Why Your Stomach Fails During Ultras and How to Train It

Most stomach problems during ultras come from three things stacking up: blood diverted away from your gut for hours, heat, and more food or fluid than your gut has been trained to handle. You can’t remove the first, but managing heat and doing two weeks of deliberate gut training both reduced symptoms in controlled trials.

Almost everyone gets something. At the 161 km Western States race, 96% of 272 runners reported gut symptoms, and about a third of those who dropped out named gut trouble as a reason, usually nausea (Stuempfle & Hoffman, 2015). Our ultra marathon nutrition guide covers the basics. This piece goes further: what each symptom is telling you, how to build a gut-training block around Malaysian weather, and what to do at a checkpoint when it’s already going wrong.

What happens to your gut in a long race?

It gets less blood, slows down and absorbs less. During hard exercise, especially when you’re also dehydrated, blood flow to the gut drops, and this is thought to be one of the main drivers of symptoms (de Oliveira et al., 2014). A systematic review from Monash University put a rough threshold on it: about two hours at 60% of maximal aerobic capacity is where meaningful gut disturbance shows up, whatever your fitness. Past that point the stomach empties more slowly, the small intestine moves food along more slowly, and some carbohydrate isn’t absorbed at all. Running is harder on the gut than cycling, and heat makes everything worse (Costa et al., 2017a).

So a 50 km trail race like the Riggers 50 at the Angsi Forest Challenge is past that threshold before most runners reach the halfway checkpoint. Fitness buys you speed. It doesn’t exempt your stomach.

Why heat makes it so much worse

Heat roughly multiplied the damage in the clearest study we have. Ten endurance runners ran two hours at a steady 60% effort, once at 35°C and once at 22°C. In the heat, core temperature rose 2.4°C instead of 1.4°C, a blood marker of intestinal injury rose 432% instead of 127%, and 90% of runners had gut symptoms instead of 70%. Symptom severity was more than ten times higher. Nausea tracked core temperature (Snipe et al., 2018).

Look at those two temperatures from a Malaysian point of view. 22°C is cooler than almost any lowland race here ever gets; 35°C is a normal hot afternoon. At Western States, too, nausea peaked in the hottest, hardest section (Stuempfle & Hoffman, 2015). That changes how you should think about cooling. Ice in your buff, water over your head at every stream crossing and easing off on exposed climbs at midday aren’t only comfort measures. They’re gut strategy.

What is your symptom telling you?

Different symptoms point to different fixes. These are starting points drawn from the causes above, not diagnoses:

One finding surprises people. In a study of 221 athletes across Ironman, marathon and cycling races, carbohydrate intake ranged from 6 to 136 g an hour. Higher intakes were linked to more nausea and wind, but in the Ironman races they were also linked to faster finishing times. The strongest predictor of gut trouble was a history of gut trouble (Pfeiffer et al., 2012). The answer usually isn’t to eat less. It’s to train your gut to cope with more, and if you’ve had problems before, you are exactly the person who needs to.

How do you train your gut?

By eating and drinking during training the way you plan to on race day, regularly, for at least two weeks. In a Monash trial, 25 runners did two weeks of repeated runs with carbohydrate. Gut symptoms in the follow-up test fell by 60% in the group that used gel discs and 63% in the group that used ordinary carbohydrate foods, while a placebo group didn’t improve. Both carbohydrate groups also ran further in a one-hour test afterwards (Costa et al., 2017b). Real food worked about as well as gels. That’s good news if your race lays out bananas and rice at the checkpoints.

Why it works isn’t fully mapped, but the gut is adaptable: stomach emptying and comfort improve with practice, and a carbohydrate-rich diet increases the transporters that absorb glucose (Jeukendrup, 2017).

A practical 14-day block for a Malaysian ultra:

• Days 1 to 7: three or four easy runs of 60 to 90 minutes, taking about 60 g of carbohydrate an hour in the form you’ll race with, and drinking roughly what you plan to drink per hour on race day.
• Days 8 to 14: step up towards your race target, often 75 to 90 g an hour, and add one long run of three hours or more that starts late enough to catch the heat. Penang Hill or Gunung Datuk at 10 am is a better test than a 6 am loop of the park.
• Every session: score your gut from 0 to 10 at the end. If a food scores badly twice, drop it.
• Race foods: find out what the organiser serves and practise with that. If the race kit says watermelon, banana and cola, eat watermelon, banana and cola on your long runs.

The honest caveat: these trials ran for two weeks, used small groups and tested runs of about three hours. Nobody has shown that gut training changes how you finish a 100 km. It’s still the best-supported thing you can do. Use our guide to carbs per hour to set your target.

What should you eat in the two days before?

