Quick answer: During intense exercise, your body pulls blood away from your digestive tract to feed working muscles. The gut lining temporarily runs short on oxygen and resources, which can cause cramping, nausea, and loose stools. Heat, dehydration, poorly timed meals, and pre-workout ibuprofen all make it worse. Athletes protect their gut by managing those factors and supporting the lining that takes the hit.
Table of Contents
- How Common Gut Problems Are in Athletes
- The Blood Flow Problem
- What Happens to the Gut Lining
- The Ibuprofen Problem
- Four Things That Make It Worse
- How Athletes Protect Their Gut
- When It's Not Just Training
- Our Pick
- Frequently Asked Questions
How Common Gut Problems Are in Athletes
If your stomach turns on you during hard training, you're in large company. A 2014 review in Sports Medicine by de Oliveira, Burini, and Jeukendrup estimated that roughly 30 to 50 percent of endurance athletes experience gastrointestinal symptoms, with rates climbing in marathons, triathlons, and ultra events. Researchers now group these symptoms under a single name, exercise-induced gastrointestinal syndrome, described in a 2017 systematic review by Costa and colleagues in Alimentary Pharmacology & Therapeutics.
The symptoms split into two groups:
- Upper GI: nausea, reflux, belching, a heavy or sloshing stomach, and sometimes vomiting.
- Lower GI: cramping, gas, an urgent need for the bathroom, and diarrhea, often called runner's trots.
It's not limited to runners. Cyclists, CrossFit athletes, rowers, and anyone doing long or high-intensity sessions can experience it.
The Blood Flow Problem
Your body has a fixed amount of blood to work with. At rest, a large share of it goes to your digestive organs. During hard exercise, that priority flips: muscles, heart, lungs, and skin (for cooling) get first call, and blood flow to the gut can drop sharply. Researchers call this splanchnic hypoperfusion.
The gut can handle a short dip. The problem is duration and intensity. The harder and longer you go, and the hotter it is, the longer the gut runs on reduced supply.
What Happens to the Gut Lining
The cells lining your intestine are some of the most active in your body and need a steady supply of oxygen. When blood flow drops, those cells get stressed. Studies by van Wijck and colleagues at Maastricht University measured markers of intestinal cell injury in healthy athletes and found they rose during an hour of intense cycling, and that the rise tracked with how much gut blood flow dropped.
Two things follow from that stress:
- Looser tight junctions: The seals between lining cells can temporarily loosen, increasing gut permeability during and shortly after exercise.
- Slower digestion upstream, faster transit downstream: Stomach emptying can slow (hence the sloshing, nauseous feeling) while the lower gut may speed up (hence the bathroom urgency).
In healthy athletes this is usually temporary, and the lining recovers after the session. But when hard sessions stack up day after day, especially in heat, the lining spends more time recovering than it otherwise would.
The Ibuprofen Problem
Many athletes take ibuprofen before a long run or race to get ahead of soreness. The research suggests that's a bad trade for the gut. In a 2012 study published in Medicine & Science in Sports & Exercise, van Wijck and colleagues found that ibuprofen taken before cycling increased small-intestinal injury markers and gut permeability beyond what exercise alone caused.
The reason is mechanical. NSAIDs like ibuprofen and naproxen block the COX-1 enzyme, which the gut uses to produce the prostaglandins that maintain its protective mucus and blood flow. Exercise already reduces that blood flow. Adding an NSAID takes away another layer of protection at the same moment. If you rely on NSAIDs around training, talk to your doctor about alternatives.
Four Things That Make It Worse
- Heat and dehydration: Both pull even more blood away from the gut toward the skin and reduce circulating volume. GI symptoms reliably spike in hot races.
- Eating too close to training: High-fat, high-fiber, or high-protein meals slow stomach emptying and sit heavy during effort.
- Untrained fueling: Gels, drinks, and chews taken for the first time on race day, or in amounts the gut isn't used to, are a common trigger.
- Pre-race nerves: Stress hormones affect gut motility and sensitivity. The same fueling that works in training can go sideways on race morning.
