Your Gut Will Quit Before Your Legs Do. Here's What to Do About It.

Most runners spend months training their legs, lungs, and mind for an ultra. Almost none of them train their gut. That is a problem, because GI distress does not just hurt your race. For a large number of runners, it ends it.

In one 161 km race, 96 percent of runners had stomach or gut problems.¹ Of the runners who did not finish, about 1 in 3 said gut trouble was one reason they dropped.

The good news is that the gut responds to training. Within two weeks of practice, gut discomfort can drop by nearly half. Keep practicing and your gut can handle more over time. But that adaptation has to be built. It does not come from fitness alone.

What an Ultramarathon Does to Your Gut

When you run hard, blood shifts away from your digestive system and toward your working muscles. At race effort, blood flow to your gut drops a lot. Digestion slows, stomach emptying slows, and the lining of the intestine becomes more permeable. Food that normally passes through smoothly starts to back up or move through too fast. Nausea, cramping, and bloating follow.

Toughness won't fix this. The harder you run, the less blood your gut gets, and the worse it works. Heat makes it worse. A 2025 study had people run at the same effort in warm and mild weather. In the warm runs, blood tests showed more damage to the gut lining, so the gut got leakier.⁵ Summer and desert ultras are harder on your gut, too.

The pounding of running also plays a part. Every foot strike shakes your gut, most of all on rocky trails. That stresses the gut lining in a way that biking and swimming do not.

What a Race Does to Your Gut Bacteria

A 2022 study in Scientific Reports followed nine ultra runners through a 96 to 99 km race.² It tested their gut bacteria before the race, right after, and ten days later. The results right after the race were stark.

Some gut bacteria make a substance called butyrate. The most common one, Faecalibacterium prausnitzii, dropped by about 45 percent on average. Three other butyrate makers also dropped. One runner's F. prausnitzii dropped by 85 percent. That runner felt sluggish and slept badly the whole time they recovered.

Butyrate matters because it is the main fuel for the cells that line your colon. When the bacteria that make it drop, the gut lining gets less support. The gut gets leakier, and gut trouble becomes more likely. The bacteria mostly bounced back by day ten. But right after a race, your gut is weaker in ways you can't always feel.

How Does Training Load Affect Gut Health?

A 2025 study from Edith Cowan University got a lot of attention in a February 2026 press release. It found that how hard elite athletes train changes their gut bacteria, and not always the way you'd guess.³

Researchers tested 23 top rowers in hard and easy training weeks. In hard weeks, the rowers had more butyric and propionic acid in their gut. Gut bacteria make both, and they help keep the gut healthy. The rowers also went to the bathroom more often. In easy weeks, they ate fewer fruits and vegetables and more takeout, and their gut bacteria changed.

Hard weeks did not look worse for these rowers' guts. In easy weeks, their diets slipped and their gut bacteria shifted. The authors say it's still unclear what these changes mean for gut health. What you eat matters in easy weeks too, not just hard ones.

What Is Gut Training and How Does It Work?

"Gut training" means teaching your gut to take in more carbs per hour without getting upset. Scientists think this works because your gut makes more of the proteins that move sugar into your blood, called SGLT1 and GLUT5. This has been shown in animals but not yet measured in people.

A 2023 review in Sports Medicine pulled together 8 studies. In two of them, two weeks of eating more carbs during training cut gut discomfort by 47 percent on average.⁴ Carbs that passed through without being absorbed dropped by 45 to 54 percent.

This is why trying to eat 90 grams of carbs per hour on race day, when you have been training on 40, does not work. The gut has not been prepared for it. You are asking it to absorb volume it has never practiced handling under the stress of racing.

For most runners, 90 grams per hour is the ceiling. You can go higher if you have trained your gut for a few cycles and use a product with a 2:1 mix of glucose and fructose. Studies show some trained athletes can reach 90 to 120 grams per hour. Most runners should not aim for that. Wait until you can take in 75 to 90 grams every time without trouble.

What Is the Gut Training Protocol for Ultramarathon Runners?

