Why Does My Stomach Hurt When I Run? A Guide to Runner's Gut
Nora FiermanLet's talk about something almost every runner has experienced but very few people want to bring up: the sudden, urgent need to find a bathroom mid-run. The moment where your pace becomes secondary to a much more pressing concern. Like, you gotta poop right now.
If this sounds familiar, you're in very good company. The majority of runners and endurance athletes experience some form of GI discomfort during training or racing. This includes everything from mild bloating and cramping to more urgent issue (gotta poop NOW!). In one widely cited statistic, GI problems are considered perhaps the most common cause of underperformance in endurance events. More athletes are slowed down or sidelined by their stomach than by their legs.
So if you've ever wondered why does my stomach hurt when I run, or why running gives you diarrhea, or why you can train for months and still feel like your gut sabotages race day, this is for you. Here's what's actually happening, why it happens, and what the research says about preventing runner's gut.
How Common Is Runner's Gut, Really?
Very common. In one study of 145 endurance runners, men experienced GI discomfort on 84 percent of training runs over the course of 30 days, while women reported symptoms 78 percent of the time. Adverse GI complaints like cramps, nausea, vomiting, and diarrhea affect up to 90% of endurance athletes.
If you've ever felt like you're the only one dealing with this, you're not. It's also not something you just have to live with. There's a real body of research on why runner's gut happens and what helps.
Why Does Running Cause Stomach Problems?
There are three main causes of GI symptoms during exercise: physiological, mechanical, and nutritional. Understanding each one helps explain why running specifically more than cycling, swimming, or hiking tends to trigger runner's gut more than other sports.
Physiological: reduced blood flow to your gut
Blood flow to the GI tract is impaired during exercise, and this is believed to contribute to the development of GI symptoms. Strenuous exercise triggers the release of stress hormones that cause blood vessels in your gut to constrict, redirecting circulation to the muscles, heart, and lungs that need it most during exercise.
In plain terms: your body is making a trade-off. When you're running hard, it prioritizes your legs and lungs over your digestive system. Less blood flow to the gut means digestion slows down and becomes less efficient, which is part of why eating the wrong thing right before or during a run causes problems it wouldn't cause at rest.
This reduced blood flow can range from mild circulatory changes to more significant tissue effects, potentially affecting the gut lining and its barrier function. Heat makes this worse. Heat exposure during exercise further redirects blood away from your internal organs toward your skin and muscles, which is part of why runner's gut tends to spike on hot race days.
Mechanical: the literal bouncing
The mechanical cause of GI symptoms is the bouncing effect of running. This is the most running-specific factor on this list, and it's a big part of why runners experience more GI distress than cyclists. The repetitive up-and-down movement jostles your intestines and sloshes around stomach acid and fluid creating that sensation of needing to use the bathroom that seems to come out of nowhere mid-run.
This is also why eating or drinking right before a run is particularly risky. Research shows that eating and drinking immediately before exercise can cause strong abdominal pain during exercise. Combine recently-eaten food with the mechanical jostling of running, and you've created ideal conditions for runner's gut.
Nutritional: what and how much you're eating
This is the cause you have the most control over and the focus of the rest of this post. Fiber, fat, protein, and fructose can all contribute to GI symptoms, and gastrointestinal complaints are more likely with concentrated carbohydrate solutions like beverages or gels with very high sugar concentrations.
This is a big part of why so many runners associate gels with stomach problems. It's not that carbohydrates themselves are the issue, optimizing carbohydrate intake during exercise according to current recommendations is actually associated with a lower incidence and severity of GI symptoms. It's the concentration and form of those carbohydrates that matters.
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What Actually Helps: Evidence-Based Strategies for Runner's Gut
The good news: this isn't a problem you just have to accept. A 2025 systematic review identified several evidence-based strategies for minimizing GI symptoms during endurance exercise, including gut training, carbohydrate strategy, and dietary approaches like low-FODMAP.