How Athletes Protect Their Gut
- Time meals: Eat a lower-fat, lower-fiber meal two to three hours before hard sessions, with a small carbohydrate snack closer to the start if you need it.
- Train your gut: Practice race-day fueling during long training sessions. Research on gut training shows athletes can improve how well they tolerate carbohydrates and fluid during exercise.
- Start hydrated and drink steadily: Especially in heat, and for sessions longer than an hour.
- Skip pre-workout NSAIDs: Given the research above, avoid taking ibuprofen or naproxen before hard efforts unless your doctor has advised it.
- Protect recovery days: Stacking hard sessions without easy days gives the lining less time to recover between hits.
- Support the lining day to day: The gut lining depends on its mucus layer as a first line of defense. Mucosal-support botanicals like DGL licorice, slippery elm, and marshmallow root are traditionally used to support that layer. They haven't been studied in athletes specifically, but they target the same tissue that hard training stresses.
When It's Not Just Training
Exercise-related GI symptoms show up during or shortly after effort and settle with rest. See a healthcare provider if you notice blood in your stool or vomit, black or tarry stools, severe or worsening abdominal pain, unexplained weight loss, or symptoms that persist on rest days. Those aren't typical training symptoms and need proper evaluation.
Our Pick
VitaProtect Daily is built for daily mucosal lining support. Each chocolate circular wafer combines GutGard® DGL licorice (75mg, standardized to 3.5% glabridin), slippery elm bark (100mg), and marshmallow root (150mg), three botanicals that work on the gut's protective mucus layer rather than on stomach acid.
For athletes, it fits into the parts of the day you already control: chew one before your pre-training meal and before other meals on heavy training days. The wafer format starts working in the mouth and upper digestive tract instead of waiting to dissolve in the stomach, and there's no extra pill to swallow on a nervous race-morning stomach. It's free of gluten, dairy, soy, and artificial additives. If your training requires regular NSAID use, talk to your doctor, since no supplement cancels out that effect.
Frequently Asked Questions
Why does my stomach hurt during or after hard workouts?
During intense exercise, blood is redirected from the digestive tract to working muscles, heart, and skin. With less blood flow, the gut lining gets less oxygen and fewer resources for its normal upkeep, which can cause cramping, nausea, and loose stools. Heat, dehydration, and eating the wrong foods too close to training make it worse.
How common are stomach problems in athletes?
Very common. Reviews of the research estimate that roughly 30 to 50 percent of endurance athletes report gastrointestinal symptoms, with higher rates in long-distance and ultra-endurance events.
Is it bad to take ibuprofen before a workout or race?
Research suggests it adds stress to the gut. A 2012 study in Medicine and Science in Sports and Exercise found that ibuprofen taken before cycling increased markers of small-intestinal injury and gut barrier dysfunction compared to exercise alone. Talk to your doctor about pain management if you rely on NSAIDs around training.
What are runner's trots?
Runner's trots is the common name for the sudden urge to have a bowel movement, or diarrhea, during or right after running. It's linked to reduced gut blood flow, the mechanical jostling of running, and changes in gut motility during intense effort.
What should I eat before a workout to avoid stomach problems?
Most athletes do best with a lower-fat, lower-fiber, easily digested meal two to three hours before hard training, and a small carbohydrate snack closer to the start if needed. Test any new food or fuel in training, never for the first time on race day.
Can you train your gut for endurance exercise?
Yes. Research on gut training shows that practicing race-day fueling during training sessions can improve how well athletes tolerate carbohydrates and fluids during exercise and reduce GI symptoms over time.
Does dehydration cause stomach problems during exercise?
Dehydration and heat both reduce blood flow to the gut further, which is why GI symptoms tend to spike in hot races and long sessions without enough fluid. Starting hydrated and drinking steadily during long efforts helps.
When should an athlete see a doctor about stomach problems?
See a healthcare provider for blood in the stool or vomit, black stools, severe or worsening abdominal pain, unexplained weight loss, or symptoms that persist outside of training. These aren't typical exercise-related symptoms and need evaluation.
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