⏵ Fueling guide · Free

How to Avoid Stomach Problems During an Ultramarathon

To keep your stomach happy in an ultra you train your gut for 6 to 8 weeks before race day. Practice your race fueling on your long runs, start near 30 grams of carbohydrate per hour, and add about 10 grams a week toward a 60 to 90 g/hr target. On race day, eat small amounts on a steady timer, do not let your drinks get too concentrated, match your fluid and sodium to your sweat rate, and stick to low-fiber, low-fat, low-acid foods. And if your stomach turns on you out there, slow down, cool off, and reset by sipping water for 30 to 60 minutes before you start fueling again.

⏵ The one thing to understand first

Why your gut fails late in a race

GI distress is the number one cause of ultramarathon DNFs. In a study of a 161 km (100 mile) race, the large majority of runners reported gut symptoms, and nausea or vomiting alone accounted for roughly a quarter of all the dropouts. This is not a willpower thing. It is physiology, and it happens to almost everyone.

When you run hard for hours your body pulls blood away from the gut and sends it to your working muscles and your skin (to shed heat). That drop in gut blood flow slows down how fast your stomach empties and how much you absorb. Pile on the jostling of running, dehydration, heat, and hours of fuel, and the stomach backs up. Here is the takeaway: by the time you feel nauseous you are usually already behind on fuel, fluid, or sodium. Almost everything below is about heading off that backup before it ever starts.

Train your gut: the week-by-week protocol

Your gut trains like anything else you work on. A 2023 systematic review in Sports Medicine found that even two weeks of repeated gut challenge cut gut discomfort by up to about 47% and reduced carbohydrate malabsorption by roughly 45 to 54%. The usual way to do it is to start 6 to 8 weeks out and ramp your hourly carb intake on your long runs.

Week	Carbs / hour	Focus
Week 1	30 g/hr	Pick a number you know causes zero gut issues. Set a 20 min eat timer and just hit it.
Week 2	40 g/hr	Add a second fuel source so you are not stuck on one flavor all day.
Week 3	50 g/hr	Mix glucose and fructose. Your gut can absorb more than one sugar at a time.
Week 4	60 g/hr	Now you are in the normal ultra band. Run it on your longest day.
Week 5	70 g/hr	Practice in heat and on the climbs. That is where your gut takes less.
Week 6	80 g/hr	A lot of runners cap here. If 80 feels hard, just hold.
Weeks 7 to 8	80 to 90 g/hr	Hit your race number on 2+ dress rehearsal long runs, then taper.

This ramp is just an example. Only move up when the step you are on feels easy, and if a week gives you problems, hold there or drop back instead of bulling through it. A lot of ultrarunners settle into the 60 to 80 g/hr range and never need 90+. The whole point is that your race number feels routine by race day, not like an experiment.

Make the practice race-realistic

Use the exact products, flavors, and bottle concentrations you are going to race with. A gut that is trained on one gel can still turn on a different brand at mile 60. Rotate sweet and savory too, so palate fatigue does not stop you from eating.

Eat small amounts on a steady timer, every 15 to 20 minutes, not big doses once an hour. Little inputs often empty and absorb a lot easier than one big slug of calories that just sits in your stomach. And practice in heat and on the climbs, because both of those shrink how much your gut will take.

How many carbs per hour should you actually target?

For anything past about 2.5 hours, the band most people land on is 60 to 90 grams of carbohydrate per hour, using a glucose plus fructose blend so you can absorb more than a single sugar lets you. Shorter stuff needs less, and only a heavily trained gut gets anything out of 90 to 120 g/hr.

Effort length	Carbs / hour	Notes
Under 1 hour	Little to none needed	Stored glycogen covers it. Water is usually all you need.
1 to 2.5 hours	30 to 60 g/hr	One carb source or a mix is fine down at the lower band.
2.5+ hours (most ultras)	60 to 90 g/hr	You want a glucose and fructose blend and a trained gut to hit the high end.
Very long / elite	90 to 120 g/hr	Only with a heavily trained gut. Most of us do not need or tolerate this.

For the whole breakdown, see our guide on how many carbs per hour you need for an ultramarathon, and put your own numbers into the free ultra fueling calculator.

Hydration, sodium, and the concentration trap

Two of the most common gut wreckers are quiet ones: too little fluid, and drinks that are too concentrated. And they come at you from the same direction.