Train your gut, not just your legs
This is one of the more counterintuitive but well-supported findings in recent research. Studies on gut training protocols have shown reductions in GI symptoms of 60 to 63 percent within just two weeks of consistent practice. The idea: just as your muscles adapt to training stress, your gut adapts to processing nutrients during exercise, but only if you practice it consistently rather than only fueling on race day.
In practice, this means eating and drinking during your training runs the same way you plan to during a race, not just on race day itself. If your gut has never processed 60 grams of carbs per hour during a run, race day is the wrong time to find out it can't, and runner's gut is the likely result.
Be strategic about timing
Eating or drinking immediately before exercise increases the risk of abdominal pain during the run. Give yourself enough time before you start, generally two to three hours for a full meal, 30 to 60 minutes for a small snack, so digestion has a head start before the mechanical jostling of running begins.
Choose real food over concentrated sugar solutions
Concentrated carbohydrate solutions with very high sugar concentrations are more likely to cause GI complaints than more moderate concentrations. This is one of the clearest, most practical takeaways from the research and it's exactly why so many runners report less runner's gut on real food than on gels, even when the total carbohydrate amount is similar.
Whole foods like bananas, dates, and smoothie pouches deliver carbohydrates in a form and concentration closer to what your gut evolved to process, which may explain why so many athletes with gel-related GI issues tolerate real food better even at similar carb loads.
Consider a low-FODMAP approach around big efforts
A diet low in certain fermentable sugars, sometimes called a low-FODMAP diet, has been shown to have a positive effect on GI symptoms during exercise. Case studies of ultra-endurance athletes using short low-FODMAP protocols before multi-stage races have shown significant reductions in digestive distress during competition.
This doesn't mean cutting out entire food groups permanently, it means being thoughtful about what you eat in the day or two leading into a big race, particularly avoiding high-FODMAP foods that are known personal triggers.
Watch out for heat and hydration
Even under relatively mild heat conditions, prolonged exercise like a marathon can cause significant GI symptoms related to dehydration and elevated body temperature. Staying ahead of hydration, not just drinking when thirsty, but proactively, helps protect gut blood flow on hot days.
Watch your breathing and drinking technique
Swallowing air while breathing hard and drinking from a bottle at the same time, known as aerophagia, can also contribute to stomach distress. Small sips rather than big gulps, and being mindful of your breathing pattern when drinking, can reduce this.
Building a Gut-Friendly Fueling Strategy
Putting this together, here's a practical framework for preventing runner's gut:
In training: practice your race-day fueling on long runs, not just on race day. Gradually build up the amount and concentration of carbohydrates your gut can handle. Work with a registered dietitian on this!
Before running: leave enough time between eating and starting. A full meal two to three hours out, a small snack 30 to 60 minutes out.
During running: use what you know works so you're not introducing more variables into the mix. Stay ahead of hydration rather than reacting to thirst.
Around big events: if you're sensitive, consider a short low-FODMAP window in the day or two before a race. Not as a permanent diet, but as a targeted strategy.
If symptoms persist: GI issues that are severe, persistent, or include blood should be discussed with a doctor. Most runner's gut is manageable with the strategies above, but it's not something to push through indefinitely if it's not improving.
The Bottom Line on Runner's Gut
Stomach issues during running are extremely common, have real physiological causes, and, importantly, respond to changes in training and nutrition. You're not broken, and you're not alone. As marathon legend Bill Rodgers famously put it, more marathons are won or lost in the portable toilets than at the dinner table, which is either deeply reassuring or deeply concerning depending on how your last long run went.
The fix isn't usually "stop eating during runs." It's eating the right things, at the right times, trained consistently so your gut shows up on race day as prepared as your legs.
🌿 Real food, gentler on your gut.
Neve smoothie pouches are made from whole fruits and vegetables, and packed with natural electrolytes. Designed with a registered dietitian for athletes who want fuel for the long haul.
Shop Neve → or start with a Variety Pack → to find what works for you.