Runner's Gut Isn't "Just Nerves." It's Blood Flow. Here's the Fix.

Runner's gut, the cramps, nausea and sudden urgent toilet stops that hit during or after a run, is mostly physiology, not weakness or nerves. When you run hard, your body diverts blood away from your gut to your muscles and skin, and the stressed gut struggles to digest and absorb. The fix is to protect gut blood flow and train your gut like any other muscle: stay hydrated, choose the right foods, and practise fuelling on your runs.

Key takeaways

• Gut symptoms are extremely common: reviews estimate 30 to 50% of athletes are affected, and some studies find up to 90% of distance runners.
• At high intensity, blood flow to the gut can fall by 80% or more, especially in the heat or when you're dehydrated.
• Jostling, high-fibre or fatty foods, concentrated drinks, FODMAPs and anti-inflammatory painkillers all make it worse.
• The gut is trainable. In one study, two weeks of practising fuelling during runs cut gut symptoms by about 60%.
• Blood in your stool, symptoms at rest or unexplained weight loss are reasons to see a doctor, not things to train through.

What counts as runner's gut

"Runner's gut" isn't a formal diagnosis. It's a catch-all for gastrointestinal symptoms linked to exercise, and they fall into two groups.

• Upper gut: nausea, reflux or heartburn, bloating, belching, stomach pain, sometimes vomiting.
• Lower gut: cramping, gas, an urgent need to go, loose stools or diarrhoea.

They're common. A 2014 review in Sports Medicine by de Oliveira, Burini and Jeukendrup concluded that generally 30 to 50% of athletes experience gut complaints during exercise, with even higher rates in ultramarathons. Most symptoms are mild and harmless, but they can wreck a race, and occasionally they're a sign of something that needs medical attention.

Why running hits your gut: the blood-flow problem

Your muscles win the blood-flow battle

When you run, your heart can only pump so much blood. Your working muscles, heart, lungs and skin (to shed heat) all demand more, so your nervous system narrows blood vessels supplying your stomach and intestines. A 2012 review in Alimentary Pharmacology & Therapeutics by ter Steege and Kolkman reported that with longer or more intense exercise, blood flow to the gut can drop by 80% or more.

That matters because the lining of your gut needs a steady oxygen supply to work. In a 2011 study published in PLOS ONE, van Wijck and colleagues had healthy men cycle for 60 minutes at 70% of their maximum workload. Gut blood flow dropped quickly, and blood markers showed small, measurable injury to the intestinal lining and a temporary increase in "leakiness." The damage correlated with how much blood flow fell. It recovered within hours, but during the session the gut was clearly struggling.

Dehydration and heat make it worse

When you're low on fluid, or running in hot weather, your body has even less blood to share with your gut. The de Oliveira review highlights that reduced gut blood flow is worse when athletes are dehydrated. That's why the same gel that sits fine on a cool morning can cause trouble at 30°C (86°F).

The other culprits

Blood flow is the main driver, but it rarely works alone.

• Mechanical jostling: running bounces your organs with every stride, which is one reason runners report more lower-gut problems than cyclists or swimmers.
• What you ate before: fibre, fat and large amounts of protein in the hours before a run slow digestion and leave more in your gut.
• Concentrated drinks and gels: very sugary, high-osmolality fluids pull water into the gut and can cause cramps and diarrhoea, especially without enough water.
• FODMAPs: fermentable carbohydrates found in onion, garlic, beans, apples, wheat and lactose can produce gas and urgency in sensitive people.
• Painkillers: in a 2012 study in Medicine & Science in Sports & Exercise, van Wijck and colleagues found that taking two 400 mg doses of ibuprofen before exercise made gut injury worse than exercise alone. The researchers concluded NSAID use by athletes should be discouraged.
• Nerves: the gut-brain connection is real, and race-day anxiety can speed up your gut. But nerves usually add to the physical causes rather than replace them.

The fix: before, during and in training

In the 24 to 48 hours before a race

• Switch to lower-fibre carbohydrate: white rice, white bread, pasta, peeled potatoes. Our modern carb-loading guide shows how to do this without overeating fibre.
• If you suspect FODMAPs, try a short low-FODMAP period. A 2018 study in Medicine & Science in Sports & Exercise by Lis and colleagues found six days of low-FODMAP eating reduced daily gut symptoms in runners prone to exercise-related problems.
• Keep dinner simple and early. See what to eat the night before a race.