Gut training does not require a separate program. It integrates into your existing long run structure. The key is practicing at race-level carbohydrate intake during runs that already stress your system.

Phase	Duration	What to Do
Foundation	Weeks 1–2	Eat 40–50g carbs/hr on every long run over 90 minutes. Focus on consistent timing, not volume. Set a watch alarm every 20 minutes.
Build	Weeks 3–5	Increase to 60g/hr. Introduce a 2:1 glucose-to-fructose product (gels or chews with maltodextrin + fructose) to open both absorption pathways.
Race Simulation	Weeks 6–8	Practice at 75–90g/hr on runs of 3 hours or more. This is where real food (bananas, potatoes, rice balls) should enter the rotation alongside gels.
Maintenance	Until race	Hold at race-target intake on all long runs. Do not back off fueling during taper long runs. The adaptation will be lost faster than it was built if you stop practicing.

A few specifics worth noting. Practice eating at race effort, not just on easy days. Your gut at threshold or race pace is a different system than your gut at conversation pace. If you only practice fueling on easy runs, you are not training the state you will be in during the race.

Also practice eating shortly after the start. The first 30 minutes of a race are the easiest time to establish a fueling rhythm. Most runners skip it because they feel good and think they do not need food yet. That decision catches up with them around mile 40 to 50, when the gut is harder to prime and blood sugar is already dipping.

What to eat during gut training

Early on, try different kinds of fuel on your long runs. Your gut may handle something fine on an easy trail run and hate it 5 hours in, at race effort, in the heat. Regular gels are easy to carry and absorb fast. High-carb gels (40 to 90 grams per packet, like PF90 or Maurten 160) mean fewer packets per hour. In races over 50 km, real food matters more late in the race. By then you'll be sick of gels, and your gut will want something more filling.

Test every fuel type you plan to use in a race before race day. Test it at race effort. This is not optional advice.

What Should You Eat Race Week and Race Day to Prevent GI Issues?

The week before

In the 3 to 5 days before a race, eat less fiber. High-fiber veggies, beans, and whole grains are great most days, but they make more gas in your colon. Race-day effort and nerves upset that process, which often leads to cramps and bloating. That is why white rice, pasta, bread, and easy proteins like eggs and chicken are the usual pre-race foods.

Race day morning

Eat 2 to 3 hours before the start if possible. A 400 to 600 calorie meal of familiar carbohydrates gives your stomach time to empty before you start moving hard. If nerves cut your appetite, a smaller meal is fine. Liquid calories (a shake or sports drink) are easier when appetite is low. Avoid high-fat, high-fiber, and high-protein foods in the final two hours.

During the race

Start eating within the first 30 minutes, before hunger arrives. Eat on a schedule, not on feeling. Nausea during a race does not automatically mean you are eating too much. Low blood sugar and low sodium are both common causes of race-day nausea. If nausea hits, try a gel plus an electrolyte tab and give it 10 to 15 minutes before stopping fueling entirely. Broth at aid stations delivers sodium quickly and is often easier on an upset stomach than solid food.

How Do Hot Races Make GI Distress Worse?

Desert and summer ultras add real gut risk. That includes Javelina, Badwater, and any race that stays above 85°F. Heat makes your gut leakier and slows how fast your stomach empties. A fuel plan that works in cool weather may fall apart in the heat.

Two things help. First, heat train for 2 to 4 weeks before a hot race. Run in the hottest part of the day, or soak in a hot bath after easy runs. Both raise your blood volume. That makes the heat easier on your heart, and it seems to make your gut less leaky in the heat too. Second, take in more electrolytes in the heat, most of all sodium. You sweat more and lose more salt. Low blood sodium (hyponatremia) is a real risk. It causes nausea and confusion that look like normal gut trouble, but it has its own fix.

The Short Version

Your gut has to be trained the same way your legs do. It responds to specific, progressive overload. The runners who finish long races are often not the ones with the strongest legs or highest VO2 max. They are the ones whose gut kept working when everyone else's quit. That is a trainable outcome. Start now, before your next long training block, not in the week before your race.