP diet help with GI distress in ultrarunning?

For runners prone to bloating, gas, or cramping, a short-term low-FODMAP diet in the 24 to 48 hours before a long effort has reduced symptoms in both case studies and controlled trials. It isn't necessary for every runner, but testing it during a training long run is a low-risk way to find out if fermentable carbohydrates are part of your problem.

Should I eat gels or real food during an ultramarathon?

Research comparing carbohydrate gels to solid, carbohydrate-dense foods has found comparable reductions in GI symptoms when either is used consistently during gut training. The better choice usually comes down to personal tolerance, race duration, and practicality on the course rather than one format being objectively superior.

Why does my stomach get worse on hot race days?

Heat increases the competition for blood flow between the skin (for cooling), working muscle, and the gut, worsening the splanchnic hypoperfusion that already happens during exercise. Dehydration compounds this by concentrating stomach contents and further reducing gut perfusion. Acclimating to heat for one to two weeks before a hot race and pacing conservatively early on both help reduce this effect.

What should I do if gut training doesn't fix my symptoms?

If nausea, cramping, or diarrhea persist after adjusting fueling dose, format, and timing, and after cutting fiber, fat, and NSAIDs pre-race, it's worth discussing underlying conditions like IBS, celiac disease, or a specific food intolerance with a doctor or sports dietitian. Continuing to cycle through products without addressing a root cause rarely solves persistent, severe symptoms.

Can I start gut training during race week?

No. Gut training relies on repeated exposure over multiple weeks to increase intestinal transporter activity, so starting during race week won't produce meaningful adaptation and risks introducing new GI symptoms right before your event. Build gut training into your final structured training block, several weeks out, and taper the intensity of sessions the same way you taper mileage.