On race morning

• Eat 2 to 4 hours before the start, a familiar low-fibre, low-fat breakfast.
• Arrive early enough to use the toilet calmly. Coffee helps many runners "go" before the start, but test it in training first.
• Skip ibuprofen and similar painkillers before and during the run.

During the run

• Start hydrated and keep drinking. Follow a plan based on your sweat rate and conditions rather than waiting until you're parched.
• Respect the heat. Slow down by 10 to 20 seconds per km (15 to 30 seconds per mile) on hot days. Lower intensity means more blood for your gut.
• Wash gels down with water. Concentrated sugar without fluid is a classic recipe for cramps.
• Use mixed sugars at higher intakes. Products combining glucose and fructose use different gut transporters, which can help you absorb more carbohydrate per hour.

Train your gut

This is the biggest and most overlooked fix. Your gut adapts to what you ask it to do. In a 2017 study in Applied Physiology, Nutrition, and Metabolism, Costa and colleagues had endurance runners practise taking carbohydrate every 20 minutes during daily one-hour runs for two weeks. Compared with a placebo group, gut symptoms fell by about 60% and performance in a one-hour distance test improved by about 5%.

A simple gut-training plan (4 to 6 weeks)

• Weeks 1 to 2: on one or two long runs per week, take about 30 g of carbohydrate per hour (roughly one gel or 500 ml of sports drink), starting 30 minutes in.
• Weeks 3 to 4: increase to 45 to 60 g per hour on long runs, using the exact gels, drinks or foods you'll race with.
• Weeks 5 to 6: practise your full race plan, up to 60 to 90 g per hour for marathons and ultras, during a long run that includes some race-pace kilometres.
• Throughout: rehearse your pre-race dinner and breakfast before your longest runs, and keep notes on what you ate and how your gut felt.

Pair this with a sensible taper so you arrive fresh; our race-week taper plan covers that side.

When to see a doctor

Runner's gut is common, but some symptoms aren't normal. See a doctor promptly if you notice:

• Blood in your stool, or black, tarry stools.
• Gut symptoms when you're not exercising, or that wake you at night.
• Unexplained weight loss, ongoing diarrhoea or severe abdominal pain.
• Persistent fatigue, which can signal iron deficiency from gut blood loss.

Conditions such as coeliac disease, inflammatory bowel disease, irritable bowel syndrome and lactose intolerance can all look like runner's gut. A doctor can test for them, and a sports dietitian can help you build a race-fuelling plan around your gut.

Find your next race on Runtra

Ready to put your gut training to the test? Browse upcoming half marathons and trail runs, or explore the full race calendar to find your next goal race.

FAQ

Why do I always need the toilet when I run?

During running, blood is diverted from your gut, and the bouncing motion stimulates your intestines. Combined with fibre, fat, coffee or FODMAPs from earlier meals, that can trigger urgency. Eating a low-fibre meal 2 to 4 hours before and building a pre-run toilet routine usually helps.

How do I stop stomach cramps during a long run?

Stay hydrated, slow down slightly in the heat, and always take gels with water. Avoid high-fibre and fatty foods in the hours before you run. Practising your fuelling on long runs over several weeks trains your gut to tolerate carbohydrate with fewer cramps.

Can you really train your gut for running?

Yes. In a 2017 study, runners who practised taking carbohydrate during daily runs for two weeks had about 60% fewer gut symptoms and ran better than a placebo group. Start with about 30 g per hour on long runs and build gradually toward your race-day target.

Is it safe to take ibuprofen before a marathon?

It's best to avoid it. Research shows ibuprofen taken before exercise worsens exercise-related injury to the intestinal lining, and NSAIDs also put extra strain on the kidneys during long, hot efforts. Talk to a doctor if you need pain relief around races.

When is runner's gut a sign of something serious?

See a doctor if you have blood in your stool, black stools, symptoms when you're not running, unexplained weight loss, ongoing diarrhoea or severe pain. These can point to conditions such as coeliac disease, inflammatory bowel disease or gut bleeding. Don't try to train through these symptoms.