Dehydration starves the gut of blood

When you run low on fluid your blood volume drops, which cuts into the gut blood flow that running is already stealing. Digestion stalls and the nausea climbs. Sip steadily to match your sweat rate instead of swinging between bone dry and chugging a whole bottle at an aid station, which just sloshes around and overwhelms a stomach that is already slow.

Over-concentrated drinks delay emptying

Dumping extra carb or electrolyte powder into a bottle raises its osmolality. A really concentrated drink empties out of your stomach slow and pulls water into your intestine, and that is a straight shot to bloating, cramping, and diarrhea. Keep your mix at a sane concentration and pair your carbs with enough plain fluid.

For sodium, a good place to start is roughly 500 mg per liter of fluid, and bump it up if you are a salty or heavy sweater or you are out in real heat. Sodium is about replacing what you sweat out and keeping your fluid balance, it is not some magic anti-cramp button. Check the sodium guide below to dial in your own number.

Go deeper: how much sodium per hour for ultra running and how to build an hour-by-hour ultramarathon fueling plan.

The easiest foods on a struggling stomach

When the gut is under stress, simpler and more liquid wins. Liquid carbs empty out fastest, so they are your safest calories late in a race. For solids, lean low-fiber, low-fat, and low-acid. And the same rule goes for your meals in the 24 hours before the race.

Category	Examples
Liquid carbs (gentlest)	Sports drink, gels with water, carb drink mix, a recovery shake, smoothies
Low-fiber, low-fat carbs	White bread, pretzels, plain bagel, boiled potatoes with salt, white rice, ramen broth
Easy fruit	Banana, applesauce pouch, oranges
Settle-the-stomach	Flat cola or ginger ale, ginger chews, broth, salty crackers
Avoid mid-race	High fiber (raw veg, whole grains, beans), fatty or greasy food, very acidic foods, big solid meals

When your stomach shuts down mid-race

Even with good prep, an ultra can turn your gut on you. The mistake is to keep forcing food into a stomach that is not moving. Do not do that. Run the reset instead.

1. Stop eating. You cannot fix a backed-up gut by adding to it. Pause all the solid fuel.
2. Slow down. Drop to a walk or an easy hike so blood goes back to your digestion. This is the most reliable fix there is.
3. Cool off. Ice in a hat or a bandana, water on your head and neck. Cooling pulls blood back toward your core too.
4. Sip, do not chug. Small sips of water or a watered-down drink for 30 to 60 minutes. Ginger, flat cola, or broth help settle the nausea.
5. Restart gently. Once the queasiness lifts and your stomach feels empty, start back in with liquid calories or the blandest carb you can handle, then build back up. Eat on the uphill hikes, where there is less jostling.

Run this reset at the first sign of trouble, not after you have already thrown up. Catch it early and a few easy minutes can save a race that would otherwise spiral into a DNF. And pacing the whole day by effort makes the trouble a lot less likely to begin with: see how to pace an ultramarathon by effort.

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Stomach and gut training FAQ

Why do I get nausea or stop being able to eat late in an ultra?

When you run hard for hours your body pulls blood away from the gut and sends it to your working muscles and your skin (to dump heat). That drop in splanchnic blood flow slows down how fast your stomach empties and how much you absorb, so food just sits there instead of moving through. Now add the constant jostling of running, dehydration (which drops gut blood flow even more), heat, and hours of fueling, and your stomach backs up. Nausea is so common it is the single biggest cause of ultra DNFs. In a study of a 161 km race, GI symptoms hit the large majority of runners and nausea or vomiting alone accounted for roughly a quarter of the dropouts. Here is the part that matters: by the time you feel sick you are usually already behind on fuel, fluid, or sodium. So you want to prevent it, not rescue it.

What is gut training and how many weeks before race day do I start?

Gut training just means you practice your race-day fueling on purpose so your stomach gets used to absorbing carbs while you run. And it works. A 2023 systematic review in Sports Medicine found that even two weeks of repeated gut challenge cut gut discomfort by up to about 47% and dropped carbohydrate malabsorption by roughly 45 to 54%. Start the real ramp about 6 to 8 weeks out from your goal race, earlier if you have the time, and do the gut-focused fueling on one or two long runs a week. You want to hit your race carb number on a couple of dress rehearsal long runs before you taper.