Less fibre and fewer fermentable foods, if you’re prone to trouble. In a small trial of 11 runners with exercise-related gut problems, six days on a low-FODMAP diet cut daily symptoms such as wind, urgency and loose stools, though it didn’t change symptoms during the runs themselves (Lis et al., 2018). That’s preliminary evidence, not a rule. In practice it means the dhal with your roti canai, onion-heavy sambal, beans and big salads can wait until after the race. Plain rice, white bread, eggs, chicken and bananas rarely cause problems.

And skip the ibuprofen. In a controlled study, 400 mg of ibuprofen taken twice before cycling raised a marker of small-intestine injury and made the gut leakier than cycling alone, and the authors concluded its use by athletes should be discouraged (van Wijck et al., 2012). Taking a painkiller at the start “just in case” adds to exactly the damage this article is about.

What to do when it goes wrong at a checkpoint

Treat it as a heat problem first and a food problem second.

1. Walk for 10 to 15 minutes and cool down. Ice, shade, water on your head and neck.
2. Pause solids. Sip water or flat cola in small mouthfuls rather than forcing food in.
3. Restart small. When the nausea eases, a few mouthfuls of something simple every 10 minutes: cola, a piece of banana, plain rice. Build back up over half an hour.
4. Don’t stop fuelling for hours. An empty stomach brings its own low later, especially if the next checkpoint is far away.

Fluid strength matters too. Some runners do better keeping bottles as plain water and taking sodium separately, from salty checkpoint food or a caplet. Elec Stamina, for example, has 220 mg of sodium per caplet, swallowed with water. Under an hour you don’t need any sodium beyond water. Between 60 and 90 minutes, only if it’s hot, you sweat heavily or saltily, or you’re training twice that day. Past 90 minutes in the heat, plan it, sized to the fluid you actually drink; our sodium per hour guide has the numbers.

Repeated vomiting, confusion or being unable to keep fluids down is a medical issue, not a fuelling one. Tell the medics. Vomiting with a headache after drinking a lot can be low blood sodium rather than dehydration; see dehydration vs hyponatremia in ultras.

Once your gut can take your target, put the full plan into our race fueling calculator so you know what to carry between checkpoints.

References

1. Stuempfle KJ, Hoffman MD (2015). Gastrointestinal distress is common during a 161-km ultramarathon. Journal of Sports Sciences, 33(17), 1814-1821. doi:10.1080/02640414.2015.1012104
2. de Oliveira EP, Burini RC, Jeukendrup A (2014). Gastrointestinal complaints during exercise: prevalence, etiology, and nutritional recommendations. Sports Medicine, 44(Suppl 1), S79-S85. doi:10.1007/s40279-014-0153-2
3. Costa RJS, Snipe RMJ, Kitic CM, Gibson PR (2017a). Systematic review: exercise-induced gastrointestinal syndrome, implications for health and intestinal disease. Alimentary Pharmacology & Therapeutics, 46(3), 246-265. doi:10.1111/apt.14157
4. Snipe RMJ, Khoo A, Kitic CM, Gibson PR, Costa RJS (2018). The impact of exertional-heat stress on gastrointestinal integrity, gastrointestinal symptoms, systemic endotoxin and cytokine profile. European Journal of Applied Physiology, 118(2), 389-400. doi:10.1007/s00421-017-3781-z
5. Pfeiffer B, Stellingwerff T, Hodgson AB, et al. (2012). Nutritional intake and gastrointestinal problems during competitive endurance events. Medicine & Science in Sports & Exercise, 44(2), 344-351. doi:10.1249/MSS.0b013e31822dc809
6. Costa RJS, Miall A, Khoo A, et al. (2017b). Gut-training: the impact of two weeks repetitive gut-challenge during exercise on gastrointestinal status, glucose availability, fuel kinetics, and running performance. Applied Physiology, Nutrition, and Metabolism, 42(5), 547-557. doi:10.1139/apnm-2016-0453
7. Jeukendrup AE (2017). Training the gut for athletes. Sports Medicine, 47(Suppl 1), 101-110. doi:10.1007/s40279-017-0690-6
8. Lis DM, Stellingwerff T, Kitic CM, Fell JW, Ahuja KDK (2018). Low FODMAP: a preliminary strategy to reduce gastrointestinal distress in athletes. Medicine & Science in Sports & Exercise, 50(1), 116-123. doi:10.1249/MSS.0000000000001419
9. van Wijck K, Lenaerts K, van Bijnen AA, et al. (2012). Aggravation of exercise-induced intestinal injury by ibuprofen in athletes. Medicine & Science in Sports & Exercise, 44(12), 2257-2262. doi:10.1249/MSS.0b013e318265dd3d

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