How do I ramp carb tolerance (start 30g/hr, add 10g/hr per week)?

Start at a number you know does not bother you, usually around 30 g of carbohydrate per hour, on your long run. Each week add about 10 g/hr on one or two key long runs, working up through 40, 50, 60, 70, and 80 g/hr until you hit your race target, which for most ultras lands somewhere in the 60 to 90 g/hr band. Use a mix of glucose and fructose so your gut can take in more than one sugar at a time, eat small amounts on a steady timer (every 15 to 20 minutes) instead of big doses once an hour, and practice in heat and on the climbs. Hit your goal number on two or more rehearsal long runs, then back off during the taper.

What should I do when my stomach shuts down mid-race?

Stop forcing solid food, right away. You cannot fix a backed-up gut by cramming more into it. Ease your effort all the way down to a walk or an easy hike so blood goes back to your digestion, cool yourself off (ice in a hat or bandana, dump water on your head and neck), and sip small amounts of water or a watered-down drink instead of gulping. Let the stomach clear for 30 to 60 minutes and use ginger, flat cola, or broth to settle the nausea. Then start fueling again with liquid calories or the blandest carb you can keep down before you work back up to solids. Eating on the uphill hikes helps too, since there is less jostling and more blood going to your gut.

Does slowing my pace actually fix GI distress?

Yes, and it is the most reliable in-race fix there is. Your gut and your muscles are fighting over the same blood, so when you run hard your digestion loses and food stops moving. Back the effort off, even down to a walk, and blood goes back to your stomach so it can empty and you can actually absorb what you put in. It feels backwards to slow down when you are already behind, but a few easy minutes now to settle your stomach is a lot cheaper than puking, bonking, or shutting down completely later. Cooling off does the same thing for the same reason, because cooling also pulls blood back toward your core.

Which foods are easiest on the stomach (low-fiber, low-fat, low-acid)?

When your gut is struggling, simpler and more liquid wins. Liquid carbs (sports drink, gels chased with water, a carb mix, a recovery shake, smoothies) are the gentlest because they empty out fastest. For solids, lean low-fiber, low-fat, low-acid: white bread, pretzels, plain bagel, boiled salted potatoes, white rice, ramen broth, banana, and applesauce. Flat cola, ginger, and broth help settle the nausea. Stay away from the opposite stuff, which slows digestion down and brings on the cramping: high-fiber foods (raw vegetables, whole grains, beans), greasy or fatty food, very acidic foods, and big solid meals. And the same low-fiber, low-fat rule goes for your meals in the 24 hours before the race.

How do dehydration and over-concentrated drinks cause stomach issues?

Both of them hit the gut from the same direction. Dehydration shrinks your blood volume, which cuts into the blood flow that running is already pulling away from your stomach, so digestion stalls and the nausea climbs. Over-concentrated drinks (too much carbohydrate or electrolyte powder per bottle) raise the osmolality of what is sitting in your gut, which slows down gastric emptying and pulls water into your intestine, and that means bloating, cramping, and diarrhea. The fix is to keep your drinks at a sane concentration, pair your carbs with enough fluid, and aim for roughly 500 mg of sodium per liter of fluid instead of just dumping in more powder. Sip steadily to match your sweat rate. Do not swing between bone dry and overloaded.

What is the reset trick (stop eating 30-60 min, sip water) when things go bad?

The reset is just a fueling timeout you take on purpose so a backed-up stomach can clear before you try again. The second the nausea or the sloshing shows up: stop eating, slow to an easy hike, cool down, and sip only small amounts of water or a watered-down drink for the next 30 to 60 minutes. Ginger, flat cola, or broth can speed up the settle. Once the queasiness lifts and your stomach feels empty instead of full, start back in slowly with liquid calories or the blandest carb you can handle, then build back to your normal plan. Do it early and this short reset can save a race that would otherwise spiral into puking and a DNF.

This guide is for general training and planning, and it reflects published sports-science consensus and experienced-coach guidance. Carb, sodium, and fluid needs are very individual, and severe or stubborn GI symptoms can be a sign of a real medical issue. Test every strategy in training, never for the first time on race day, and talk to a sports dietitian or a physician for personal medical advice.